* However, this may also mean there's a bug in the test, so Google Mock shouldn't be silent either. If the user believes these calls are harmless, they can add an `EXPECT_CALL()` to suppress the warning.
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.
}
```
There are some caveats though (I don't like them just as much as the
next guy, but sadly they are side effects of C++'s limitations):
1.`NiceMock<MockFoo>` and `StrictMock<MockFoo>` only work for mock methods defined using the `MOCK_METHOD*` family of macros **directly** in the `MockFoo` class. If a mock method is defined in a **base class** of `MockFoo`, the "nice" or "strict" modifier may not affect it, depending on the compiler. In particular, nesting `NiceMock` and `StrictMock` (e.g. `NiceMock<StrictMock<MockFoo> >`) is **not** supported.
1. The constructors of the base mock (`MockFoo`) cannot have arguments passed by non-const reference, which happens to be banned by the [Google C++ style guide](https://google.github.io/styleguide/cppguide.html).
1. 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 on `this` object. (This behavior, however, is consistent with C++'s general rule: if a constructor or destructor calls a virtual method of `this` 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 should'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
By defining a new mock method with a trimmed argument list, we make
the mock class much more user-friendly.
## 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 of `Concrete`, 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 near `Concrete` (perhaps in a `contrib` 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:
virtual char DoThis(int n) {
return (n > 0) ? '+' :
(n <0)?'-':'0';
}
virtual void DoThat(const char* s, int* p) {
*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 Google Mock, you can have it
delegate its default action to a fake class you already have, using
this pattern:
```
using ::testing::_;
using ::testing::Invoke;
class MockFoo : public Foo {
public:
// Normal mock method definitions using Google Mock.
// 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(Invoke(&fake_, &FakeFoo::DoThis));
ON_CALL(*this, DoThat(_, _))
.WillByDefault(Invoke(&fake_, &FakeFoo::DoThat));
}
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()` in `EXPECT_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()`), see the "Selecting Between Overloaded Functions" section on this page; to disambiguate a fake function (the one you place inside `Invoke()`), use a `static_cast` to specify the function's type. For instance, if class `Foo` has methods `char DoThis(int n)` and `bool DoThis(double x) const`, and you want to invoke the latter, you need to write `Invoke(&fake_, static_cast<bool (FakeFoo::*)(double) const>(&FakeFoo::DoThis))` instead of `Invoke(&fake_, &FakeFoo::DoThis)` (The strange-looking thing inside the angled brackets of `static_cast` is the type of a function pointer to the second `DoThis()` 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::_;
using ::testing::AtLeast;
using ::testing::Invoke;
class MockFoo : public Foo {
public:
MockFoo() {
// By default, all calls are delegated to the real object.
ON_CALL(*this, DoThis())
.WillByDefault(Invoke(&real_, &Foo::DoThis));
ON_CALL(*this, DoThat(_))
.WillByDefault(Invoke(&real_, &Foo::DoThat));
...
}
MOCK_METHOD0(DoThis, ...);
MOCK_METHOD1(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, Google Mock 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_METHOD1(Pure, void(int n));
// Mocking a concrete method. Foo::Concrete() is shadowed.
MOCK_METHOD1(Concrete, int(const char* str));
};
```
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_METHOD1(Pure, void(int n));
// Mocking a concrete method. Foo::Concrete() is shadowed.
MOCK_METHOD1(Concrete, int(const char* str));
// 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:
```
using ::testing::_;
using ::testing::Invoke;
...
EXPECT_CALL(foo, Concrete(_))
.WillOnce(Invoke(&foo, &MockFoo::FooConcrete));
```
or tell the mock object that you don't want to mock `Concrete()`:
(Why don't we just write `Invoke(&foo, &Foo::Concrete)`? 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::Ge;
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:
```
using ::testing::_;
using ::testing::NotNull;
...
EXPECT_CALL(foo, DoThat(_, NotNull()));
```
## Combining Matchers ##
You can build complex matchers from existing ones using `AllOf()`,
`AnyOf()`, 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 ##
Google Mock 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` before giving it to the
matcher.
To support this need, Google Mock gives you the
`SafeMatcherCast<T>(m)` function. It casts a matcher `m` to type
`Matcher<T>`. To ensure safety, Google Mock checks that (let `U` be the
type `m` accepts):
1. Type `T` can be implicitly cast to type `U`;
1. When both `T` and `U` are built-in arithmetic types (`bool`, integers, and floating-point numbers), the conversion from `T` to `U` is not lossy (in other words, any value representable by `T` can also be represented by `U`); and
1. When `U` is a reference, `T` must also be a reference (as the underlying matcher may be interested in the address of the `U` value).
*`ElementsAre*()` can be used to match _any_ container that implements the STL iterator pattern (i.e. it has a `const_iterator` type and supports `begin()/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*()`. Therefore don't use it with containers whose element order is undefined (e.g. `hash_map`).
## Sharing Matchers ##
Under the hood, a Google Mock 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,
```
Matcher<int> in_range = AllOf(Gt(5), Le(10));
... use in_range as a matcher in multiple EXPECT_CALLs ...
```
# Setting Expectations #
## Knowing When to Expect ##
`ON_CALL` is likely the single most under-utilized construct in Google Mock.
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,
Google Mock will perform its default action to allow the test program
to continue. If you are not happy with the default action taken by
Google Mock, you can override it using `DefaultValue<T>::Set()`
(described later in this document) 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 Google Mock. _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. (Google Mock'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 Google Mock, `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.
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 Google Mock 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.
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_CALLs` is important, as a newer `EXPECT_CALL` takes precedence over an older one.
## 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 Google Mock doesn't know
about. You can do this using the `::testing::DefaultValue` class
template:
```
class MockFoo : public Foo {
public:
MOCK_METHOD0(CalculateBar, Bar());
};
...
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 news 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.
## Using Functions/Methods/Functors as Actions ##
If the built-in actions don't suit you, you can easily use an existing
* The inner action used in `WithArgs` and friends does not have to be `Invoke()` -- 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` and `my_action` takes a `double`, `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, method, or functor 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,
... 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
exihibit 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
```
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.
```
# Misc Recipes on Using Google Mock #
## Mocking Methods That Use Move-Only Types ##
C++11 introduced <em>move-only types</em>. 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.
Let’s say we are working on a fictional project that lets one post and share snippets called “buzzes”. Your code uses these types:
virtual bool ShareBuzz(std::unique_ptr<Buzz> buzz, Time timestamp) = 0;
...
};
```
A `Buzz` object represents a snippet being posted. A class that implements the `Buzzer` interface is capable of creating and sharing `Buzz`. 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 returns a move-only type, you just use the familiar `MOCK_METHOD` syntax as usual:
However, if you attempt to use the same `MOCK_METHOD` pattern to mock a method that takes a move-only parameter, you’ll get a compiler error currently:
```
// Does NOT compile!
MOCK_METHOD2(ShareBuzz, bool(std::unique_ptr<Buzz> buzz, Time timestamp));
```
While it’s highly desirable to make this syntax just work, it’s not trivial and the work hasn’t been done yet. Fortunately, there is a trick you can apply today to get something that works nearly as well as this.
The trick, is to delegate the `ShareBuzz()` method to a mock method (let’s call it `DoShareBuzz()`) that does not take move-only parameters:
MOCK_METHOD2(DoShareBuzz, bool(Buzz* buzz, Time timestamp));
bool ShareBuzz(std::unique_ptr<Buzz> buzz, Time timestamp) {
return DoShareBuzz(buzz.get(), timestamp);
}
};
```
Note that there's no need to define or declare `DoShareBuzz()` in a base class. You only need to define it as a `MOCK_METHOD` in the mock class.
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 `.WillRepeated()` 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:
If you are not happy with the default action, you can tweak it. Depending on what you need, you may either tweak the default action for a specific (mock object, mock method) combination using `ON_CALL()`, or you may tweak the default action for all mock methods that return a specific type. The usage of `ON_CALL()` is similar to `EXPECT_CALL()`, so we’ll skip it and just explain how to do the latter (tweaking the default action for a specific return type). You do this via the `DefaultValue<>::SetFactory()` and `DefaultValue<>::Clear()` API:
```
// Sets the default action for return type std::unique_ptr<Buzz> to
// Resets the default action for return type std::unique_ptr<Buzz>,
// to avoid interfere with other tests.
DefaultValue<std::unique_ptr<Buzz>>::Clear();
```
What if you want the method to do something other than the default action? 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
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 `Invoke()` to call a lambda or a callable object, which can do pretty much anything you want:
Every time this `EXPECT_CALL` fires, a new `unique_ptr<Buzz>` will be created and returned. You cannot do this with `Return(ByMove(...))`.
Now there’s one topic we haven’t covered: how do you set expectations on `ShareBuzz()`, which takes a move-only-typed parameter? The answer is you don’t. Instead, you set expectations on the `DoShareBuzz()` mock method (remember that we defined a `MOCK_METHOD` for `DoShareBuzz()`, not `ShareBuzz()`):
Some of you may have spotted one problem with this approach: the `DoShareBuzz()` mock method differs from the real `ShareBuzz()` method in that it cannot take ownership of the buzz parameter - `ShareBuzz()` will always delete buzz after `DoShareBuzz()` returns. What if you need to save the buzz object somewhere for later use when `ShareBuzz()` is called? Indeed, you'd be stuck.
Another problem with the `DoShareBuzz()` we had is that it can surprise people reading or maintaining the test, as one would expect that `DoShareBuzz()` has (logically) the same contract as `ShareBuzz()`.
Fortunately, these problems can be fixed with a bit more code. Let's try to get it right this time:
```
class MockBuzzer : public Buzzer {
public:
MockBuzzer() {
// Since DoShareBuzz(buzz, time) is supposed to take ownership of
// buzz, define a default behavior for DoShareBuzz(buzz, time) to
// delete buzz.
ON_CALL(*this, DoShareBuzz(_, _))
.WillByDefault(Invoke([](Buzz* buzz, Time timestamp) {
Using the tricks covered in this recipe, you are now able to mock methods that take and/or return move-only types. Put your newly-acquired power to good use - when you design a new API, you can now feel comfortable using `unique_ptrs` as appropriate, without fearing that doing so will compromise your tests.
## 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 `.cpp` 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_METHOD0(DoThis, int());
MOCK_METHOD1(DoThat, bool(const char* str));
... 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_METHOD0(DoThis, int());
MOCK_METHOD1(DoThat, bool(const char* str));
... more mock methods ...
};
```
and
```
// File mock_foo.cpp.
#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
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 may not be 100% reliable. So, sometimes you do want
to _force_ Google Mock 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(stringcheck_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_METHOD0` macro doesn't work for it:
```
MOCK_METHOD0(~MockFoo, void()); // 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_METHOD0(Die, void());
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 Google Mock and Threads ##
**IMPORTANT NOTE:** What we describe in this recipe is **ONLY** true on
platforms where Google Mock is thread-safe. Currently these are only
platforms that support the pthreads library (this includes Linux and Mac).
To make it thread-safe on other platforms we only need to implement
some synchronization operations in `"gtest/internal/gtest-port.h"`.
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 many programs are multi-threaded, and sometimes to test something
we need to pound on it from more than one thread. Google Mock works
for this purpose too.
Remember the steps for using a mock:
1. Create a mock object `foo`.
1. Set its default actions and expectations using `ON_CALL()` and `EXPECT_CALL()`.
1. The code under test calls methods of `foo`.
1. Optionally, verify and reset the mock.
1. 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. Google Mock 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.
Google Mock 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,
Google Mock will execute `action1` in thread 1 and `action2` in thread
2.
Google Mock 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 Google Mock Prints ##
When Google Mock 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 and the return value. 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 and the return value). Clearly, one size doesn't fit
all.
You can control how much Google Mock tells you using the
`--gmock_verbose=LEVEL` command-line flag, where `LEVEL` is a string
with three possible values:
*`info`: Google Mock will print all informational messages, warnings, and errors (most verbose). At this setting, Google Mock will also log any calls to the `ON_CALL/EXPECT_CALL` macros.
*`warning`: Google Mock will print both warnings and errors (less verbose). This is the default.
*`error`: Google Mock 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";
```
Now, judiciously use the right flag to enable Google Mock serve you better!
## Gaining Super Vision into Mock Calls ##
You have a test using Google Mock. It fails: Google Mock tells you
that 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` Google Mock thinks it matches?
You can unlock this power by running your test with the
`--gmock_verbose=info` flag. For example, given the test program:
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
Google Mock already prints it for you.
**Notes:**
1. 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.
1. Google Mock doesn't guarantee when or how many times a matcher will be invoked. Therefore the matcher logic must be _purely functional_ (i.e. it cannot have any side effect, and the result must not depend on anything other than the value being matched and the matcher parameters). This requirement must be satisfied no matter how you define the matcher (e.g. using one of the methods described in the following recipes). In particular, a matcher can never call a mock function, as that will affect the state of the mock object and Google Mock.
## Writing New Parameterized Matchers Quickly ##
Sometimes you'll want to define a matcher that has parameters. For that you
