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gMock Cookbook
You can find recipes for using gMock here. If you haven't yet, please read this first to make sure you understand the basics.
Note: gMock lives in the testing
name space. For readability, it is
recommended to write using ::testing::Foo;
once in your file before using the
name Foo
defined by gMock. We omit such using
statements in this section for
brevity, but you should do it in your own code.
Creating Mock Classes
Mock classes are defined as normal classes, using the MOCK_METHOD
macro to
generate mocked methods. The macro gets 3 or 4 parameters:
class MyMock {
public:
MOCK_METHOD(ReturnType, MethodName, (Args...));
MOCK_METHOD(ReturnType, MethodName, (Args...), (Specs...));
};
The first 3 parameters are simply the method declaration, split into 3 parts. The 4th parameter accepts a closed list of qualifiers, which affect the generated method:
const
- Makes the mocked method aconst
method. Required if overriding aconst
method.override
- Marks the method withoverride
. Recommended if overriding avirtual
method.noexcept
- Marks the method withnoexcept
. Required if overriding anoexcept
method.Calltype(...)
- Sets the call type for the method (e.g. toSTDMETHODCALLTYPE
), useful in Windows.
Dealing with unprotected commas
Unprotected commas, i.e. commas which are not surrounded by parentheses, prevent
MOCK_METHOD
from parsing its arguments correctly:
class MockFoo {
public:
MOCK_METHOD(std::pair<bool, int>, GetPair, ()); // Won't compile!
MOCK_METHOD(bool, CheckMap, (std::map<int, double>, bool)); // Won't compile!
};
Solution 1 - wrap with parentheses:
class MockFoo {
public:
MOCK_METHOD((std::pair<bool, int>), GetPair, ());
MOCK_METHOD(bool, CheckMap, ((std::map<int, double>), bool));
};
Note that wrapping a return or argument type with parentheses is, in general,
invalid C++. MOCK_METHOD
removes the parentheses.
Solution 2 - define an alias:
class MockFoo {
public:
using BoolAndInt = std::pair<bool, int>;
MOCK_METHOD(BoolAndInt, GetPair, ());
using MapIntDouble = std::map<int, double>;
MOCK_METHOD(bool, CheckMap, (MapIntDouble, bool));
};
Mocking Private or Protected Methods
You must always put a mock method definition (MOCK_METHOD
) in a public:
section of the mock class, regardless of the method being mocked being public
,
protected
, or private
in the base class. This allows ON_CALL
and
EXPECT_CALL
to reference the mock function from outside of the mock class.
(Yes, C++ allows a subclass to change the access level of a virtual function in
the base class.) Example:
class Foo {
public:
...
virtual bool Transform(Gadget* g) = 0;
protected:
virtual void Resume();
private:
virtual int GetTimeOut();
};
class MockFoo : public Foo {
public:
...
MOCK_METHOD(bool, Transform, (Gadget* g), (override));
// The following must be in the public section, even though the
// methods are protected or private in the base class.
MOCK_METHOD(void, Resume, (), (override));
MOCK_METHOD(int, GetTimeOut, (), (override));
};
Mocking Overloaded Methods
You can mock overloaded functions as usual. No special attention is required:
class Foo {
...
// Must be virtual as we'll inherit from Foo.
virtual ~Foo();
// Overloaded on the types and/or numbers of arguments.
virtual int Add(Element x);
virtual int Add(int times, Element x);
// Overloaded on the const-ness of this object.
virtual Bar& GetBar();
virtual const Bar& GetBar() const;
};
class MockFoo : public Foo {
...
MOCK_METHOD(int, Add, (Element x), (override));
MOCK_METHOD(int, Add, (int times, Element x), (override));
MOCK_METHOD(Bar&, GetBar, (), (override));
MOCK_METHOD(const Bar&, GetBar, (), (const, override));
};
Note: if you don't mock all versions of the overloaded method, the compiler
will give you a warning about some methods in the base class being hidden. To
fix that, use using
to bring them in scope:
class MockFoo : public Foo {
...
using Foo::Add;
MOCK_METHOD(int, Add, (Element x), (override));
// We don't want to mock int Add(int times, Element x);
...
};
Mocking Class Templates
You can mock class templates just like any class.
template <typename Elem>
class StackInterface {
...
// Must be virtual as we'll inherit from StackInterface.
virtual ~StackInterface();
virtual int GetSize() const = 0;
virtual void Push(const Elem& x) = 0;
};
template <typename Elem>
class MockStack : public StackInterface<Elem> {
...
MOCK_METHOD(int, GetSize, (), (override));
MOCK_METHOD(void, Push, (const Elem& x), (override));
};
Mocking Non-virtual Methods
gMock can mock non-virtual functions to be used in Hi-perf dependency injection..
In this case, instead of sharing a common base class with the real class, your
mock class will be unrelated to the real class, but contain methods with the
same signatures. The syntax for mocking non-virtual methods is the same as
mocking virtual methods (just don't add override
):
// A simple packet stream class. None of its members is virtual.
class ConcretePacketStream {
public:
void AppendPacket(Packet* new_packet);
const Packet* GetPacket(size_t packet_number) const;
size_t NumberOfPackets() const;
...
};
// A mock packet stream class. It inherits from no other, but defines
// GetPacket() and NumberOfPackets().
class MockPacketStream {
public:
MOCK_METHOD(const Packet*, GetPacket, (size_t packet_number), (const));
MOCK_METHOD(size_t, NumberOfPackets, (), (const));
...
};
Note that the mock class doesn't define AppendPacket()
, unlike the real class.
That's fine as long as the test doesn't need to call it.
Next, you need a way to say that you want to use ConcretePacketStream
in
production code, and use MockPacketStream
in tests. Since the functions are
not virtual and the two classes are unrelated, you must specify your choice at
compile time (as opposed to run time).
One way to do it is to templatize your code that needs to use a packet stream.
More specifically, you will give your code a template type argument for the type
of the packet stream. In production, you will instantiate your template with
ConcretePacketStream
as the type argument. In tests, you will instantiate the
same template with MockPacketStream
. For example, you may write:
template <class PacketStream>
void CreateConnection(PacketStream* stream) { ... }
template <class PacketStream>
class PacketReader {
public:
void ReadPackets(PacketStream* stream, size_t packet_num);
};
Then you can use CreateConnection<ConcretePacketStream>()
and
PacketReader<ConcretePacketStream>
in production code, and use
CreateConnection<MockPacketStream>()
and PacketReader<MockPacketStream>
in
tests.
MockPacketStream mock_stream;
EXPECT_CALL(mock_stream, ...)...;
.. set more expectations on mock_stream ...
PacketReader<MockPacketStream> reader(&mock_stream);
... exercise reader ...
Mocking Free Functions
It's possible to use gMock to mock a free function (i.e. a C-style function or a static method). You just need to rewrite your code to use an interface (abstract class).
Instead of calling a free function (say, OpenFile
) directly, introduce an
interface for it and have a concrete subclass that calls the free function:
class FileInterface {
public:
...
virtual bool Open(const char* path, const char* mode) = 0;
};
class File : public FileInterface {
public:
...
virtual bool Open(const char* path, const char* mode) {
return OpenFile(path, mode);
}
};
Your code should talk to FileInterface
to open a file. Now it's easy to mock
out the function.
This may seem like a lot of hassle, but in practice you often have multiple related functions that you can put in the same interface, so the per-function syntactic overhead will be much lower.
If you are concerned about the performance overhead incurred by virtual functions, and profiling confirms your concern, you can combine this with the recipe for mocking non-virtual methods.
Old-Style MOCK_METHODn
Macros
Before the generic MOCK_METHOD
macro was introduced, mocks where created using
a family of macros collectively called MOCK_METHODn
. These macros are still
supported, though migration to the new MOCK_METHOD
is recommended.
The macros in the MOCK_METHODn
family differ from MOCK_METHOD
:
- The general structure is
MOCK_METHODn(MethodName, ReturnType(Args))
, instead ofMOCK_METHOD(ReturnType, MethodName, (Args))
. - The number
n
must equal the number of arguments. - When mocking a const method, one must use
MOCK_CONST_METHODn
. - When mocking a class template, the macro name must be suffixed with
_T
. - In order to specify the call type, the macro name must be suffixed with
_WITH_CALLTYPE
, and the call type is the first macro argument.
Old macros and their new equivalents:
Simple | |
---|---|
Old | `MOCK_METHOD1(Foo, bool(int))` |
New | `MOCK_METHOD(bool, Foo, (int))` |
Const Method | |
Old | `MOCK_CONST_METHOD1(Foo, bool(int))` |
New | `MOCK_METHOD(bool, Foo, (int), (const))` |
Method in a Class Template | |
Old | `MOCK_METHOD1_T(Foo, bool(int))` |
New | `MOCK_METHOD(bool, Foo, (int))` |
Const Method in a Class Template | |
Old | `MOCK_CONST_METHOD1_T(Foo, bool(int))` |
New | `MOCK_METHOD(bool, Foo, (int), (const))` |
Method with Call Type | |
Old | `MOCK_METHOD1_WITH_CALLTYPE(STDMETHODCALLTYPE, Foo, bool(int))` |
New | `MOCK_METHOD(bool, Foo, (int), (Calltype(STDMETHODCALLTYPE)))` |
Const Method with Call Type | |
Old | `MOCK_CONST_METHOD1_WITH_CALLTYPE(STDMETHODCALLTYPE, Foo, bool(int))` |
New | `MOCK_METHOD(bool, Foo, (int), (const, Calltype(STDMETHODCALLTYPE)))` |
Method with Call Type in a Class Template | |
Old | `MOCK_METHOD1_T_WITH_CALLTYPE(STDMETHODCALLTYPE, Foo, bool(int))` |
New | `MOCK_METHOD(bool, Foo, (int), (Calltype(STDMETHODCALLTYPE)))` |
Const Method with Call Type in a Class Template | |
Old | `MOCK_CONST_METHOD1_T_WITH_CALLTYPE(STDMETHODCALLTYPE, Foo, bool(int))` |
New | `MOCK_METHOD(bool, Foo, (int), (const, Calltype(STDMETHODCALLTYPE)))` |
The Nice, the Strict, and the Naggy
If a mock method has no EXPECT_CALL
spec but is called, we say that it's an
"uninteresting call", and the default action (which can be specified using
ON_CALL()
) of the method will be taken. Currently, an uninteresting call will
also by default cause gMock to print a warning. (In the future, we might remove
this warning by default.)
However, sometimes you may want to ignore these uninteresting calls, and sometimes you may want to treat them as errors. gMock lets you make the decision on a per-mock-object basis.
Suppose your test uses a mock class MockFoo
:
TEST(...) {
MockFoo mock_foo;
EXPECT_CALL(mock_foo, DoThis());
... code that uses mock_foo ...
}
If a method of mock_foo
other than DoThis()
is called, you will get a
warning. However, if you rewrite your test to use NiceMock<MockFoo>
instead,
you can suppress the warning:
using ::testing::NiceMock;
TEST(...) {
NiceMock<MockFoo> mock_foo;
EXPECT_CALL(mock_foo, DoThis());
... code that uses mock_foo ...
}
NiceMock<MockFoo>
is a subclass of MockFoo
, so it can be used wherever
MockFoo
is accepted.
It also works if MockFoo
's constructor takes some arguments, as
NiceMock<MockFoo>
"inherits" MockFoo
's constructors:
using ::testing::NiceMock;
TEST(...) {
NiceMock<MockFoo> mock_foo(5, "hi"); // Calls MockFoo(5, "hi").
EXPECT_CALL(mock_foo, DoThis());
... code that uses mock_foo ...
}
The usage of StrictMock
is similar, except that it makes all uninteresting
calls failures:
using ::testing::StrictMock;
TEST(...) {
StrictMock<MockFoo> mock_foo;
EXPECT_CALL(mock_foo, DoThis());
... code that uses mock_foo ...
// The test will fail if a method of mock_foo other than DoThis()
// is called.
}
NOTE: NiceMock
and StrictMock
only affects uninteresting calls (calls of
methods with no expectations); they do not affect unexpected calls (calls of
methods with expectations, but they don't match). See
Understanding Uninteresting vs Unexpected Calls.
There are some caveats though (I dislike them just as much as the next guy, but sadly they are side effects of C++'s limitations):
NiceMock<MockFoo>
andStrictMock<MockFoo>
only work for mock methods defined using theMOCK_METHOD
macro directly in theMockFoo
class. If a mock method is defined in a base class ofMockFoo
, the "nice" or "strict" modifier may not affect it, depending on the compiler. In particular, nestingNiceMock
andStrictMock
(e.g.NiceMock<StrictMock<MockFoo> >
) is not supported.NiceMock<MockFoo>
andStrictMock<MockFoo>
may not work correctly if the destructor ofMockFoo
is not virtual. We would like to fix this, but it requires cleaning up existing tests. http://b/28934720 tracks the issue.- During the constructor or destructor of
MockFoo
, the mock object is not nice or strict. This may cause surprises if the constructor or destructor calls a mock method onthis
object. (This behavior, however, is consistent with C++'s general rule: if a constructor or destructor calls a virtual method ofthis
object, that method is treated as non-virtual. In other words, to the base class's constructor or destructor,this
object behaves like an instance of the base class, not the derived class. This rule is required for safety. Otherwise a base constructor may use members of a derived class before they are initialized, or a base destructor may use members of a derived class after they have been destroyed.)
Finally, you should be very cautious about when to use naggy or strict mocks, as they tend to make tests more brittle and harder to maintain. When you refactor your code without changing its externally visible behavior, ideally you shouldn't need to update any tests. If your code interacts with a naggy mock, however, you may start to get spammed with warnings as the result of your change. Worse, if your code interacts with a strict mock, your tests may start to fail and you'll be forced to fix them. Our general recommendation is to use nice mocks (not yet the default) most of the time, use naggy mocks (the current default) when developing or debugging tests, and use strict mocks only as the last resort.
Simplifying the Interface without Breaking Existing Code
Sometimes a method has a long list of arguments that is mostly uninteresting. For example:
class LogSink {
public:
...
virtual void send(LogSeverity severity, const char* full_filename,
const char* base_filename, int line,
const struct tm* tm_time,
const char* message, size_t message_len) = 0;
};
This method's argument list is lengthy and hard to work with (the message
argument is not even 0-terminated). If we mock it as is, using the mock will be
awkward. If, however, we try to simplify this interface, we'll need to fix all
clients depending on it, which is often infeasible.
The trick is to redispatch the method in the mock class:
class ScopedMockLog : public LogSink {
public:
...
virtual void send(LogSeverity severity, const char* full_filename,
const char* base_filename, int line, const tm* tm_time,
const char* message, size_t message_len) {
// We are only interested in the log severity, full file name, and
// log message.
Log(severity, full_filename, std::string(message, message_len));
}
// Implements the mock method:
//
// void Log(LogSeverity severity,
// const string& file_path,
// const string& message);
MOCK_METHOD(void, Log,
(LogSeverity severity, const string& file_path,
const string& message));
};
By defining a new mock method with a trimmed argument list, we make the mock class more user-friendly.
This technique may also be applied to make overloaded methods more amenable to mocking. For example, when overloads have been used to implement default arguments:
class MockTurtleFactory : public TurtleFactory {
public:
Turtle* MakeTurtle(int length, int weight) override { ... }
Turtle* MakeTurtle(int length, int weight, int speed) override { ... }
// the above methods delegate to this one:
MOCK_METHOD(Turtle*, DoMakeTurtle, ());
};
This allows tests that don't care which overload was invoked to avoid specifying argument matchers:
ON_CALL(factory, DoMakeTurtle)
.WillByDefault(MakeMockTurtle());
Alternative to Mocking Concrete Classes
Often you may find yourself using classes that don't implement interfaces. In
order to test your code that uses such a class (let's call it Concrete
), you
may be tempted to make the methods of Concrete
virtual and then mock it.
Try not to do that.
Making a non-virtual function virtual is a big decision. It creates an extension point where subclasses can tweak your class' behavior. This weakens your control on the class because now it's harder to maintain the class invariants. You should make a function virtual only when there is a valid reason for a subclass to override it.
Mocking concrete classes directly is problematic as it creates a tight coupling between the class and the tests - any small change in the class may invalidate your tests and make test maintenance a pain.
To avoid such problems, many programmers have been practicing "coding to
interfaces": instead of talking to the Concrete
class, your code would define
an interface and talk to it. Then you implement that interface as an adaptor on
top of Concrete
. In tests, you can easily mock that interface to observe how
your code is doing.
This technique incurs some overhead:
- You pay the cost of virtual function calls (usually not a problem).
- There is more abstraction for the programmers to learn.
However, it can also bring significant benefits in addition to better testability:
Concrete
's API may not fit your problem domain very well, as you may not be the only client it tries to serve. By designing your own interface, you have a chance to tailor it to your need - you may add higher-level functionalities, rename stuff, etc instead of just trimming the class. This allows you to write your code (user of the interface) in a more natural way, which means it will be more readable, more maintainable, and you'll be more productive.- If
Concrete
's implementation ever has to change, you don't have to rewrite everywhere it is used. Instead, you can absorb the change in your implementation of the interface, and your other code and tests will be insulated from this change.
Some people worry that if everyone is practicing this technique, they will end up writing lots of redundant code. This concern is totally understandable. However, there are two reasons why it may not be the case:
- Different projects may need to use
Concrete
in different ways, so the best interfaces for them will be different. Therefore, each of them will have its own domain-specific interface on top ofConcrete
, and they will not be the same code. - If enough projects want to use the same interface, they can always share it,
just like they have been sharing
Concrete
. You can check in the interface and the adaptor somewhere nearConcrete
(perhaps in acontrib
sub-directory) and let many projects use it.
You need to weigh the pros and cons carefully for your particular problem, but I'd like to assure you that the Java community has been practicing this for a long time and it's a proven effective technique applicable in a wide variety of situations. :-)
Delegating Calls to a Fake
Some times you have a non-trivial fake implementation of an interface. For example:
class Foo {
public:
virtual ~Foo() {}
virtual char DoThis(int n) = 0;
virtual void DoThat(const char* s, int* p) = 0;
};
class FakeFoo : public Foo {
public:
char DoThis(int n) override {
return (n > 0) ? '+' :
(n < 0) ? '-' : '0';
}
void DoThat(const char* s, int* p) override {
*p = strlen(s);
}
};
Now you want to mock this interface such that you can set expectations on it.
However, you also want to use FakeFoo
for the default behavior, as duplicating
it in the mock object is, well, a lot of work.
When you define the mock class using gMock, you can have it delegate its default action to a fake class you already have, using this pattern:
class MockFoo : public Foo {
public:
// Normal mock method definitions using gMock.
MOCK_METHOD(char, DoThis, (int n), (override));
MOCK_METHOD(void, DoThat, (const char* s, int* p), (override));
// Delegates the default actions of the methods to a FakeFoo object.
// This must be called *before* the custom ON_CALL() statements.
void DelegateToFake() {
ON_CALL(*this, DoThis).WillByDefault([this](int n) {
return fake_.DoThis(n);
});
ON_CALL(*this, DoThat).WillByDefault([this](const char* s, int* p) {
fake_.DoThat(s, p);
});
}
private:
FakeFoo fake_; // Keeps an instance of the fake in the mock.
};
With that, you can use MockFoo
in your tests as usual. Just remember that if
you don't explicitly set an action in an ON_CALL()
or EXPECT_CALL()
, the
fake will be called upon to do it.:
using ::testing::_;
TEST(AbcTest, Xyz) {
MockFoo foo;
foo.DelegateToFake(); // Enables the fake for delegation.
// Put your ON_CALL(foo, ...)s here, if any.
// No action specified, meaning to use the default action.
EXPECT_CALL(foo, DoThis(5));
EXPECT_CALL(foo, DoThat(_, _));
int n = 0;
EXPECT_EQ('+', foo.DoThis(5)); // FakeFoo::DoThis() is invoked.
foo.DoThat("Hi", &n); // FakeFoo::DoThat() is invoked.
EXPECT_EQ(2, n);
}
Some tips:
-
If you want, you can still override the default action by providing your own
ON_CALL()
or using.WillOnce()
/.WillRepeatedly()
inEXPECT_CALL()
. -
In
DelegateToFake()
, you only need to delegate the methods whose fake implementation you intend to use. -
The general technique discussed here works for overloaded methods, but you'll need to tell the compiler which version you mean. To disambiguate a mock function (the one you specify inside the parentheses of
ON_CALL()
), use this technique; to disambiguate a fake function (the one you place insideInvoke()
), use astatic_cast
to specify the function's type. For instance, if classFoo
has methodschar DoThis(int n)
andbool DoThis(double x) const
, and you want to invoke the latter, you need to writeInvoke(&fake_, static_cast<bool (FakeFoo::*)(double) const>(&FakeFoo::DoThis))
instead ofInvoke(&fake_, &FakeFoo::DoThis)
(The strange-looking thing inside the angled brackets ofstatic_cast
is the type of a function pointer to the secondDoThis()
method.). -
Having to mix a mock and a fake is often a sign of something gone wrong. Perhaps you haven't got used to the interaction-based way of testing yet. Or perhaps your interface is taking on too many roles and should be split up. Therefore, don't abuse this. We would only recommend to do it as an intermediate step when you are refactoring your code.
Regarding the tip on mixing a mock and a fake, here's an example on why it may
be a bad sign: Suppose you have a class System
for low-level system
operations. In particular, it does file and I/O operations. And suppose you want
to test how your code uses System
to do I/O, and you just want the file
operations to work normally. If you mock out the entire System
class, you'll
have to provide a fake implementation for the file operation part, which
suggests that System
is taking on too many roles.
Instead, you can define a FileOps
interface and an IOOps
interface and split
System
's functionalities into the two. Then you can mock IOOps
without
mocking FileOps
.
Delegating Calls to a Real Object
When using testing doubles (mocks, fakes, stubs, and etc), sometimes their behaviors will differ from those of the real objects. This difference could be either intentional (as in simulating an error such that you can test the error handling code) or unintentional. If your mocks have different behaviors than the real objects by mistake, you could end up with code that passes the tests but fails in production.
You can use the delegating-to-real technique to ensure that your mock has the same behavior as the real object while retaining the ability to validate calls. This technique is very similar to the delegating-to-fake technique, the difference being that we use a real object instead of a fake. Here's an example:
using ::testing::AtLeast;
class MockFoo : public Foo {
public:
MockFoo() {
// By default, all calls are delegated to the real object.
ON_CALL(*this, DoThis).WillByDefault([this](int n) {
return real_.DoThis(n);
});
ON_CALL(*this, DoThat).WillByDefault([this](const char* s, int* p) {
real_.DoThat(s, p);
});
...
}
MOCK_METHOD(char, DoThis, ...);
MOCK_METHOD(void, DoThat, ...);
...
private:
Foo real_;
};
...
MockFoo mock;
EXPECT_CALL(mock, DoThis())
.Times(3);
EXPECT_CALL(mock, DoThat("Hi"))
.Times(AtLeast(1));
... use mock in test ...
With this, gMock will verify that your code made the right calls (with the right arguments, in the right order, called the right number of times, etc), and a real object will answer the calls (so the behavior will be the same as in production). This gives you the best of both worlds.
Delegating Calls to a Parent Class
Ideally, you should code to interfaces, whose methods are all pure virtual. In reality, sometimes you do need to mock a virtual method that is not pure (i.e, it already has an implementation). For example:
class Foo {
public:
virtual ~Foo();
virtual void Pure(int n) = 0;
virtual int Concrete(const char* str) { ... }
};
class MockFoo : public Foo {
public:
// Mocking a pure method.
MOCK_METHOD(void, Pure, (int n), (override));
// Mocking a concrete method. Foo::Concrete() is shadowed.
MOCK_METHOD(int, Concrete, (const char* str), (override));
};
Sometimes you may want to call Foo::Concrete()
instead of
MockFoo::Concrete()
. Perhaps you want to do it as part of a stub action, or
perhaps your test doesn't need to mock Concrete()
at all (but it would be
oh-so painful to have to define a new mock class whenever you don't need to mock
one of its methods).
The trick is to leave a back door in your mock class for accessing the real methods in the base class:
class MockFoo : public Foo {
public:
// Mocking a pure method.
MOCK_METHOD(void, Pure, (int n), (override));
// Mocking a concrete method. Foo::Concrete() is shadowed.
MOCK_METHOD(int, Concrete, (const char* str), (override));
// Use this to call Concrete() defined in Foo.
int FooConcrete(const char* str) { return Foo::Concrete(str); }
};
Now, you can call Foo::Concrete()
inside an action by:
...
EXPECT_CALL(foo, Concrete).WillOnce([&foo](const char* str) {
return foo.FooConcrete(str);
});
or tell the mock object that you don't want to mock Concrete()
:
...
ON_CALL(foo, Concrete).WillByDefault([&foo](const char* str) {
return foo.FooConcrete(str);
});
(Why don't we just write { return foo.Concrete(str); }
? If you do that,
MockFoo::Concrete()
will be called (and cause an infinite recursion) since
Foo::Concrete()
is virtual. That's just how C++ works.)
Using Matchers
Matching Argument Values Exactly
You can specify exactly which arguments a mock method is expecting:
using ::testing::Return;
...
EXPECT_CALL(foo, DoThis(5))
.WillOnce(Return('a'));
EXPECT_CALL(foo, DoThat("Hello", bar));
Using Simple Matchers
You can use matchers to match arguments that have a certain property:
using ::testing::NotNull;
using ::testing::Return;
...
EXPECT_CALL(foo, DoThis(Ge(5))) // The argument must be >= 5.
.WillOnce(Return('a'));
EXPECT_CALL(foo, DoThat("Hello", NotNull()));
// The second argument must not be NULL.
A frequently used matcher is _
, which matches anything:
EXPECT_CALL(foo, DoThat(_, NotNull()));
Combining Matchers
You can build complex matchers from existing ones using AllOf()
,
AllOfArray()
, AnyOf()
, AnyOfArray()
and Not()
:
using ::testing::AllOf;
using ::testing::Gt;
using ::testing::HasSubstr;
using ::testing::Ne;
using ::testing::Not;
...
// The argument must be > 5 and != 10.
EXPECT_CALL(foo, DoThis(AllOf(Gt(5),
Ne(10))));
// The first argument must not contain sub-string "blah".
EXPECT_CALL(foo, DoThat(Not(HasSubstr("blah")),
NULL));
Casting Matchers
gMock matchers are statically typed, meaning that the compiler can catch your
mistake if you use a matcher of the wrong type (for example, if you use Eq(5)
to match a string
argument). Good for you!
Sometimes, however, you know what you're doing and want the compiler to give you
some slack. One example is that you have a matcher for long
and the argument
you want to match is int
. While the two types aren't exactly the same, there
is nothing really wrong with using a Matcher<long>
to match an int
- after
all, we can first convert the int
argument to a long
losslessly before
giving it to the matcher.
To support this need, gMock gives you the SafeMatcherCast<T>(m)
function. It
casts a matcher m
to type Matcher<T>
. To ensure safety, gMock checks that
(let U
be the type m
accepts :
- Type
T
can be implicitly cast to typeU
; - When both
T
andU
are built-in arithmetic types (bool
, integers, and floating-point numbers), the conversion fromT
toU
is not lossy (in other words, any value representable byT
can also be represented byU
); and - When
U
is a reference,T
must also be a reference (as the underlying matcher may be interested in the address of theU
value).
The code won't compile if any of these conditions isn't met.
Here's one example:
using ::testing::SafeMatcherCast;
// A base class and a child class.
class Base { ... };
class Derived : public Base { ... };
class MockFoo : public Foo {
public:
MOCK_METHOD(void, DoThis, (Derived* derived), (override));
};
...
MockFoo foo;
// m is a Matcher<Base*> we got from somewhere.
EXPECT_CALL(foo, DoThis(SafeMatcherCast<Derived*>(m)));
If you find SafeMatcherCast<T>(m)
too limiting, you can use a similar function
MatcherCast<T>(m)
. The difference is that MatcherCast
works as long as you
can static_cast
type T
to type U
.
MatcherCast
essentially lets you bypass C++'s type system (static_cast
isn't
always safe as it could throw away information, for example), so be careful not
to misuse/abuse it.
Selecting Between Overloaded Functions
If you expect an overloaded function to be called, the compiler may need some help on which overloaded version it is.
To disambiguate functions overloaded on the const-ness of this object, use the
Const()
argument wrapper.
using ::testing::ReturnRef;
class MockFoo : public Foo {
...
MOCK_METHOD(Bar&, GetBar, (), (override));
MOCK_METHOD(const Bar&, GetBar, (), (const, override));
};
...
MockFoo foo;
Bar bar1, bar2;
EXPECT_CALL(foo, GetBar()) // The non-const GetBar().
.WillOnce(ReturnRef(bar1));
EXPECT_CALL(Const(foo), GetBar()) // The const GetBar().
.WillOnce(ReturnRef(bar2));
(Const()
is defined by gMock and returns a const
reference to its argument.)
To disambiguate overloaded functions with the same number of arguments but
different argument types, you may need to specify the exact type of a matcher,
either by wrapping your matcher in Matcher<type>()
, or using a matcher whose
type is fixed (TypedEq<type>
, An<type>()
, etc):
using ::testing::An;
using ::testing::Matcher;
using ::testing::TypedEq;
class MockPrinter : public Printer {
public:
MOCK_METHOD(void, Print, (int n), (override));
MOCK_METHOD(void, Print, (char c), (override));
};
TEST(PrinterTest, Print) {
MockPrinter printer;
EXPECT_CALL(printer, Print(An<int>())); // void Print(int);
EXPECT_CALL(printer, Print(Matcher<int>(Lt(5)))); // void Print(int);
EXPECT_CALL(printer, Print(TypedEq<char>('a'))); // void Print(char);
printer.Print(3);
printer.Print(6);
printer.Print('a');
}
Performing Different Actions Based on the Arguments
When a mock method is called, the last matching expectation that's still active will be selected (think "newer overrides older"). So, you can make a method do different things depending on its argument values like this:
using ::testing::_;
using ::testing::Lt;
using ::testing::Return;
...
// The default case.
EXPECT_CALL(foo, DoThis(_))
.WillRepeatedly(Return('b'));
// The more specific case.
EXPECT_CALL(foo, DoThis(Lt(5)))
.WillRepeatedly(Return('a'));
Now, if foo.DoThis()
is called with a value less than 5, 'a'
will be
returned; otherwise 'b'
will be returned.
Matching Multiple Arguments as a Whole
Sometimes it's not enough to match the arguments individually. For example, we
may want to say that the first argument must be less than the second argument.
The With()
clause allows us to match all arguments of a mock function as a
whole. For example,
using ::testing::_;
using ::testing::Ne;
using ::testing::Lt;
...
EXPECT_CALL(foo, InRange(Ne(0), _))
.With(Lt());
says that the first argument of InRange()
must not be 0, and must be less than
the second argument.
The expression inside With()
must be a matcher of type
Matcher< ::std::tuple<A1, ..., An> >
, where A1
, ..., An
are the types of
the function arguments.
You can also write AllArgs(m)
instead of m
inside .With()
. The two forms
are equivalent, but .With(AllArgs(Lt()))
is more readable than .With(Lt())
.
You can use Args<k1, ..., kn>(m)
to match the n
selected arguments (as a
tuple) against m
. For example,
using ::testing::_;
using ::testing::AllOf;
using ::testing::Args;
using ::testing::Lt;
...
EXPECT_CALL(foo, Blah)
.With(AllOf(Args<0, 1>(Lt()), Args<1, 2>(Lt())));
says that Blah
will be called with arguments x
, y
, and z
where x < y < z
. Note that in this example, it wasn't necessary specify the positional
matchers.
As a convenience and example, gMock provides some matchers for 2-tuples,
including the Lt()
matcher above. See here for the
complete list.
Note that if you want to pass the arguments to a predicate of your own (e.g.
.With(Args<0, 1>(Truly(&MyPredicate)))
), that predicate MUST be written to
take a ::std::tuple
as its argument; gMock will pass the n
selected
arguments as one single tuple to the predicate.
Using Matchers as Predicates
Have you noticed that a matcher is just a fancy predicate that also knows how to
describe itself? Many existing algorithms take predicates as arguments (e.g.
those defined in STL's <algorithm>
header), and it would be a shame if gMock
matchers were not allowed to participate.
Luckily, you can use a matcher where a unary predicate functor is expected by
wrapping it inside the Matches()
function. For example,
#include <algorithm>
#include <vector>
using ::testing::Matches;
using ::testing::Ge;
vector<int> v;
...
// How many elements in v are >= 10?
const int count = count_if(v.begin(), v.end(), Matches(Ge(10)));
Since you can build complex matchers from simpler ones easily using gMock, this
gives you a way to conveniently construct composite predicates (doing the same
using STL's <functional>
header is just painful). For example, here's a
predicate that's satisfied by any number that is >= 0, <= 100, and != 50:
using testing::AllOf;
using testing::Ge;
using testing::Le;
using testing::Matches;
using testing::Ne;
...
Matches(AllOf(Ge(0), Le(100), Ne(50)))
Using Matchers in googletest Assertions
Since matchers are basically predicates that also know how to describe
themselves, there is a way to take advantage of them in googletest assertions.
It's called ASSERT_THAT
and EXPECT_THAT
:
ASSERT_THAT(value, matcher); // Asserts that value matches matcher.
EXPECT_THAT(value, matcher); // The non-fatal version.
For example, in a googletest test you can write:
#include "gmock/gmock.h"
using ::testing::AllOf;
using ::testing::Ge;
using ::testing::Le;
using ::testing::MatchesRegex;
using ::testing::StartsWith;
...
EXPECT_THAT(Foo(), StartsWith("Hello"));
EXPECT_THAT(Bar(), MatchesRegex("Line \\d+"));
ASSERT_THAT(Baz(), AllOf(Ge(5), Le(10)));
which (as you can probably guess) executes Foo()
, Bar()
, and Baz()
, and
verifies that:
Foo()
returns a string that starts with"Hello"
.Bar()
returns a string that matches regular expression"Line \\d+"
.Baz()
returns a number in the range [5, 10].
The nice thing about these macros is that they read like English. They
generate informative messages too. For example, if the first EXPECT_THAT()
above fails, the message will be something like:
Value of: Foo()
Actual: "Hi, world!"
Expected: starts with "Hello"
Credit: The idea of (ASSERT|EXPECT)_THAT
was borrowed from Joe Walnes'
Hamcrest project, which adds assertThat()
to JUnit.
Using Predicates as Matchers
gMock provides a built-in set of matchers. In case you find them
lacking, you can use an arbitrary unary predicate function or functor as a
matcher - as long as the predicate accepts a value of the type you want. You do
this by wrapping the predicate inside the Truly()
function, for example:
using ::testing::Truly;
int IsEven(int n) { return (n % 2) == 0 ? 1 : 0; }
...
// Bar() must be called with an even number.
EXPECT_CALL(foo, Bar(Truly(IsEven)));
Note that the predicate function / functor doesn't have to return bool
. It
works as long as the return value can be used as the condition in in statement
if (condition) ...
.
Matching Arguments that Are Not Copyable
When you do an EXPECT_CALL(mock_obj, Foo(bar))
, gMock saves away a copy of
bar
. When Foo()
is called later, gMock compares the argument to Foo()
with
the saved copy of bar
. This way, you don't need to worry about bar
being
modified or destroyed after the EXPECT_CALL()
is executed. The same is true
when you use matchers like Eq(bar)
, Le(bar)
, and so on.
But what if bar
cannot be copied (i.e. has no copy constructor)? You could
define your own matcher function or callback and use it with Truly()
, as the
previous couple of recipes have shown. Or, you may be able to get away from it
if you can guarantee that bar
won't be changed after the EXPECT_CALL()
is
executed. Just tell gMock that it should save a reference to bar
, instead of a
copy of it. Here's how:
using ::testing::ByRef;
using ::testing::Eq;
using ::testing::Lt;
...
// Expects that Foo()'s argument == bar.
EXPECT_CALL(mock_obj, Foo(Eq(ByRef(bar))));
// Expects that Foo()'s argument < bar.
EXPECT_CALL(mock_obj, Foo(Lt(ByRef(bar))));
Remember: if you do this, don't change bar
after the EXPECT_CALL()
, or the
result is undefined.
Validating a Member of an Object
Often a mock function takes a reference to object as an argument. When matching
the argument, you may not want to compare the entire object against a fixed
object, as that may be over-specification. Instead, you may need to validate a
certain member variable or the result of a certain getter method of the object.
You can do this with Field()
and Property()
. More specifically,
Field(&Foo::bar, m)
is a matcher that matches a Foo
object whose bar
member variable satisfies
matcher m
.
Property(&Foo::baz, m)
is a matcher that matches a Foo
object whose baz()
method returns a value
that satisfies matcher m
.
For example:
Expression | Description |
---|---|
Field(&Foo::number, Ge(3)) |
Matches x where x.number >= 3 . |
Property(&Foo::name, StartsWith("John ")) |
Matches x where x.name() starts with "John " . |
Note that in Property(&Foo::baz, ...)
, method baz()
must take no argument
and be declared as const
.
BTW, Field()
and Property()
can also match plain pointers to objects. For
instance,
using ::testing::Field;
using ::testing::Ge;
...
Field(&Foo::number, Ge(3))
matches a plain pointer p
where p->number >= 3
. If p
is NULL
, the match
will always fail regardless of the inner matcher.
What if you want to validate more than one members at the same time? Remember
that there are AllOf()
and AllOfArray()
.
Finally Field()
and Property()
provide overloads that take the field or
property names as the first argument to include it in the error message. This
can be useful when creating combined matchers.
using ::testing::AllOf;
using ::testing::Field;
using ::testing::Matcher;
using ::testing::SafeMatcherCast;
Matcher<Foo> IsFoo(const Foo& foo) {
return AllOf(Field("some_field", &Foo::some_field, foo.some_field),
Field("other_field", &Foo::other_field, foo.other_field),
Field("last_field", &Foo::last_field, foo.last_field));
}
Validating the Value Pointed to by a Pointer Argument
C++ functions often take pointers as arguments. You can use matchers like
IsNull()
, NotNull()
, and other comparison matchers to match a pointer, but
what if you want to make sure the value pointed to by the pointer, instead of
the pointer itself, has a certain property? Well, you can use the Pointee(m)
matcher.
Pointee(m)
matches a pointer if m
matches the value the pointer points to.
For example:
using ::testing::Ge;
using ::testing::Pointee;
...
EXPECT_CALL(foo, Bar(Pointee(Ge(3))));
expects foo.Bar()
to be called with a pointer that points to a value greater
than or equal to 3.
One nice thing about Pointee()
is that it treats a NULL
pointer as a match
failure, so you can write Pointee(m)
instead of
using ::testing::AllOf;
using ::testing::NotNull;
using ::testing::Pointee;
...
AllOf(NotNull(), Pointee(m))
without worrying that a NULL
pointer will crash your test.
Also, did we tell you that Pointee()
works with both raw pointers and
smart pointers (std::unique_ptr
, std::shared_ptr
, etc)?
What if you have a pointer to pointer? You guessed it - you can use nested
Pointee()
to probe deeper inside the value. For example,
Pointee(Pointee(Lt(3)))
matches a pointer that points to a pointer that points
to a number less than 3 (what a mouthful...).
Testing a Certain Property of an Object
Sometimes you want to specify that an object argument has a certain property, but there is no existing matcher that does this. If you want good error messages, you should define a matcher. If you want to do it quick and dirty, you could get away with writing an ordinary function.
Let's say you have a mock function that takes an object of type Foo
, which has
an int bar()
method and an int baz()
method, and you want to constrain that
the argument's bar()
value plus its baz()
value is a given number. Here's
how you can define a matcher to do it:
using ::testing::Matcher;
using ::testing::MatcherInterface;
using ::testing::MatchResultListener;
class BarPlusBazEqMatcher : public MatcherInterface<const Foo&> {
public:
explicit BarPlusBazEqMatcher(int expected_sum)
: expected_sum_(expected_sum) {}
bool MatchAndExplain(const Foo& foo,
MatchResultListener* /* listener */) const override {
return (foo.bar() + foo.baz()) == expected_sum_;
}
void DescribeTo(::std::ostream* os) const override {
*os << "bar() + baz() equals " << expected_sum_;
}
void DescribeNegationTo(::std::ostream* os) const override {
*os << "bar() + baz() does not equal " << expected_sum_;
}
private:
const int expected_sum_;
};
Matcher<const Foo&> BarPlusBazEq(int expected_sum) {
return MakeMatcher(new BarPlusBazEqMatcher(expected_sum));
}
...
EXPECT_CALL(..., DoThis(BarPlusBazEq(5)))...;
Matching Containers
Sometimes an STL container (e.g. list, vector, map, ...) is passed to a mock
function and you may want to validate it. Since most STL containers support the
==
operator, you can write Eq(expected_container)
or simply
expected_container
to match a container exactly.
Sometimes, though, you may want to be more flexible (for example, the first element must be an exact match, but the second element can be any positive number, and so on). Also, containers used in tests often have a small number of elements, and having to define the expected container out-of-line is a bit of a hassle.
You can use the ElementsAre()
or UnorderedElementsAre()
matcher in such
cases:
using ::testing::_;
using ::testing::ElementsAre;
using ::testing::Gt;
...
MOCK_METHOD(void, Foo, (const vector<int>& numbers), (override));
...
EXPECT_CALL(mock, Foo(ElementsAre(1, Gt(0), _, 5)));
The above matcher says that the container must have 4 elements, which must be 1, greater than 0, anything, and 5 respectively.
If you instead write:
using ::testing::_;
using ::testing::Gt;
using ::testing::UnorderedElementsAre;
...
MOCK_METHOD(void, Foo, (const vector<int>& numbers), (override));
...
EXPECT_CALL(mock, Foo(UnorderedElementsAre(1, Gt(0), _, 5)));
It means that the container must have 4 elements, which (under some permutation) must be 1, greater than 0, anything, and 5 respectively.
As an alternative you can place the arguments in a C-style array and use
ElementsAreArray()
or UnorderedElementsAreArray()
instead:
using ::testing::ElementsAreArray;
...
// ElementsAreArray accepts an array of element values.
const int expected_vector1[] = {1, 5, 2, 4, ...};
EXPECT_CALL(mock, Foo(ElementsAreArray(expected_vector1)));
// Or, an array of element matchers.
Matcher<int> expected_vector2[] = {1, Gt(2), _, 3, ...};
EXPECT_CALL(mock, Foo(ElementsAreArray(expected_vector2)));
In case the array needs to be dynamically created (and therefore the array size
cannot be inferred by the compiler), you can give ElementsAreArray()
an
additional argument to specify the array size:
using ::testing::ElementsAreArray;
...
int* const expected_vector3 = new int[count];
... fill expected_vector3 with values ...
EXPECT_CALL(mock, Foo(ElementsAreArray(expected_vector3, count)));
Use Pair
when comparing maps or other associative containers.
using testing::ElementsAre;
using testing::Pair;
...
std::map<string, int> m = {{"a", 1}, {"b", 2}, {"c", 3}};
EXPECT_THAT(m, ElementsAre(Pair("a", 1), Pair("b", 2), Pair("c", 3)));
Tips:
ElementsAre*()
can be used to match any container that implements the STL iterator pattern (i.e. it has aconst_iterator
type and supportsbegin()/end()
), not just the ones defined in STL. It will even work with container types yet to be written - as long as they follows the above pattern.- You can use nested
ElementsAre*()
to match nested (multi-dimensional) containers. - If the container is passed by pointer instead of by reference, just write
Pointee(ElementsAre*(...))
. - The order of elements matters for
ElementsAre*()
. If you are using it with containers whose element order are undefined (e.g.hash_map
) you should useWhenSorted
aroundElementsAre
.
Sharing Matchers
Under the hood, a gMock matcher object consists of a pointer to a ref-counted implementation object. Copying matchers is allowed and very efficient, as only the pointer is copied. When the last matcher that references the implementation object dies, the implementation object will be deleted.
Therefore, if you have some complex matcher that you want to use again and again, there is no need to build it everytime. Just assign it to a matcher variable and use that variable repeatedly! For example,
using ::testing::AllOf;
using ::testing::Gt;
using ::testing::Le;
using ::testing::Matcher;
...
Matcher<int> in_range = AllOf(Gt(5), Le(10));
... use in_range as a matcher in multiple EXPECT_CALLs ...
Matchers must have no side-effects
WARNING: gMock does not guarantee when or how many times a matcher will be invoked. Therefore, all matchers must be purely functional: they cannot have any side effects, and the match result must not depend on anything other than the matcher's parameters and the value being matched.
This requirement must be satisfied no matter how a matcher is defined (e.g., if it is one of the standard matchers, or a custom matcher). In particular, a matcher can never call a mock function, as that will affect the state of the mock object and gMock.
Setting Expectations
Knowing When to Expect
ON_CALL
is likely the single most under-utilized construct in gMock.
There are basically two constructs for defining the behavior of a mock object:
ON_CALL
and EXPECT_CALL
. The difference? ON_CALL
defines what happens when
a mock method is called, but doesn't imply any expectation on the method
being called. EXPECT_CALL
not only defines the behavior, but also sets an
expectation that the method will be called with the given arguments, for the
given number of times (and in the given order when you specify the order
too).
Since EXPECT_CALL
does more, isn't it better than ON_CALL
? Not really. Every
EXPECT_CALL
adds a constraint on the behavior of the code under test. Having
more constraints than necessary is baaad - even worse than not having enough
constraints.
This may be counter-intuitive. How could tests that verify more be worse than tests that verify less? Isn't verification the whole point of tests?
The answer lies in what a test should verify. A good test verifies the contract of the code. If a test over-specifies, it doesn't leave enough freedom to the implementation. As a result, changing the implementation without breaking the contract (e.g. refactoring and optimization), which should be perfectly fine to do, can break such tests. Then you have to spend time fixing them, only to see them broken again the next time the implementation is changed.
Keep in mind that one doesn't have to verify more than one property in one test. In fact, it's a good style to verify only one thing in one test. If you do that, a bug will likely break only one or two tests instead of dozens (which case would you rather debug?). If you are also in the habit of giving tests descriptive names that tell what they verify, you can often easily guess what's wrong just from the test log itself.
So use ON_CALL
by default, and only use EXPECT_CALL
when you actually intend
to verify that the call is made. For example, you may have a bunch of ON_CALL
s
in your test fixture to set the common mock behavior shared by all tests in the
same group, and write (scarcely) different EXPECT_CALL
s in different TEST_F
s
to verify different aspects of the code's behavior. Compared with the style
where each TEST
has many EXPECT_CALL
s, this leads to tests that are more
resilient to implementational changes (and thus less likely to require
maintenance) and makes the intent of the tests more obvious (so they are easier
to maintain when you do need to maintain them).
If you are bothered by the "Uninteresting mock function call" message printed
when a mock method without an EXPECT_CALL
is called, you may use a NiceMock
instead to suppress all such messages for the mock object, or suppress the
message for specific methods by adding EXPECT_CALL(...).Times(AnyNumber())
. DO
NOT suppress it by blindly adding an EXPECT_CALL(...)
, or you'll have a test
that's a pain to maintain.
Ignoring Uninteresting Calls
If you are not interested in how a mock method is called, just don't say
anything about it. In this case, if the method is ever called, gMock will
perform its default action to allow the test program to continue. If you are not
happy with the default action taken by gMock, you can override it using
DefaultValue<T>::Set()
(described here) or ON_CALL()
.
Please note that once you expressed interest in a particular mock method (via
EXPECT_CALL()
), all invocations to it must match some expectation. If this
function is called but the arguments don't match any EXPECT_CALL()
statement,
it will be an error.
Disallowing Unexpected Calls
If a mock method shouldn't be called at all, explicitly say so:
using ::testing::_;
...
EXPECT_CALL(foo, Bar(_))
.Times(0);
If some calls to the method are allowed, but the rest are not, just list all the expected calls:
using ::testing::AnyNumber;
using ::testing::Gt;
...
EXPECT_CALL(foo, Bar(5));
EXPECT_CALL(foo, Bar(Gt(10)))
.Times(AnyNumber());
A call to foo.Bar()
that doesn't match any of the EXPECT_CALL()
statements
will be an error.
Understanding Uninteresting vs Unexpected Calls
Uninteresting calls and unexpected calls are different concepts in gMock. Very different.
A call x.Y(...)
is uninteresting if there's not even a single
EXPECT_CALL(x, Y(...))
set. In other words, the test isn't interested in the
x.Y()
method at all, as evident in that the test doesn't care to say anything
about it.
A call x.Y(...)
is unexpected if there are some EXPECT_CALL(x, Y(...))
s set, but none of them matches the call. Put another way, the test is
interested in the x.Y()
method (therefore it explicitly sets some
EXPECT_CALL
to verify how it's called); however, the verification fails as the
test doesn't expect this particular call to happen.
An unexpected call is always an error, as the code under test doesn't behave the way the test expects it to behave.
By default, an uninteresting call is not an error, as it violates no constraint specified by the test. (gMock's philosophy is that saying nothing means there is no constraint.) However, it leads to a warning, as it might indicate a problem (e.g. the test author might have forgotten to specify a constraint).
In gMock, NiceMock
and StrictMock
can be used to make a mock class "nice" or
"strict". How does this affect uninteresting calls and unexpected calls?
A nice mock suppresses uninteresting call warnings. It is less chatty than the default mock, but otherwise is the same. If a test fails with a default mock, it will also fail using a nice mock instead. And vice versa. Don't expect making a mock nice to change the test's result.
A strict mock turns uninteresting call warnings into errors. So making a mock strict may change the test's result.
Let's look at an example:
TEST(...) {
NiceMock<MockDomainRegistry> mock_registry;
EXPECT_CALL(mock_registry, GetDomainOwner("google.com"))
.WillRepeatedly(Return("Larry Page"));
// Use mock_registry in code under test.
... &mock_registry ...
}
The sole EXPECT_CALL
here says that all calls to GetDomainOwner()
must have
"google.com"
as the argument. If GetDomainOwner("yahoo.com")
is called, it
will be an unexpected call, and thus an error. Having a nice mock doesn't
change the severity of an unexpected call.
So how do we tell gMock that GetDomainOwner()
can be called with some other
arguments as well? The standard technique is to add a "catch all" EXPECT_CALL
:
EXPECT_CALL(mock_registry, GetDomainOwner(_))
.Times(AnyNumber()); // catches all other calls to this method.
EXPECT_CALL(mock_registry, GetDomainOwner("google.com"))
.WillRepeatedly(Return("Larry Page"));
Remember that _
is the wildcard matcher that matches anything. With this, if
GetDomainOwner("google.com")
is called, it will do what the second
EXPECT_CALL
says; if it is called with a different argument, it will do what
the first EXPECT_CALL
says.
Note that the order of the two EXPECT_CALL
s is important, as a newer
EXPECT_CALL
takes precedence over an older one.
For more on uninteresting calls, nice mocks, and strict mocks, read "The Nice, the Strict, and the Naggy".
Ignoring Uninteresting Arguments
If your test doesn't care about the parameters (it only cares about the number or order of calls), you can often simply omit the parameter list:
// Expect foo.Bar( ... ) twice with any arguments.
EXPECT_CALL(foo, Bar).Times(2);
// Delegate to the given method whenever the factory is invoked.
ON_CALL(foo_factory, MakeFoo)
.WillByDefault(&BuildFooForTest);
This functionality is only available when a method is not overloaded; to prevent unexpected behavior it is a compilation error to try to set an expectation on a method where the specific overload is ambiguous. You can work around this by supplying a simpler mock interface than the mocked class provides.
This pattern is also useful when the arguments are interesting, but match logic is substantially complex. You can leave the argument list unspecified and use SaveArg actions to save the values for later verification. If you do that, you can easily differentiate calling the method the wrong number of times from calling it with the wrong arguments.
Expecting Ordered Calls
Although an EXPECT_CALL()
statement defined earlier takes precedence when
gMock tries to match a function call with an expectation, by default calls don't
have to happen in the order EXPECT_CALL()
statements are written. For example,
if the arguments match the matchers in the third EXPECT_CALL()
, but not those
in the first two, then the third expectation will be used.
If you would rather have all calls occur in the order of the expectations, put
the EXPECT_CALL()
statements in a block where you define a variable of type
InSequence
:
using ::testing::_;
using ::testing::InSequence;
{
InSequence s;
EXPECT_CALL(foo, DoThis(5));
EXPECT_CALL(bar, DoThat(_))
.Times(2);
EXPECT_CALL(foo, DoThis(6));
}
In this example, we expect a call to foo.DoThis(5)
, followed by two calls to
bar.DoThat()
where the argument can be anything, which are in turn followed by
a call to foo.DoThis(6)
. If a call occurred out-of-order, gMock will report an
error.
Expecting Partially Ordered Calls
Sometimes requiring everything to occur in a predetermined order can lead to
brittle tests. For example, we may care about A
occurring before both B
and
C
, but aren't interested in the relative order of B
and C
. In this case,
the test should reflect our real intent, instead of being overly constraining.
gMock allows you to impose an arbitrary DAG (directed acyclic graph) on the
calls. One way to express the DAG is to use the After clause of
EXPECT_CALL
.
Another way is via the InSequence()
clause (not the same as the InSequence
class), which we borrowed from jMock 2. It's less flexible than After()
, but
more convenient when you have long chains of sequential calls, as it doesn't
require you to come up with different names for the expectations in the chains.
Here's how it works:
If we view EXPECT_CALL()
statements as nodes in a graph, and add an edge from
node A to node B wherever A must occur before B, we can get a DAG. We use the
term "sequence" to mean a directed path in this DAG. Now, if we decompose the
DAG into sequences, we just need to know which sequences each EXPECT_CALL()
belongs to in order to be able to reconstruct the original DAG.
So, to specify the partial order on the expectations we need to do two things:
first to define some Sequence
objects, and then for each EXPECT_CALL()
say
which Sequence
objects it is part of.
Expectations in the same sequence must occur in the order they are written. For example,
using ::testing::Sequence;
...
Sequence s1, s2;
EXPECT_CALL(foo, A())
.InSequence(s1, s2);
EXPECT_CALL(bar, B())
.InSequence(s1);
EXPECT_CALL(bar, C())
.InSequence(s2);
EXPECT_CALL(foo, D())
.InSequence(s2);
specifies the following DAG (where s1
is A -> B
, and s2
is A -> C -> D
):
+---> B
|
A ---|
|
+---> C ---> D
This means that A must occur before B and C, and C must occur before D. There's no restriction about the order other than these.
Controlling When an Expectation Retires
When a mock method is called, gMock only considers expectations that are still active. An expectation is active when created, and becomes inactive (aka retires) when a call that has to occur later has occurred. For example, in
using ::testing::_;
using ::testing::Sequence;
...
Sequence s1, s2;
EXPECT_CALL(log, Log(WARNING, _, "File too large.")) // #1
.Times(AnyNumber())
.InSequence(s1, s2);
EXPECT_CALL(log, Log(WARNING, _, "Data set is empty.")) // #2
.InSequence(s1);
EXPECT_CALL(log, Log(WARNING, _, "User not found.")) // #3
.InSequence(s2);
as soon as either #2 or #3 is matched, #1 will retire. If a warning "File too large."
is logged after this, it will be an error.
Note that an expectation doesn't retire automatically when it's saturated. For example,
using ::testing::_;
...
EXPECT_CALL(log, Log(WARNING, _, _)); // #1
EXPECT_CALL(log, Log(WARNING, _, "File too large.")); // #2
says that there will be exactly one warning with the message "File too large."
. If the second warning contains this message too, #2 will match again
and result in an upper-bound-violated error.
If this is not what you want, you can ask an expectation to retire as soon as it becomes saturated:
using ::testing::_;
...
EXPECT_CALL(log, Log(WARNING, _, _)); // #1
EXPECT_CALL(log, Log(WARNING, _, "File too large.")) // #2
.RetiresOnSaturation();
Here #2 can be used only once, so if you have two warnings with the message
"File too large."
, the first will match #2 and the second will match #1 -
there will be no error.
Using Actions
Returning References from Mock Methods
If a mock function's return type is a reference, you need to use ReturnRef()
instead of Return()
to return a result:
using ::testing::ReturnRef;
class MockFoo : public Foo {
public:
MOCK_METHOD(Bar&, GetBar, (), (override));
};
...
MockFoo foo;
Bar bar;
EXPECT_CALL(foo, GetBar())
.WillOnce(ReturnRef(bar));
...
Returning Live Values from Mock Methods
The Return(x)
action saves a copy of x
when the action is created, and
always returns the same value whenever it's executed. Sometimes you may want to
instead return the live value of x
(i.e. its value at the time when the
action is executed.). Use either ReturnRef()
or ReturnPointee()
for this
purpose.
If the mock function's return type is a reference, you can do it using
ReturnRef(x)
, as shown in the previous recipe ("Returning References from Mock
Methods"). However, gMock doesn't let you use ReturnRef()
in a mock function
whose return type is not a reference, as doing that usually indicates a user
error. So, what shall you do?
Though you may be tempted, DO NOT use ByRef()
:
using testing::ByRef;
using testing::Return;
class MockFoo : public Foo {
public:
MOCK_METHOD(int, GetValue, (), (override));
};
...
int x = 0;
MockFoo foo;
EXPECT_CALL(foo, GetValue())
.WillRepeatedly(Return(ByRef(x))); // Wrong!
x = 42;
EXPECT_EQ(42, foo.GetValue());
Unfortunately, it doesn't work here. The above code will fail with error:
Value of: foo.GetValue()
Actual: 0
Expected: 42
The reason is that Return(*value*)
converts value
to the actual return type
of the mock function at the time when the action is created, not when it is
executed. (This behavior was chosen for the action to be safe when value
is
a proxy object that references some temporary objects.) As a result, ByRef(x)
is converted to an int
value (instead of a const int&
) when the expectation
is set, and Return(ByRef(x))
will always return 0.
ReturnPointee(pointer)
was provided to solve this problem specifically. It
returns the value pointed to by pointer
at the time the action is executed:
using testing::ReturnPointee;
...
int x = 0;
MockFoo foo;
EXPECT_CALL(foo, GetValue())
.WillRepeatedly(ReturnPointee(&x)); // Note the & here.
x = 42;
EXPECT_EQ(42, foo.GetValue()); // This will succeed now.
Combining Actions
Want to do more than one thing when a function is called? That's fine. DoAll()
allow you to do sequence of actions every time. Only the return value of the
last action in the sequence will be used.
using ::testing::_;
using ::testing::DoAll;
class MockFoo : public Foo {
public:
MOCK_METHOD(bool, Bar, (int n), (override));
};
...
EXPECT_CALL(foo, Bar(_))
.WillOnce(DoAll(action_1,
action_2,
...
action_n));
Verifying Complex Arguments
If you want to verify that a method is called with a particular argument but the match criteria is complex, it can be difficult to distinguish between cardinality failures (calling the method the wrong number of times) and argument match failures. Similarly, if you are matching multiple parameters, it may not be easy to distinguishing which argument failed to match. For example:
// Not ideal: this could fail because of a problem with arg1 or arg2, or maybe
// just the method wasn't called.
EXPECT_CALL(foo, SendValues(_, ElementsAre(1, 4, 4, 7), EqualsProto( ... )));
You can instead save the arguments and test them individually:
EXPECT_CALL(foo, SendValues)
.WillOnce(DoAll(SaveArg<1>(&actual_array), SaveArg<2>(&actual_proto)));
... run the test
EXPECT_THAT(actual_array, ElementsAre(1, 4, 4, 7));
EXPECT_THAT(actual_proto, EqualsProto( ... ));
Mocking Side Effects
Sometimes a method exhibits its effect not via returning a value but via side
effects. For example, it may change some global state or modify an output
argument. To mock side effects, in general you can define your own action by
implementing ::testing::ActionInterface
.
If all you need to do is to change an output argument, the built-in
SetArgPointee()
action is convenient:
using ::testing::_;
using ::testing::SetArgPointee;
class MockMutator : public Mutator {
public:
MOCK_METHOD(void, Mutate, (bool mutate, int* value), (override));
...
}
...
MockMutator mutator;
EXPECT_CALL(mutator, Mutate(true, _))
.WillOnce(SetArgPointee<1>(5));
In this example, when mutator.Mutate()
is called, we will assign 5 to the
int
variable pointed to by argument #1 (0-based).
SetArgPointee()
conveniently makes an internal copy of the value you pass to
it, removing the need to keep the value in scope and alive. The implication
however is that the value must have a copy constructor and assignment operator.
If the mock method also needs to return a value as well, you can chain
SetArgPointee()
with Return()
using DoAll()
, remembering to put the
Return()
statement last:
using ::testing::_;
using ::testing::Return;
using ::testing::SetArgPointee;
class MockMutator : public Mutator {
public:
...
MOCK_METHOD(bool, MutateInt, (int* value), (override));
}
...
MockMutator mutator;
EXPECT_CALL(mutator, MutateInt(_))
.WillOnce(DoAll(SetArgPointee<0>(5),
Return(true)));
Note, however, that if you use the ReturnOKWith()
method, it will override the
values provided by SetArgPointee()
in the response parameters of your function
call.
If the output argument is an array, use the SetArrayArgument<N>(first, last)
action instead. It copies the elements in source range [first, last)
to the
array pointed to by the N
-th (0-based) argument:
using ::testing::NotNull;
using ::testing::SetArrayArgument;
class MockArrayMutator : public ArrayMutator {
public:
MOCK_METHOD(void, Mutate, (int* values, int num_values), (override));
...
}
...
MockArrayMutator mutator;
int values[5] = {1, 2, 3, 4, 5};
EXPECT_CALL(mutator, Mutate(NotNull(), 5))
.WillOnce(SetArrayArgument<0>(values, values + 5));
This also works when the argument is an output iterator:
using ::testing::_;
using ::testing::SetArrayArgument;
class MockRolodex : public Rolodex {
public:
MOCK_METHOD(void, GetNames, (std::back_insert_iterator<vector<string>>),
(override));
...
}
...
MockRolodex rolodex;
vector<string> names;
names.push_back("George");
names.push_back("John");
names.push_back("Thomas");
EXPECT_CALL(rolodex, GetNames(_))
.WillOnce(SetArrayArgument<0>(names.begin(), names.end()));
Changing a Mock Object's Behavior Based on the State
If you expect a call to change the behavior of a mock object, you can use
::testing::InSequence
to specify different behaviors before and after the
call:
using ::testing::InSequence;
using ::testing::Return;
...
{
InSequence seq;
EXPECT_CALL(my_mock, IsDirty())
.WillRepeatedly(Return(true));
EXPECT_CALL(my_mock, Flush());
EXPECT_CALL(my_mock, IsDirty())
.WillRepeatedly(Return(false));
}
my_mock.FlushIfDirty();
This makes my_mock.IsDirty()
return true
before my_mock.Flush()
is called
and return false
afterwards.
If the behavior change is more complex, you can store the effects in a variable and make a mock method get its return value from that variable:
using ::testing::_;
using ::testing::SaveArg;
using ::testing::Return;
ACTION_P(ReturnPointee, p) { return *p; }
...
int previous_value = 0;
EXPECT_CALL(my_mock, GetPrevValue)
.WillRepeatedly(ReturnPointee(&previous_value));
EXPECT_CALL(my_mock, UpdateValue)
.WillRepeatedly(SaveArg<0>(&previous_value));
my_mock.DoSomethingToUpdateValue();
Here my_mock.GetPrevValue()
will always return the argument of the last
UpdateValue()
call.
Setting the Default Value for a Return Type
If a mock method's return type is a built-in C++ type or pointer, by default it will return 0 when invoked. Also, in C++ 11 and above, a mock method whose return type has a default constructor will return a default-constructed value by default. You only need to specify an action if this default value doesn't work for you.
Sometimes, you may want to change this default value, or you may want to specify
a default value for types gMock doesn't know about. You can do this using the
::testing::DefaultValue
class template:
using ::testing::DefaultValue;
class MockFoo : public Foo {
public:
MOCK_METHOD(Bar, CalculateBar, (), (override));
};
...
Bar default_bar;
// Sets the default return value for type Bar.
DefaultValue<Bar>::Set(default_bar);
MockFoo foo;
// We don't need to specify an action here, as the default
// return value works for us.
EXPECT_CALL(foo, CalculateBar());
foo.CalculateBar(); // This should return default_bar.
// Unsets the default return value.
DefaultValue<Bar>::Clear();
Please note that changing the default value for a type can make you tests hard
to understand. We recommend you to use this feature judiciously. For example,
you may want to make sure the Set()
and Clear()
calls are right next to the
code that uses your mock.
Setting the Default Actions for a Mock Method
You've learned how to change the default value of a given type. However, this
may be too coarse for your purpose: perhaps you have two mock methods with the
same return type and you want them to have different behaviors. The ON_CALL()
macro allows you to customize your mock's behavior at the method level:
using ::testing::_;
using ::testing::AnyNumber;
using ::testing::Gt;
using ::testing::Return;
...
ON_CALL(foo, Sign(_))
.WillByDefault(Return(-1));
ON_CALL(foo, Sign(0))
.WillByDefault(Return(0));
ON_CALL(foo, Sign(Gt(0)))
.WillByDefault(Return(1));
EXPECT_CALL(foo, Sign(_))
.Times(AnyNumber());
foo.Sign(5); // This should return 1.
foo.Sign(-9); // This should return -1.
foo.Sign(0); // This should return 0.
As you may have guessed, when there are more than one ON_CALL()
statements,
the newer ones in the order take precedence over the older ones. In other words,
the last one that matches the function arguments will be used. This matching
order allows you to set up the common behavior in a mock object's constructor or
the test fixture's set-up phase and specialize the mock's behavior later.
Note that both ON_CALL
and EXPECT_CALL
have the same "later statements take
precedence" rule, but they don't interact. That is, EXPECT_CALL
s have their
own precedence order distinct from the ON_CALL
precedence order.
Using Functions/Methods/Functors/Lambdas as Actions
If the built-in actions don't suit you, you can use an existing callable
(function, std::function
, method, functor, lambda as an action.
using ::testing::_; using ::testing::Invoke;
class MockFoo : public Foo {
public:
MOCK_METHOD(int, Sum, (int x, int y), (override));
MOCK_METHOD(bool, ComplexJob, (int x), (override));
};
int CalculateSum(int x, int y) { return x + y; }
int Sum3(int x, int y, int z) { return x + y + z; }
class Helper {
public:
bool ComplexJob(int x);
};
...
MockFoo foo;
Helper helper;
EXPECT_CALL(foo, Sum(_, _))
.WillOnce(&CalculateSum)
.WillRepeatedly(Invoke(NewPermanentCallback(Sum3, 1)));
EXPECT_CALL(foo, ComplexJob(_))
.WillOnce(Invoke(&helper, &Helper::ComplexJob));
.WillRepeatedly([](int x) { return x > 0; });
foo.Sum(5, 6); // Invokes CalculateSum(5, 6).
foo.Sum(2, 3); // Invokes Sum3(1, 2, 3).
foo.ComplexJob(10); // Invokes helper.ComplexJob(10).
foo.ComplexJob(-1); // Invokes the inline lambda.
The only requirement is that the type of the function, etc must be compatible with the signature of the mock function, meaning that the latter's arguments can be implicitly converted to the corresponding arguments of the former, and the former's return type can be implicitly converted to that of the latter. So, you can invoke something whose type is not exactly the same as the mock function, as long as it's safe to do so - nice, huh?
Note:
{.escaped}
-
The action takes ownership of the callback and will delete it when the action itself is destructed.
-
If the type of a callback is derived from a base callback type
C
, you need to implicitly cast it toC
to resolve the overloading, e.g.using ::testing::Invoke; ... ResultCallback<bool>* is_ok = ...; ... Invoke(is_ok) ...; // This works. BlockingClosure* done = new BlockingClosure; ... Invoke(implicit_cast<Closure*>(done)) ...; // The cast is necessary.
Using Functions with Extra Info as Actions
The function or functor you call using Invoke()
must have the same number of
arguments as the mock function you use it for. Sometimes you may have a function
that takes more arguments, and you are willing to pass in the extra arguments
yourself to fill the gap. You can do this in gMock using callbacks with
pre-bound arguments. Here's an example:
using ::testing::Invoke;
class MockFoo : public Foo {
public:
MOCK_METHOD(char, DoThis, (int n), (override));
};
char SignOfSum(int x, int y) {
const int sum = x + y;
return (sum > 0) ? '+' : (sum < 0) ? '-' : '0';
}
TEST_F(FooTest, Test) {
MockFoo foo;
EXPECT_CALL(foo, DoThis(2))
.WillOnce(Invoke(NewPermanentCallback(SignOfSum, 5)));
EXPECT_EQ('+', foo.DoThis(2)); // Invokes SignOfSum(5, 2).
}
Invoking a Function/Method/Functor/Lambda/Callback Without Arguments
Invoke()
is very useful for doing actions that are more complex. It passes the
mock function's arguments to the function, etc being invoked such that the
callee has the full context of the call to work with. If the invoked function is
not interested in some or all of the arguments, it can simply ignore them.
Yet, a common pattern is that a test author wants to invoke a function without
the arguments of the mock function. Invoke()
allows her to do that using a
wrapper function that throws away the arguments before invoking an underlining
nullary function. Needless to say, this can be tedious and obscures the intent
of the test.
InvokeWithoutArgs()
solves this problem. It's like Invoke()
except that it
doesn't pass the mock function's arguments to the callee. Here's an example:
using ::testing::_;
using ::testing::InvokeWithoutArgs;
class MockFoo : public Foo {
public:
MOCK_METHOD(bool, ComplexJob, (int n), (override));
};
bool Job1() { ... }
bool Job2(int n, char c) { ... }
...
MockFoo foo;
EXPECT_CALL(foo, ComplexJob(_))
.WillOnce(InvokeWithoutArgs(Job1))
.WillOnce(InvokeWithoutArgs(NewPermanentCallback(Job2, 5, 'a')));
foo.ComplexJob(10); // Invokes Job1().
foo.ComplexJob(20); // Invokes Job2(5, 'a').
Note:
{.escaped}
-
The action takes ownership of the callback and will delete it when the action itself is destructed.
-
If the type of a callback is derived from a base callback type
C
, you need to implicitly cast it toC
to resolve the overloading, e.g.using ::testing::InvokeWithoutArgs; ... ResultCallback<bool>* is_ok = ...; ... InvokeWithoutArgs(is_ok) ...; // This works. BlockingClosure* done = ...; ... InvokeWithoutArgs(implicit_cast<Closure*>(done)) ...; // The cast is necessary.
Invoking an Argument of the Mock Function
Sometimes a mock function will receive a function pointer, a functor (in other words, a "callable") as an argument, e.g.
class MockFoo : public Foo {
public:
MOCK_METHOD(bool, DoThis, (int n, (ResultCallback1<bool, int>* callback)),
(override));
};
and you may want to invoke this callable argument:
using ::testing::_;
...
MockFoo foo;
EXPECT_CALL(foo, DoThis(_, _))
.WillOnce(...);
// Will execute callback->Run(5), where callback is the
// second argument DoThis() receives.
NOTE: The section below is legacy documentation from before C++ had lambdas:
Arghh, you need to refer to a mock function argument but C++ has no lambda (yet), so you have to define your own action. :-( Or do you really?
Well, gMock has an action to solve exactly this problem:
InvokeArgument<N>(arg_1, arg_2, ..., arg_m)
will invoke the N
-th (0-based) argument the mock function receives, with
arg_1
, arg_2
, ..., and arg_m
. No matter if the argument is a function
pointer, a functor, or a callback. gMock handles them all.
With that, you could write:
using ::testing::_;
using ::testing::InvokeArgument;
...
EXPECT_CALL(foo, DoThis(_, _))
.WillOnce(InvokeArgument<1>(5));
// Will execute callback->Run(5), where callback is the
// second argument DoThis() receives.
What if the callable takes an argument by reference? No problem - just wrap it
inside ByRef()
:
...
MOCK_METHOD(bool, Bar,
((ResultCallback2<bool, int, const Helper&>* callback)),
(override));
...
using ::testing::_;
using ::testing::ByRef;
using ::testing::InvokeArgument;
...
MockFoo foo;
Helper helper;
...
EXPECT_CALL(foo, Bar(_))
.WillOnce(InvokeArgument<0>(5, ByRef(helper)));
// ByRef(helper) guarantees that a reference to helper, not a copy of it,
// will be passed to the callback.
What if the callable takes an argument by reference and we do not wrap the
argument in ByRef()
? Then InvokeArgument()
will make a copy of the
argument, and pass a reference to the copy, instead of a reference to the
original value, to the callable. This is especially handy when the argument is a
temporary value:
...
MOCK_METHOD(bool, DoThat, (bool (*f)(const double& x, const string& s)),
(override));
...
using ::testing::_;
using ::testing::InvokeArgument;
...
MockFoo foo;
...
EXPECT_CALL(foo, DoThat(_))
.WillOnce(InvokeArgument<0>(5.0, string("Hi")));
// Will execute (*f)(5.0, string("Hi")), where f is the function pointer
// DoThat() receives. Note that the values 5.0 and string("Hi") are
// temporary and dead once the EXPECT_CALL() statement finishes. Yet
// it's fine to perform this action later, since a copy of the values
// are kept inside the InvokeArgument action.
Ignoring an Action's Result
Sometimes you have an action that returns something, but you need an action
that returns void
(perhaps you want to use it in a mock function that returns
void
, or perhaps it needs to be used in DoAll()
and it's not the last in the
list). IgnoreResult()
lets you do that. For example:
using ::testing::_;
using ::testing::DoAll;
using ::testing::IgnoreResult;
using ::testing::Return;
int Process(const MyData& data);
string DoSomething();
class MockFoo : public Foo {
public:
MOCK_METHOD(void, Abc, (const MyData& data), (override));
MOCK_METHOD(bool, Xyz, (), (override));
};
...
MockFoo foo;
EXPECT_CALL(foo, Abc(_))
// .WillOnce(Invoke(Process));
// The above line won't compile as Process() returns int but Abc() needs
// to return void.
.WillOnce(IgnoreResult(Process));
EXPECT_CALL(foo, Xyz())
.WillOnce(DoAll(IgnoreResult(DoSomething),
// Ignores the string DoSomething() returns.
Return(true)));
Note that you cannot use IgnoreResult()
on an action that already returns
void
. Doing so will lead to ugly compiler errors.
Selecting an Action's Arguments
Say you have a mock function Foo()
that takes seven arguments, and you have a
custom action that you want to invoke when Foo()
is called. Trouble is, the
custom action only wants three arguments:
using ::testing::_;
using ::testing::Invoke;
...
MOCK_METHOD(bool, Foo,
(bool visible, const string& name, int x, int y,
(const map<pair<int, int>>), double& weight, double min_weight,
double max_wight));
...
bool IsVisibleInQuadrant1(bool visible, int x, int y) {
return visible && x >= 0 && y >= 0;
}
...
EXPECT_CALL(mock, Foo)
.WillOnce(Invoke(IsVisibleInQuadrant1)); // Uh, won't compile. :-(
To please the compiler God, you need to define an "adaptor" that has the same
signature as Foo()
and calls the custom action with the right arguments:
using ::testing::_;
using ::testing::Invoke;
...
bool MyIsVisibleInQuadrant1(bool visible, const string& name, int x, int y,
const map<pair<int, int>, double>& weight,
double min_weight, double max_wight) {
return IsVisibleInQuadrant1(visible, x, y);
}
...
EXPECT_CALL(mock, Foo)
.WillOnce(Invoke(MyIsVisibleInQuadrant1)); // Now it works.
But isn't this awkward?
gMock provides a generic action adaptor, so you can spend your time minding more important business than writing your own adaptors. Here's the syntax:
WithArgs<N1, N2, ..., Nk>(action)
creates an action that passes the arguments of the mock function at the given
indices (0-based) to the inner action
and performs it. Using WithArgs
, our
original example can be written as:
using ::testing::_;
using ::testing::Invoke;
using ::testing::WithArgs;
...
EXPECT_CALL(mock, Foo)
.WillOnce(WithArgs<0, 2, 3>(Invoke(IsVisibleInQuadrant1))); // No need to define your own adaptor.
For better readability, gMock also gives you:
WithoutArgs(action)
when the inneraction
takes no argument, andWithArg<N>(action)
(nos
afterArg
) when the inneraction
takes one argument.
As you may have realized, InvokeWithoutArgs(...)
is just syntactic sugar for
WithoutArgs(Invoke(...))
.
Here are more tips:
- The inner action used in
WithArgs
and friends does not have to beInvoke()
-- it can be anything. - You can repeat an argument in the argument list if necessary, e.g.
WithArgs<2, 3, 3, 5>(...)
. - You can change the order of the arguments, e.g.
WithArgs<3, 2, 1>(...)
. - The types of the selected arguments do not have to match the signature of
the inner action exactly. It works as long as they can be implicitly
converted to the corresponding arguments of the inner action. For example,
if the 4-th argument of the mock function is an
int
andmy_action
takes adouble
,WithArg<4>(my_action)
will work.
Ignoring Arguments in Action Functions
The selecting-an-action's-arguments recipe showed us one way
to make a mock function and an action with incompatible argument lists fit
together. The downside is that wrapping the action in WithArgs<...>()
can get
tedious for people writing the tests.
If you are defining a function (or method, functor, lambda, callback) to be used
with Invoke*()
, and you are not interested in some of its arguments, an
alternative to WithArgs
is to declare the uninteresting arguments as Unused
.
This makes the definition less cluttered and less fragile in case the types of
the uninteresting arguments change. It could also increase the chance the action
function can be reused. For example, given
public:
MOCK_METHOD(double, Foo, double(const string& label, double x, double y),
(override));
MOCK_METHOD(double, Bar, (int index, double x, double y), (override));
instead of
using ::testing::_;
using ::testing::Invoke;
double DistanceToOriginWithLabel(const string& label, double x, double y) {
return sqrt(x*x + y*y);
}
double DistanceToOriginWithIndex(int index, double x, double y) {
return sqrt(x*x + y*y);
}
...
EXPECT_CALL(mock, Foo("abc", _, _))
.WillOnce(Invoke(DistanceToOriginWithLabel));
EXPECT_CALL(mock, Bar(5, _, _))
.WillOnce(Invoke(DistanceToOriginWithIndex));
you could write
using ::testing::_;
using ::testing::Invoke;
using ::testing::Unused;
double DistanceToOrigin(Unused, double x, double y) {
return sqrt(x*x + y*y);
}
...
EXPECT_CALL(mock, Foo("abc", _, _))
.WillOnce(Invoke(DistanceToOrigin));
EXPECT_CALL(mock, Bar(5, _, _))
.WillOnce(Invoke(DistanceToOrigin));
Sharing Actions
Just like matchers, a gMock action object consists of a pointer to a ref-counted implementation object. Therefore copying actions is also allowed and very efficient. When the last action that references the implementation object dies, the implementation object will be deleted.
If you have some complex action that you want to use again and again, you may not have to build it from scratch everytime. If the action doesn't have an internal state (i.e. if it always does the same thing no matter how many times it has been called), you can assign it to an action variable and use that variable repeatedly. For example:
using ::testing::Action;
using ::testing::DoAll;
using ::testing::Return;
using ::testing::SetArgPointee;
...
Action<bool(int*)> set_flag = DoAll(SetArgPointee<0>(5),
Return(true));
... use set_flag in .WillOnce() and .WillRepeatedly() ...
However, if the action has its own state, you may be surprised if you share the
action object. Suppose you have an action factory IncrementCounter(init)
which
creates an action that increments and returns a counter whose initial value is
init
, using two actions created from the same expression and using a shared
action will exhibit different behaviors. Example:
EXPECT_CALL(foo, DoThis())
.WillRepeatedly(IncrementCounter(0));
EXPECT_CALL(foo, DoThat())
.WillRepeatedly(IncrementCounter(0));
foo.DoThis(); // Returns 1.
foo.DoThis(); // Returns 2.
foo.DoThat(); // Returns 1 - Blah() uses a different
// counter than Bar()'s.
versus
using ::testing::Action;
...
Action<int()> increment = IncrementCounter(0);
EXPECT_CALL(foo, DoThis())
.WillRepeatedly(increment);
EXPECT_CALL(foo, DoThat())
.WillRepeatedly(increment);
foo.DoThis(); // Returns 1.
foo.DoThis(); // Returns 2.
foo.DoThat(); // Returns 3 - the counter is shared.
Testing Asynchronous Behavior
One oft-encountered problem with gMock is that it can be hard to test
asynchronous behavior. Suppose you had a EventQueue
class that you wanted to
test, and you created a separate EventDispatcher
interface so that you could
easily mock it out. However, the implementation of the class fired all the
events on a background thread, which made test timings difficult. You could just
insert sleep()
statements and hope for the best, but that makes your test
behavior nondeterministic. A better way is to use gMock actions and
Notification
objects to force your asynchronous test to behave synchronously.
using ::testing::DoAll;
using ::testing::InvokeWithoutArgs;
using ::testing::Return;
class MockEventDispatcher : public EventDispatcher {
MOCK_METHOD(bool, DispatchEvent, (int32), (override));
};
ACTION_P(Notify, notification) {
notification->Notify();
}
TEST(EventQueueTest, EnqueueEventTest) {
MockEventDispatcher mock_event_dispatcher;
EventQueue event_queue(&mock_event_dispatcher);
const int32 kEventId = 321;
Notification done;
EXPECT_CALL(mock_event_dispatcher, DispatchEvent(kEventId))
.WillOnce(Notify(&done));
event_queue.EnqueueEvent(kEventId);
done.WaitForNotification();
}
In the example above, we set our normal gMock expectations, but then add an
additional action to notify the Notification
object. Now we can just call
Notification::WaitForNotification()
in the main thread to wait for the
asynchronous call to finish. After that, our test suite is complete and we can
safely exit.
Note: this example has a downside: namely, if the expectation is not satisfied,
our test will run forever. It will eventually time-out and fail, but it will
take longer and be slightly harder to debug. To alleviate this problem, you can
use WaitForNotificationWithTimeout(ms)
instead of WaitForNotification()
.
Misc Recipes on Using gMock
Mocking Methods That Use Move-Only Types
C++11 introduced move-only types. A move-only-typed value can be moved from
one object to another, but cannot be copied. std::unique_ptr<T>
is probably
the most commonly used move-only type.
Mocking a method that takes and/or returns move-only types presents some challenges, but nothing insurmountable. This recipe shows you how you can do it. Note that the support for move-only method arguments was only introduced to gMock in April 2017; in older code, you may find more complex workarounds for lack of this feature.
Let’s say we are working on a fictional project that lets one post and share snippets called “buzzes”. Your code uses these types:
enum class AccessLevel { kInternal, kPublic };
class Buzz {
public:
explicit Buzz(AccessLevel access) { ... }
...
};
class Buzzer {
public:
virtual ~Buzzer() {}
virtual std::unique_ptr<Buzz> MakeBuzz(StringPiece text) = 0;
virtual bool ShareBuzz(std::unique_ptr<Buzz> buzz, int64_t timestamp) = 0;
...
};
A Buzz
object represents a snippet being posted. A class that implements the
Buzzer
interface is capable of creating and sharing Buzz
es. Methods in
Buzzer
may return a unique_ptr<Buzz>
or take a unique_ptr<Buzz>
. Now we
need to mock Buzzer
in our tests.
To mock a method that accepts or returns move-only types, you just use the
familiar MOCK_METHOD
syntax as usual:
class MockBuzzer : public Buzzer {
public:
MOCK_METHOD(std::unique_ptr<Buzz>, MakeBuzz, (StringPiece text), (override));
MOCK_METHOD(bool, ShareBuzz, (std::unique_ptr<Buzz> buzz, int64_t timestamp),
(override));
};
Now that we have the mock class defined, we can use it in tests. In the
following code examples, we assume that we have defined a MockBuzzer
object
named mock_buzzer_
:
MockBuzzer mock_buzzer_;
First let’s see how we can set expectations on the MakeBuzz()
method, which
returns a unique_ptr<Buzz>
.
As usual, if you set an expectation without an action (i.e. the .WillOnce()
or
.WillRepeatedly()
clause), when that expectation fires, the default action for
that method will be taken. Since unique_ptr<>
has a default constructor that
returns a null unique_ptr
, that’s what you’ll get if you don’t specify an
action:
// Use the default action.
EXPECT_CALL(mock_buzzer_, MakeBuzz("hello"));
// Triggers the previous EXPECT_CALL.
EXPECT_EQ(nullptr, mock_buzzer_.MakeBuzz("hello"));
If you are not happy with the default action, you can tweak it as usual; see Setting Default Actions.
If you just need to return a pre-defined move-only value, you can use the
Return(ByMove(...))
action:
// When this fires, the unique_ptr<> specified by ByMove(...) will
// be returned.
EXPECT_CALL(mock_buzzer_, MakeBuzz("world"))
.WillOnce(Return(ByMove(MakeUnique<Buzz>(AccessLevel::kInternal))));
EXPECT_NE(nullptr, mock_buzzer_.MakeBuzz("world"));
Note that ByMove()
is essential here - if you drop it, the code won’t compile.
Quiz time! What do you think will happen if a Return(ByMove(...))
action is
performed more than once (e.g. you write ... .WillRepeatedly(Return(ByMove(...)));
)? Come think of it, after the first time
the action runs, the source value will be consumed (since it’s a move-only
value), so the next time around, there’s no value to move from -- you’ll get a
run-time error that Return(ByMove(...))
can only be run once.
If you need your mock method to do more than just moving a pre-defined value, remember that you can always use a lambda or a callable object, which can do pretty much anything you want:
EXPECT_CALL(mock_buzzer_, MakeBuzz("x"))
.WillRepeatedly([](StringPiece text) {
return MakeUnique<Buzz>(AccessLevel::kInternal);
});
EXPECT_NE(nullptr, mock_buzzer_.MakeBuzz("x"));
EXPECT_NE(nullptr, mock_buzzer_.MakeBuzz("x"));
Every time this EXPECT_CALL
fires, a new unique_ptr<Buzz>
will be created
and returned. You cannot do this with Return(ByMove(...))
.
That covers returning move-only values; but how do we work with methods
accepting move-only arguments? The answer is that they work normally, although
some actions will not compile when any of method's arguments are move-only. You
can always use Return
, or a lambda or functor:
using ::testing::Unused;
EXPECT_CALL(mock_buzzer_, ShareBuzz(NotNull(), _)).WillOnce(Return(true));
EXPECT_TRUE(mock_buzzer_.ShareBuzz(MakeUnique<Buzz>(AccessLevel::kInternal)),
0);
EXPECT_CALL(mock_buzzer_, ShareBuzz(_, _)).WillOnce(
[](std::unique_ptr<Buzz> buzz, Unused) { return buzz != nullptr; });
EXPECT_FALSE(mock_buzzer_.ShareBuzz(nullptr, 0));
Many built-in actions (WithArgs
, WithoutArgs
,DeleteArg
, SaveArg
, ...)
could in principle support move-only arguments, but the support for this is not
implemented yet. If this is blocking you, please file a bug.
A few actions (e.g. DoAll
) copy their arguments internally, so they can never
work with non-copyable objects; you'll have to use functors instead.
Legacy workarounds for move-only types
Support for move-only function arguments was only introduced to gMock in April 2017. In older code, you may encounter the following workaround for the lack of this feature (it is no longer necessary - we're including it just for reference):
class MockBuzzer : public Buzzer {
public:
MOCK_METHOD(bool, DoShareBuzz, (Buzz* buzz, Time timestamp));
bool ShareBuzz(std::unique_ptr<Buzz> buzz, Time timestamp) override {
return DoShareBuzz(buzz.get(), timestamp);
}
};
The trick is to delegate the ShareBuzz()
method to a mock method (let’s call
it DoShareBuzz()
) that does not take move-only parameters. Then, instead of
setting expectations on ShareBuzz()
, you set them on the DoShareBuzz()
mock
method:
MockBuzzer mock_buzzer_;
EXPECT_CALL(mock_buzzer_, DoShareBuzz(NotNull(), _));
// When one calls ShareBuzz() on the MockBuzzer like this, the call is
// forwarded to DoShareBuzz(), which is mocked. Therefore this statement
// will trigger the above EXPECT_CALL.
mock_buzzer_.ShareBuzz(MakeUnique<Buzz>(AccessLevel::kInternal), 0);
Making the Compilation Faster
Believe it or not, the vast majority of the time spent on compiling a mock class is in generating its constructor and destructor, as they perform non-trivial tasks (e.g. verification of the expectations). What's more, mock methods with different signatures have different types and thus their constructors/destructors need to be generated by the compiler separately. As a result, if you mock many different types of methods, compiling your mock class can get really slow.
If you are experiencing slow compilation, you can move the definition of your
mock class' constructor and destructor out of the class body and into a .cc
file. This way, even if you #include
your mock class in N files, the compiler
only needs to generate its constructor and destructor once, resulting in a much
faster compilation.
Let's illustrate the idea using an example. Here's the definition of a mock class before applying this recipe:
// File mock_foo.h.
...
class MockFoo : public Foo {
public:
// Since we don't declare the constructor or the destructor,
// the compiler will generate them in every translation unit
// where this mock class is used.
MOCK_METHOD(int, DoThis, (), (override));
MOCK_METHOD(bool, DoThat, (const char* str), (override));
... more mock methods ...
};
After the change, it would look like:
// File mock_foo.h.
...
class MockFoo : public Foo {
public:
// The constructor and destructor are declared, but not defined, here.
MockFoo();
virtual ~MockFoo();
MOCK_METHOD(int, DoThis, (), (override));
MOCK_METHOD(bool, DoThat, (const char* str), (override));
... more mock methods ...
};
and
// File mock_foo.cc.
#include "path/to/mock_foo.h"
// The definitions may appear trivial, but the functions actually do a
// lot of things through the constructors/destructors of the member
// variables used to implement the mock methods.
MockFoo::MockFoo() {}
MockFoo::~MockFoo() {}
Forcing a Verification
When it's being destroyed, your friendly mock object will automatically verify that all expectations on it have been satisfied, and will generate googletest failures if not. This is convenient as it leaves you with one less thing to worry about. That is, unless you are not sure if your mock object will be destroyed.
How could it be that your mock object won't eventually be destroyed? Well, it might be created on the heap and owned by the code you are testing. Suppose there's a bug in that code and it doesn't delete the mock object properly - you could end up with a passing test when there's actually a bug.
Using a heap checker is a good idea and can alleviate the concern, but its
implementation is not 100% reliable. So, sometimes you do want to force gMock
to verify a mock object before it is (hopefully) destructed. You can do this
with Mock::VerifyAndClearExpectations(&mock_object)
:
TEST(MyServerTest, ProcessesRequest) {
using ::testing::Mock;
MockFoo* const foo = new MockFoo;
EXPECT_CALL(*foo, ...)...;
// ... other expectations ...
// server now owns foo.
MyServer server(foo);
server.ProcessRequest(...);
// In case that server's destructor will forget to delete foo,
// this will verify the expectations anyway.
Mock::VerifyAndClearExpectations(foo);
} // server is destroyed when it goes out of scope here.
Tip: The Mock::VerifyAndClearExpectations()
function returns a bool
to
indicate whether the verification was successful (true
for yes), so you can
wrap that function call inside a ASSERT_TRUE()
if there is no point going
further when the verification has failed.
Using Check Points
Sometimes you may want to "reset" a mock object at various check points in your test: at each check point, you verify that all existing expectations on the mock object have been satisfied, and then you set some new expectations on it as if it's newly created. This allows you to work with a mock object in "phases" whose sizes are each manageable.
One such scenario is that in your test's SetUp()
function, you may want to put
the object you are testing into a certain state, with the help from a mock
object. Once in the desired state, you want to clear all expectations on the
mock, such that in the TEST_F
body you can set fresh expectations on it.
As you may have figured out, the Mock::VerifyAndClearExpectations()
function
we saw in the previous recipe can help you here. Or, if you are using
ON_CALL()
to set default actions on the mock object and want to clear the
default actions as well, use Mock::VerifyAndClear(&mock_object)
instead. This
function does what Mock::VerifyAndClearExpectations(&mock_object)
does and
returns the same bool
, plus it clears the ON_CALL()
statements on
mock_object
too.
Another trick you can use to achieve the same effect is to put the expectations in sequences and insert calls to a dummy "check-point" function at specific places. Then you can verify that the mock function calls do happen at the right time. For example, if you are exercising code:
Foo(1);
Foo(2);
Foo(3);
and want to verify that Foo(1)
and Foo(3)
both invoke mock.Bar("a")
, but
Foo(2)
doesn't invoke anything. You can write:
using ::testing::MockFunction;
TEST(FooTest, InvokesBarCorrectly) {
MyMock mock;
// Class MockFunction<F> has exactly one mock method. It is named
// Call() and has type F.
MockFunction<void(string check_point_name)> check;
{
InSequence s;
EXPECT_CALL(mock, Bar("a"));
EXPECT_CALL(check, Call("1"));
EXPECT_CALL(check, Call("2"));
EXPECT_CALL(mock, Bar("a"));
}
Foo(1);
check.Call("1");
Foo(2);
check.Call("2");
Foo(3);
}
The expectation spec says that the first Bar("a")
must happen before check
point "1", the second Bar("a")
must happen after check point "2", and nothing
should happen between the two check points. The explicit check points make it
easy to tell which Bar("a")
is called by which call to Foo()
.
Mocking Destructors
Sometimes you want to make sure a mock object is destructed at the right time,
e.g. after bar->A()
is called but before bar->B()
is called. We already know
that you can specify constraints on the order of mock function
calls, so all we need to do is to mock the destructor of the mock function.
This sounds simple, except for one problem: a destructor is a special function
with special syntax and special semantics, and the MOCK_METHOD
macro doesn't
work for it:
MOCK_METHOD(void, ~MockFoo, ()); // Won't compile!
The good news is that you can use a simple pattern to achieve the same effect.
First, add a mock function Die()
to your mock class and call it in the
destructor, like this:
class MockFoo : public Foo {
...
// Add the following two lines to the mock class.
MOCK_METHOD(void, Die, ());
virtual ~MockFoo() { Die(); }
};
(If the name Die()
clashes with an existing symbol, choose another name.) Now,
we have translated the problem of testing when a MockFoo
object dies to
testing when its Die()
method is called:
MockFoo* foo = new MockFoo;
MockBar* bar = new MockBar;
...
{
InSequence s;
// Expects *foo to die after bar->A() and before bar->B().
EXPECT_CALL(*bar, A());
EXPECT_CALL(*foo, Die());
EXPECT_CALL(*bar, B());
}
And that's that.
Using gMock and Threads
In a unit test, it's best if you could isolate and test a piece of code in a single-threaded context. That avoids race conditions and dead locks, and makes debugging your test much easier.
Yet most programs are multi-threaded, and sometimes to test something we need to pound on it from more than one thread. gMock works for this purpose too.
Remember the steps for using a mock:
- Create a mock object
foo
. - Set its default actions and expectations using
ON_CALL()
andEXPECT_CALL()
. - The code under test calls methods of
foo
. - Optionally, verify and reset the mock.
- Destroy the mock yourself, or let the code under test destroy it. The destructor will automatically verify it.
If you follow the following simple rules, your mocks and threads can live happily together:
- Execute your test code (as opposed to the code being tested) in one thread. This makes your test easy to follow.
- Obviously, you can do step #1 without locking.
- When doing step #2 and #5, make sure no other thread is accessing
foo
. Obvious too, huh? - #3 and #4 can be done either in one thread or in multiple threads - anyway you want. gMock takes care of the locking, so you don't have to do any - unless required by your test logic.
If you violate the rules (for example, if you set expectations on a mock while another thread is calling its methods), you get undefined behavior. That's not fun, so don't do it.
gMock guarantees that the action for a mock function is done in the same thread that called the mock function. For example, in
EXPECT_CALL(mock, Foo(1))
.WillOnce(action1);
EXPECT_CALL(mock, Foo(2))
.WillOnce(action2);
if Foo(1)
is called in thread 1 and Foo(2)
is called in thread 2, gMock will
execute action1
in thread 1 and action2
in thread 2.
gMock does not impose a sequence on actions performed in different threads
(doing so may create deadlocks as the actions may need to cooperate). This means
that the execution of action1
and action2
in the above example may
interleave. If this is a problem, you should add proper synchronization logic to
action1
and action2
to make the test thread-safe.
Also, remember that DefaultValue<T>
is a global resource that potentially
affects all living mock objects in your program. Naturally, you won't want to
mess with it from multiple threads or when there still are mocks in action.
Controlling How Much Information gMock Prints
When gMock sees something that has the potential of being an error (e.g. a mock function with no expectation is called, a.k.a. an uninteresting call, which is allowed but perhaps you forgot to explicitly ban the call), it prints some warning messages, including the arguments of the function, the return value, and the stack trace. Hopefully this will remind you to take a look and see if there is indeed a problem.
Sometimes you are confident that your tests are correct and may not appreciate such friendly messages. Some other times, you are debugging your tests or learning about the behavior of the code you are testing, and wish you could observe every mock call that happens (including argument values, the return value, and the stack trace). Clearly, one size doesn't fit all.
You can control how much gMock tells you using the --gmock_verbose=LEVEL
command-line flag, where LEVEL
is a string with three possible values:
info
: gMock will print all informational messages, warnings, and errors (most verbose). At this setting, gMock will also log any calls to theON_CALL/EXPECT_CALL
macros. It will include a stack trace in "uninteresting call" warnings.warning
: gMock will print both warnings and errors (less verbose); it will omit the stack traces in "uninteresting call" warnings. This is the default.error
: gMock will print errors only (least verbose).
Alternatively, you can adjust the value of that flag from within your tests like so:
::testing::FLAGS_gmock_verbose = "error";
If you find gMock printing too many stack frames with its informational or
warning messages, remember that you can control their amount with the
--gtest_stack_trace_depth=max_depth
flag.
Now, judiciously use the right flag to enable gMock serve you better!
Gaining Super Vision into Mock Calls
You have a test using gMock. It fails: gMock tells you some expectations aren't
satisfied. However, you aren't sure why: Is there a typo somewhere in the
matchers? Did you mess up the order of the EXPECT_CALL
s? Or is the code under
test doing something wrong? How can you find out the cause?
Won't it be nice if you have X-ray vision and can actually see the trace of all
EXPECT_CALL
s and mock method calls as they are made? For each call, would you
like to see its actual argument values and which EXPECT_CALL
gMock thinks it
matches? If you still need some help to figure out who made these calls, how
about being able to see the complete stack trace at each mock call?
You can unlock this power by running your test with the --gmock_verbose=info
flag. For example, given the test program:
#include "gmock/gmock.h"
using testing::_;
using testing::HasSubstr;
using testing::Return;
class MockFoo {
public:
MOCK_METHOD(void, F, (const string& x, const string& y));
};
TEST(Foo, Bar) {
MockFoo mock;
EXPECT_CALL(mock, F(_, _)).WillRepeatedly(Return());
EXPECT_CALL(mock, F("a", "b"));
EXPECT_CALL(mock, F("c", HasSubstr("d")));
mock.F("a", "good");
mock.F("a", "b");
}
if you run it with --gmock_verbose=info
, you will see this output:
[ RUN ] Foo.Bar
foo_test.cc:14: EXPECT_CALL(mock, F(_, _)) invoked
Stack trace: ...
foo_test.cc:15: EXPECT_CALL(mock, F("a", "b")) invoked
Stack trace: ...
foo_test.cc:16: EXPECT_CALL(mock, F("c", HasSubstr("d"))) invoked
Stack trace: ...
foo_test.cc:14: Mock function call matches EXPECT_CALL(mock, F(_, _))...
Function call: F(@0x7fff7c8dad40"a",@0x7fff7c8dad10"good")
Stack trace: ...
foo_test.cc:15: Mock function call matches EXPECT_CALL(mock, F("a", "b"))...
Function call: F(@0x7fff7c8dada0"a",@0x7fff7c8dad70"b")
Stack trace: ...
foo_test.cc:16: Failure
Actual function call count doesn't match EXPECT_CALL(mock, F("c", HasSubstr("d")))...
Expected: to be called once
Actual: never called - unsatisfied and active
[ FAILED ] Foo.Bar
Suppose the bug is that the "c"
in the third EXPECT_CALL
is a typo and
should actually be "a"
. With the above message, you should see that the actual
F("a", "good")
call is matched by the first EXPECT_CALL
, not the third as
you thought. From that it should be obvious that the third EXPECT_CALL
is
written wrong. Case solved.
If you are interested in the mock call trace but not the stack traces, you can
combine --gmock_verbose=info
with --gtest_stack_trace_depth=0
on the test
command line.
Running Tests in Emacs
If you build and run your tests in Emacs using the M-x google-compile
command
(as many googletest users do), the source file locations of gMock and googletest
errors will be highlighted. Just press <Enter>
on one of them and you'll be
taken to the offending line. Or, you can just type `C-x`` to jump to the next
error.
To make it even easier, you can add the following lines to your ~/.emacs
file:
(global-set-key "\M-m" 'google-compile) ; m is for make
(global-set-key [M-down] 'next-error)
(global-set-key [M-up] '(lambda () (interactive) (next-error -1)))
Then you can type M-m
to start a build (if you want to run the test as well,
just make sure foo_test.run
or runtests
is in the build command you supply
after typing M-m
), or M-up
/M-down
to move back and forth between errors.
Extending gMock
Writing New Matchers Quickly
WARNING: gMock does not guarantee when or how many times a matcher will be invoked. Therefore, all matchers must be functionally pure. See this section for more details.
The MATCHER*
family of macros can be used to define custom matchers easily.
The syntax:
MATCHER(name, description_string_expression) { statements; }
will define a matcher with the given name that executes the statements, which
must return a bool
to indicate if the match succeeds. Inside the statements,
you can refer to the value being matched by arg
, and refer to its type by
arg_type
.
The description string is a string
-typed expression that documents what the
matcher does, and is used to generate the failure message when the match fails.
It can (and should) reference the special bool
variable negation
, and should
evaluate to the description of the matcher when negation
is false
, or that
of the matcher's negation when negation
is true
.
For convenience, we allow the description string to be empty (""
), in which
case gMock will use the sequence of words in the matcher name as the
description.
For example:
MATCHER(IsDivisibleBy7, "") { return (arg % 7) == 0; }
allows you to write
// Expects mock_foo.Bar(n) to be called where n is divisible by 7.
EXPECT_CALL(mock_foo, Bar(IsDivisibleBy7()));
or,
using ::testing::Not;
...
// Verifies that two values are divisible by 7.
EXPECT_THAT(some_expression, IsDivisibleBy7());
EXPECT_THAT(some_other_expression, Not(IsDivisibleBy7()));
If the above assertions fail, they will print something like:
Value of: some_expression
Expected: is divisible by 7
Actual: 27
...
Value of: some_other_expression
Expected: not (is divisible by 7)
Actual: 21
where the descriptions "is divisible by 7"
and "not (is divisible by 7)"
are
automatically calculated from the matcher name IsDivisibleBy7
.
As you may have noticed, the auto-generated descriptions (especially those for
the negation) may not be so great. You can always override them with a string
expression of your own:
MATCHER(IsDivisibleBy7,
absl::StrCat(negation ? "isn't" : "is", " divisible by 7")) {
return (arg % 7) == 0;
}
Optionally, you can stream additional information to a hidden argument named
result_listener
to explain the match result. For example, a better definition
of IsDivisibleBy7
is:
MATCHER(IsDivisibleBy7, "") {
if ((arg % 7) == 0)
return true;
*result_listener << "the remainder is " << (arg % 7);
return false;
}
With this definition, the above assertion will give a better message:
Value of: some_expression
Expected: is divisible by 7
Actual: 27 (the remainder is 6)
You should let MatchAndExplain()
print any additional information that can
help a user understand the match result. Note that it should explain why the
match succeeds in case of a success (unless it's obvious) - this is useful when
the matcher is used inside Not()
. There is no need to print the argument value
itself, as gMock already prints it for you.
NOTE: The type of the value being matched (arg_type
) is determined by the
context in which you use the matcher and is supplied to you by the compiler, so
you don't need to worry about declaring it (nor can you). This allows the
matcher to be polymorphic. For example, IsDivisibleBy7()
can be used to match
any type where the value of (arg % 7) == 0
can be implicitly converted to a
bool
. In the Bar(IsDivisibleBy7())
example above, if method Bar()
takes an
int
, arg_type
will be int
; if it takes an unsigned long
, arg_type
will
be unsigned long
; and so on.
Writing New Parameterized Matchers Quickly
Sometimes you'll want to define a matcher that has parameters. For that you can use the macro:
MATCHER_P(name, param_name, description_string) { statements; }
where the description string can be either ""
or a string
expression that
references negation
and param_name
.
For example:
MATCHER_P(HasAbsoluteValue, value, "") { return abs(arg) == value; }
will allow you to write:
EXPECT_THAT(Blah("a"), HasAbsoluteValue(n));
which may lead to this message (assuming n
is 10):
Value of: Blah("a")
Expected: has absolute value 10
Actual: -9
Note that both the matcher description and its parameter are printed, making the message human-friendly.
In the matcher definition body, you can write foo_type
to reference the type
of a parameter named foo
. For example, in the body of
MATCHER_P(HasAbsoluteValue, value)
above, you can write value_type
to refer
to the type of value
.
gMock also provides MATCHER_P2
, MATCHER_P3
, ..., up to MATCHER_P10
to
support multi-parameter matchers:
MATCHER_Pk(name, param_1, ..., param_k, description_string) { statements; }
Please note that the custom description string is for a particular instance of the matcher, where the parameters have been bound to actual values. Therefore usually you'll want the parameter values to be part of the description. gMock lets you do that by referencing the matcher parameters in the description string expression.
For example,
using ::testing::PrintToString;
MATCHER_P2(InClosedRange, low, hi,
absl::StrFormat("%s in range [%s, %s]", negation ? "isn't" : "is",
PrintToString(low), PrintToString(hi))) {
return low <= arg && arg <= hi;
}
...
EXPECT_THAT(3, InClosedRange(4, 6));
would generate a failure that contains the message:
Expected: is in range [4, 6]
If you specify ""
as the description, the failure message will contain the
sequence of words in the matcher name followed by the parameter values printed
as a tuple. For example,
MATCHER_P2(InClosedRange, low, hi, "") { ... }
...
EXPECT_THAT(3, InClosedRange(4, 6));
would generate a failure that contains the text:
Expected: in closed range (4, 6)
For the purpose of typing, you can view
MATCHER_Pk(Foo, p1, ..., pk, description_string) { ... }
as shorthand for
template <typename p1_type, ..., typename pk_type>
FooMatcherPk<p1_type, ..., pk_type>
Foo(p1_type p1, ..., pk_type pk) { ... }
When you write Foo(v1, ..., vk)
, the compiler infers the types of the
parameters v1
, ..., and vk
for you. If you are not happy with the result of
the type inference, you can specify the types by explicitly instantiating the
template, as in Foo<long, bool>(5, false)
. As said earlier, you don't get to
(or need to) specify arg_type
as that's determined by the context in which the
matcher is used.
You can assign the result of expression Foo(p1, ..., pk)
to a variable of type
FooMatcherPk<p1_type, ..., pk_type>
. This can be useful when composing
matchers. Matchers that don't have a parameter or have only one parameter have
special types: you can assign Foo()
to a FooMatcher
-typed variable, and
assign Foo(p)
to a FooMatcherP<p_type>
-typed variable.
While you can instantiate a matcher template with reference types, passing the parameters by pointer usually makes your code more readable. If, however, you still want to pass a parameter by reference, be aware that in the failure message generated by the matcher you will see the value of the referenced object but not its address.
You can overload matchers with different numbers of parameters:
MATCHER_P(Blah, a, description_string_1) { ... }
MATCHER_P2(Blah, a, b, description_string_2) { ... }
While it's tempting to always use the MATCHER*
macros when defining a new
matcher, you should also consider implementing MatcherInterface
or using
MakePolymorphicMatcher()
instead (see the recipes that follow), especially if
you need to use the matcher a lot. While these approaches require more work,
they give you more control on the types of the value being matched and the
matcher parameters, which in general leads to better compiler error messages
that pay off in the long run. They also allow overloading matchers based on
parameter types (as opposed to just based on the number of parameters).
Writing New Monomorphic Matchers
A matcher of argument type T
implements ::testing::MatcherInterface<T>
and
does two things: it tests whether a value of type T
matches the matcher, and
can describe what kind of values it matches. The latter ability is used for
generating readable error messages when expectations are violated.
The interface looks like this:
class MatchResultListener {
public:
...
// Streams x to the underlying ostream; does nothing if the ostream
// is NULL.
template <typename T>
MatchResultListener& operator<<(const T& x);
// Returns the underlying ostream.
::std::ostream* stream();
};
template <typename T>
class MatcherInterface {
public:
virtual ~MatcherInterface();
// Returns true if the matcher matches x; also explains the match
// result to 'listener'.
virtual bool MatchAndExplain(T x, MatchResultListener* listener) const = 0;
// Describes this matcher to an ostream.
virtual void DescribeTo(::std::ostream* os) const = 0;
// Describes the negation of this matcher to an ostream.
virtual void DescribeNegationTo(::std::ostream* os) const;
};
If you need a custom matcher but Truly()
is not a good option (for example,
you may not be happy with the way Truly(predicate)
describes itself, or you
may want your matcher to be polymorphic as Eq(value)
is), you can define a
matcher to do whatever you want in two steps: first implement the matcher
interface, and then define a factory function to create a matcher instance. The
second step is not strictly needed but it makes the syntax of using the matcher
nicer.
For example, you can define a matcher to test whether an int
is divisible by 7
and then use it like this:
using ::testing::MakeMatcher;
using ::testing::Matcher;
using ::testing::MatcherInterface;
using ::testing::MatchResultListener;
class DivisibleBy7Matcher : public MatcherInterface<int> {
public:
bool MatchAndExplain(int n,
MatchResultListener* /* listener */) const override {
return (n % 7) == 0;
}
void DescribeTo(::std::ostream* os) const override {
*os << "is divisible by 7";
}
void DescribeNegationTo(::std::ostream* os) const override {
*os << "is not divisible by 7";
}
};
Matcher<int> DivisibleBy7() {
return MakeMatcher(new DivisibleBy7Matcher);
}
...
EXPECT_CALL(foo, Bar(DivisibleBy7()));
You may improve the matcher message by streaming additional information to the
listener
argument in MatchAndExplain()
:
class DivisibleBy7Matcher : public MatcherInterface<int> {
public:
bool MatchAndExplain(int n,
MatchResultListener* listener) const override {
const int remainder = n % 7;
if (remainder != 0) {
*listener << "the remainder is " << remainder;
}
return remainder == 0;
}
...
};
Then, EXPECT_THAT(x, DivisibleBy7());
may generate a message like this:
Value of: x
Expected: is divisible by 7
Actual: 23 (the remainder is 2)
Writing New Polymorphic Matchers
You've learned how to write your own matchers in the previous recipe. Just one
problem: a matcher created using MakeMatcher()
only works for one particular
type of arguments. If you want a polymorphic matcher that works with arguments
of several types (for instance, Eq(x)
can be used to match a value
as long
as value == x
compiles -- value
and x
don't have to share the same
type), you can learn the trick from testing/base/public/gmock-matchers.h
but
it's a bit involved.
Fortunately, most of the time you can define a polymorphic matcher easily with
the help of MakePolymorphicMatcher()
. Here's how you can define NotNull()
as
an example:
using ::testing::MakePolymorphicMatcher;
using ::testing::MatchResultListener;
using ::testing::PolymorphicMatcher;
class NotNullMatcher {
public:
// To implement a polymorphic matcher, first define a COPYABLE class
// that has three members MatchAndExplain(), DescribeTo(), and
// DescribeNegationTo(), like the following.
// In this example, we want to use NotNull() with any pointer, so
// MatchAndExplain() accepts a pointer of any type as its first argument.
// In general, you can define MatchAndExplain() as an ordinary method or
// a method template, or even overload it.
template <typename T>
bool MatchAndExplain(T* p,
MatchResultListener* /* listener */) const {
return p != NULL;
}
// Describes the property of a value matching this matcher.
void DescribeTo(::std::ostream* os) const { *os << "is not NULL"; }
// Describes the property of a value NOT matching this matcher.
void DescribeNegationTo(::std::ostream* os) const { *os << "is NULL"; }
};
// To construct a polymorphic matcher, pass an instance of the class
// to MakePolymorphicMatcher(). Note the return type.
PolymorphicMatcher<NotNullMatcher> NotNull() {
return MakePolymorphicMatcher(NotNullMatcher());
}
...
EXPECT_CALL(foo, Bar(NotNull())); // The argument must be a non-NULL pointer.
Note: Your polymorphic matcher class does not need to inherit from
MatcherInterface
or any other class, and its methods do not need to be
virtual.
Like in a monomorphic matcher, you may explain the match result by streaming
additional information to the listener
argument in MatchAndExplain()
.
Writing New Cardinalities
A cardinality is used in Times()
to tell gMock how many times you expect a
call to occur. It doesn't have to be exact. For example, you can say
AtLeast(5)
or Between(2, 4)
.
If the built-in set of cardinalities doesn't
suit you, you are free to define your own by implementing the following
interface (in namespace testing
):
class CardinalityInterface {
public:
virtual ~CardinalityInterface();
// Returns true if call_count calls will satisfy this cardinality.
virtual bool IsSatisfiedByCallCount(int call_count) const = 0;
// Returns true if call_count calls will saturate this cardinality.
virtual bool IsSaturatedByCallCount(int call_count) const = 0;
// Describes self to an ostream.
virtual void DescribeTo(::std::ostream* os) const = 0;
};
For example, to specify that a call must occur even number of times, you can write
using ::testing::Cardinality;
using ::testing::CardinalityInterface;
using ::testing::MakeCardinality;
class EvenNumberCardinality : public CardinalityInterface {
public:
bool IsSatisfiedByCallCount(int call_count) const override {
return (call_count % 2) == 0;
}
bool IsSaturatedByCallCount(int call_count) const override {
return false;
}
void DescribeTo(::std::ostream* os) const {
*os << "called even number of times";
}
};
Cardinality EvenNumber() {
return MakeCardinality(new EvenNumberCardinality);
}
...
EXPECT_CALL(foo, Bar(3))
.Times(EvenNumber());
Writing New Actions Quickly
If the built-in actions don't work for you, you can easily define your own one. Just define a functor class with a (possibly templated) call operator, matching the signature of your action.
struct Increment {
template <typename T>
T operator()(T* arg) {
return ++(*arg);
}
}
The same approach works with stateful functors (or any callable, really):
struct MultiplyBy {
template <typename T>
T operator()(T arg) { return arg * multiplier; }
int multiplier;
}
// Then use:
// EXPECT_CALL(...).WillOnce(MultiplyBy{7});
Legacy macro-based Actions
Before C++11, the functor-based actions were not supported; the old way of
writing actions was through a set of ACTION*
macros. We suggest to avoid them
in new code; they hide a lot of logic behind the macro, potentially leading to
harder-to-understand compiler errors. Nevertheless, we cover them here for
completeness.
By writing
ACTION(name) { statements; }
in a namespace scope (i.e. not inside a class or function), you will define an
action with the given name that executes the statements. The value returned by
statements
will be used as the return value of the action. Inside the
statements, you can refer to the K-th (0-based) argument of the mock function as
argK
. For example:
ACTION(IncrementArg1) { return ++(*arg1); }
allows you to write
... WillOnce(IncrementArg1());
Note that you don't need to specify the types of the mock function arguments.
Rest assured that your code is type-safe though: you'll get a compiler error if
*arg1
doesn't support the ++
operator, or if the type of ++(*arg1)
isn't
compatible with the mock function's return type.
Another example:
ACTION(Foo) {
(*arg2)(5);
Blah();
*arg1 = 0;
return arg0;
}
defines an action Foo()
that invokes argument #2 (a function pointer) with 5,
calls function Blah()
, sets the value pointed to by argument #1 to 0, and
returns argument #0.
For more convenience and flexibility, you can also use the following pre-defined
symbols in the body of ACTION
:
argK_type |
The type of the K-th (0-based) argument of the mock function |
---|---|
args |
All arguments of the mock function as a tuple |
args_type |
The type of all arguments of the mock function as a tuple |
return_type |
The return type of the mock function |
function_type |
The type of the mock function |
For example, when using an ACTION
as a stub action for mock function:
int DoSomething(bool flag, int* ptr);
we have:
Pre-defined Symbol | Is Bound To |
---|---|
arg0 |
the value of flag |
arg0_type |
the type bool |
arg1 |
the value of ptr |
arg1_type |
the type int* |
args |
the tuple (flag, ptr) |
args_type |
the type std::tuple<bool, int*> |
return_type |
the type int |
function_type |
the type int(bool, int*) |
Legacy macro-based parameterized Actions
Sometimes you'll want to parameterize an action you define. For that we have another macro
ACTION_P(name, param) { statements; }
For example,
ACTION_P(Add, n) { return arg0 + n; }
will allow you to write
// Returns argument #0 + 5.
... WillOnce(Add(5));
For convenience, we use the term arguments for the values used to invoke the mock function, and the term parameters for the values used to instantiate an action.
Note that you don't need to provide the type of the parameter either. Suppose
the parameter is named param
, you can also use the gMock-defined symbol
param_type
to refer to the type of the parameter as inferred by the compiler.
For example, in the body of ACTION_P(Add, n)
above, you can write n_type
for
the type of n
.
gMock also provides ACTION_P2
, ACTION_P3
, and etc to support multi-parameter
actions. For example,
ACTION_P2(ReturnDistanceTo, x, y) {
double dx = arg0 - x;
double dy = arg1 - y;
return sqrt(dx*dx + dy*dy);
}
lets you write
... WillOnce(ReturnDistanceTo(5.0, 26.5));
You can view ACTION
as a degenerated parameterized action where the number of
parameters is 0.
You can also easily define actions overloaded on the number of parameters:
ACTION_P(Plus, a) { ... }
ACTION_P2(Plus, a, b) { ... }
Restricting the Type of an Argument or Parameter in an ACTION
For maximum brevity and reusability, the ACTION*
macros don't ask you to
provide the types of the mock function arguments and the action parameters.
Instead, we let the compiler infer the types for us.
Sometimes, however, we may want to be more explicit about the types. There are several tricks to do that. For example:
ACTION(Foo) {
// Makes sure arg0 can be converted to int.
int n = arg0;
... use n instead of arg0 here ...
}
ACTION_P(Bar, param) {
// Makes sure the type of arg1 is const char*.
::testing::StaticAssertTypeEq<const char*, arg1_type>();
// Makes sure param can be converted to bool.
bool flag = param;
}
where StaticAssertTypeEq
is a compile-time assertion in googletest that
verifies two types are the same.
Writing New Action Templates Quickly
Sometimes you want to give an action explicit template parameters that cannot be
inferred from its value parameters. ACTION_TEMPLATE()
supports that and can be
viewed as an extension to ACTION()
and ACTION_P*()
.
The syntax:
ACTION_TEMPLATE(ActionName,
HAS_m_TEMPLATE_PARAMS(kind1, name1, ..., kind_m, name_m),
AND_n_VALUE_PARAMS(p1, ..., p_n)) { statements; }
defines an action template that takes m explicit template parameters and n
value parameters, where m is in [1, 10] and n is in [0, 10]. name_i
is the
name of the i-th template parameter, and kind_i
specifies whether it's a
typename
, an integral constant, or a template. p_i
is the name of the i-th
value parameter.
Example:
// DuplicateArg<k, T>(output) converts the k-th argument of the mock
// function to type T and copies it to *output.
ACTION_TEMPLATE(DuplicateArg,
// Note the comma between int and k:
HAS_2_TEMPLATE_PARAMS(int, k, typename, T),
AND_1_VALUE_PARAMS(output)) {
*output = T(::std::get<k>(args));
}
To create an instance of an action template, write:
ActionName<t1, ..., t_m>(v1, ..., v_n)
where the t
s are the template arguments and the v
s are the value arguments.
The value argument types are inferred by the compiler. For example:
using ::testing::_;
...
int n;
EXPECT_CALL(mock, Foo).WillOnce(DuplicateArg<1, unsigned char>(&n));
If you want to explicitly specify the value argument types, you can provide additional template arguments:
ActionName<t1, ..., t_m, u1, ..., u_k>(v1, ..., v_n)
where u_i
is the desired type of v_i
.
ACTION_TEMPLATE
and ACTION
/ACTION_P*
can be overloaded on the number of
value parameters, but not on the number of template parameters. Without the
restriction, the meaning of the following is unclear:
OverloadedAction<int, bool>(x);
Are we using a single-template-parameter action where bool
refers to the type
of x
, or a two-template-parameter action where the compiler is asked to infer
the type of x
?
Using the ACTION Object's Type
If you are writing a function that returns an ACTION
object, you'll need to
know its type. The type depends on the macro used to define the action and the
parameter types. The rule is relatively simple:
Given Definition | Expression | Has Type |
---|---|---|
ACTION(Foo) |
Foo() |
FooAction |
ACTION_TEMPLATE(Foo, |
`Foo<t1, ..., | `FooAction<t1, ..., |
- :
HAS_m_TEMPLATE_PARAMS(...),
: t_m>(): t_m>
: AND_0_VALUE_PARAMS())
: : : |ACTION_P(Bar, param)
|Bar(int_value)
|BarActionP<int>
| |ACTION_TEMPLATE(Bar,
|Bar<t1, ..., t_m>
| `FooActionP<t1, ..., |HAS_m_TEMPLATE_PARAMS(...),
:(int_value)
: t_m, int>` :AND_1_VALUE_PARAMS(p1))
: : : |ACTION_P2(Baz, p1, p2)
|Baz(bool_value,
| `BazActionP2<bool, |: `int_value)` : int>` :
- |
ACTION_TEMPLATE(Baz,
|Baz<t1, ..., t_m>
| `FooActionP2<t1, ..., | HAS_m_TEMPLATE_PARAMS(...),
:(bool_value,
: t_m,AND_2_VALUE_PARAMS(p1, p2))
:int_value)
: : | ... | ... | ... |
Note that we have to pick different suffixes (Action
, ActionP
, ActionP2
,
and etc) for actions with different numbers of value parameters, or the action
definitions cannot be overloaded on the number of them.
Writing New Monomorphic Actions
While the ACTION*
macros are very convenient, sometimes they are
inappropriate. For example, despite the tricks shown in the previous recipes,
they don't let you directly specify the types of the mock function arguments and
the action parameters, which in general leads to unoptimized compiler error
messages that can baffle unfamiliar users. They also don't allow overloading
actions based on parameter types without jumping through some hoops.
An alternative to the ACTION*
macros is to implement
::testing::ActionInterface<F>
, where F
is the type of the mock function in
which the action will be used. For example:
template <typename F>
class ActionInterface {
public:
virtual ~ActionInterface();
// Performs the action. Result is the return type of function type
// F, and ArgumentTuple is the tuple of arguments of F.
//
// For example, if F is int(bool, const string&), then Result would
// be int, and ArgumentTuple would be ::std::tuple<bool, const string&>.
virtual Result Perform(const ArgumentTuple& args) = 0;
};
using ::testing::_;
using ::testing::Action;
using ::testing::ActionInterface;
using ::testing::MakeAction;
typedef int IncrementMethod(int*);
class IncrementArgumentAction : public ActionInterface<IncrementMethod> {
public:
int Perform(const ::std::tuple<int*>& args) override {
int* p = ::std::get<0>(args); // Grabs the first argument.
return *p++;
}
};
Action<IncrementMethod> IncrementArgument() {
return MakeAction(new IncrementArgumentAction);
}
...
EXPECT_CALL(foo, Baz(_))
.WillOnce(IncrementArgument());
int n = 5;
foo.Baz(&n); // Should return 5 and change n to 6.
Writing New Polymorphic Actions
The previous recipe showed you how to define your own action. This is all good,
except that you need to know the type of the function in which the action will
be used. Sometimes that can be a problem. For example, if you want to use the
action in functions with different types (e.g. like Return()
and
SetArgPointee()
).
If an action can be used in several types of mock functions, we say it's
polymorphic. The MakePolymorphicAction()
function template makes it easy to
define such an action:
namespace testing {
template <typename Impl>
PolymorphicAction<Impl> MakePolymorphicAction(const Impl& impl);
} // namespace testing
As an example, let's define an action that returns the second argument in the mock function's argument list. The first step is to define an implementation class:
class ReturnSecondArgumentAction {
public:
template <typename Result, typename ArgumentTuple>
Result Perform(const ArgumentTuple& args) const {
// To get the i-th (0-based) argument, use ::std::get(args).
return ::std::get<1>(args);
}
};
This implementation class does not need to inherit from any particular class.
What matters is that it must have a Perform()
method template. This method
template takes the mock function's arguments as a tuple in a single
argument, and returns the result of the action. It can be either const
or not,
but must be invokable with exactly one template argument, which is the result
type. In other words, you must be able to call Perform<R>(args)
where R
is
the mock function's return type and args
is its arguments in a tuple.
Next, we use MakePolymorphicAction()
to turn an instance of the implementation
class into the polymorphic action we need. It will be convenient to have a
wrapper for this:
using ::testing::MakePolymorphicAction;
using ::testing::PolymorphicAction;
PolymorphicAction<ReturnSecondArgumentAction> ReturnSecondArgument() {
return MakePolymorphicAction(ReturnSecondArgumentAction());
}
Now, you can use this polymorphic action the same way you use the built-in ones:
using ::testing::_;
class MockFoo : public Foo {
public:
MOCK_METHOD(int, DoThis, (bool flag, int n), (override));
MOCK_METHOD(string, DoThat, (int x, const char* str1, const char* str2),
(override));
};
...
MockFoo foo;
EXPECT_CALL(foo, DoThis).WillOnce(ReturnSecondArgument());
EXPECT_CALL(foo, DoThat).WillOnce(ReturnSecondArgument());
...
foo.DoThis(true, 5); // Will return 5.
foo.DoThat(1, "Hi", "Bye"); // Will return "Hi".
Teaching gMock How to Print Your Values
When an uninteresting or unexpected call occurs, gMock prints the argument
values and the stack trace to help you debug. Assertion macros like
EXPECT_THAT
and EXPECT_EQ
also print the values in question when the
assertion fails. gMock and googletest do this using googletest's user-extensible
value printer.
This printer knows how to print built-in C++ types, native arrays, STL
containers, and any type that supports the <<
operator. For other types, it
prints the raw bytes in the value and hopes that you the user can figure it out.
googletest's advanced guide
explains how to extend the printer to do a better job at printing your
particular type than to dump the bytes.
Useful Mocks Created Using gMock
Mock std::function
std::function
is a general function type introduced in C++11. It is a
preferred way of passing callbacks to new interfaces. Functions are copiable,
and are not usually passed around by pointer, which makes them tricky to mock.
But fear not - MockFunction
can help you with that.
MockFunction<R(T1, ..., Tn)>
has a mock method Call()
with the signature:
R Call(T1, ..., Tn);
It also has a AsStdFunction()
method, which creates a std::function
proxy
forwarding to Call:
std::function<R(T1, ..., Tn)> AsStdFunction();
To use MockFunction
, first create MockFunction
object and set up
expectations on its Call
method. Then pass proxy obtained from
AsStdFunction()
to the code you are testing. For example:
TEST(FooTest, RunsCallbackWithBarArgument) {
// 1. Create a mock object.
MockFunction<int(string)> mock_function;
// 2. Set expectations on Call() method.
EXPECT_CALL(mock_function, Call("bar")).WillOnce(Return(1));
// 3. Exercise code that uses std::function.
Foo(mock_function.AsStdFunction());
// Foo's signature can be either of:
// void Foo(const std::function<int(string)>& fun);
// void Foo(std::function<int(string)> fun);
// 4. All expectations will be verified when mock_function
// goes out of scope and is destroyed.
}
Remember that function objects created with AsStdFunction()
are just
forwarders. If you create multiple of them, they will share the same set of
expectations.
Although std::function
supports unlimited number of arguments, MockFunction
implementation is limited to ten. If you ever hit that limit... well, your
callback has bigger problems than being mockable. :-)