gMock Cheat Sheet

Defining a Mock Class

Mocking a Normal Class

Given

class Foo {
  ...
  virtual ~Foo();
  virtual int GetSize() const = 0;
  virtual string Describe(const char* name) = 0;
  virtual string Describe(int type) = 0;
  virtual bool Process(Bar elem, int count) = 0;
};

(note that ~Foo() must be virtual) we can define its mock as

#include "gmock/gmock.h"

class MockFoo : public Foo {
  ...
  MOCK_METHOD(int, GetSize, (), (const, override));
  MOCK_METHOD(string, Describe, (const char* name), (override));
  MOCK_METHOD(string, Describe, (int type), (override));
  MOCK_METHOD(bool, Process, (Bar elem, int count), (override));
};

To create a “nice” mock, which ignores all uninteresting calls, a “naggy” mock, which warns on all uninteresting calls, or a “strict” mock, which treats them as failures:

using ::testing::NiceMock;
using ::testing::NaggyMock;
using ::testing::StrictMock;

NiceMock<MockFoo> nice_foo;      // The type is a subclass of MockFoo.
NaggyMock<MockFoo> naggy_foo;    // The type is a subclass of MockFoo.
StrictMock<MockFoo> strict_foo;  // The type is a subclass of MockFoo.

{: .callout .note} Note: A mock object is currently naggy by default. We may make it nice by default in the future.

Mocking a Class Template

Class templates can be mocked just like any class.

To mock

template <typename Elem>
class StackInterface {
  ...
  virtual ~StackInterface();
  virtual int GetSize() const = 0;
  virtual void Push(const Elem& x) = 0;
};

(note that all member functions that are mocked, including ~StackInterface() must be virtual).

template <typename Elem>
class MockStack : public StackInterface<Elem> {
  ...
  MOCK_METHOD(int, GetSize, (), (const, override));
  MOCK_METHOD(void, Push, (const Elem& x), (override));
};

Specifying Calling Conventions for Mock Functions

If your mock function doesn't use the default calling convention, you can specify it by adding Calltype(convention) to MOCK_METHOD's 4th parameter. For example,

  MOCK_METHOD(bool, Foo, (int n), (Calltype(STDMETHODCALLTYPE)));
  MOCK_METHOD(int, Bar, (double x, double y),
              (const, Calltype(STDMETHODCALLTYPE)));

where STDMETHODCALLTYPE is defined by <objbase.h> on Windows.

Using Mocks in Tests

The typical work flow is:

Import the gMock names you need to use. All gMock symbols are in the testing namespace unless they are macros or otherwise noted.
Create the mock objects.
Optionally, set the default actions of the mock objects.
Set your expectations on the mock objects (How will they be called? What will they do?).
Exercise code that uses the mock objects; if necessary, check the result using googletest assertions.
When a mock object is destructed, gMock automatically verifies that all expectations on it have been satisfied.

Here's an example:

using ::testing::Return;                          // #1

TEST(BarTest, DoesThis) {
  MockFoo foo;                                    // #2

  ON_CALL(foo, GetSize())                         // #3
      .WillByDefault(Return(1));
  // ... other default actions ...

  EXPECT_CALL(foo, Describe(5))                   // #4
      .Times(3)
      .WillRepeatedly(Return("Category 5"));
  // ... other expectations ...

  EXPECT_EQ(MyProductionFunction(&foo), "good");  // #5
}                                                 // #6

Setting Default Actions

gMock has a built-in default action for any function that returns void, bool, a numeric value, or a pointer. In C++11, it will additionally returns the default-constructed value, if one exists for the given type.

To customize the default action for functions with return type T:

using ::testing::DefaultValue;

// Sets the default value to be returned. T must be CopyConstructible.
DefaultValue<T>::Set(value);
// Sets a factory. Will be invoked on demand. T must be MoveConstructible.
//  T MakeT();
DefaultValue<T>::SetFactory(&MakeT);
// ... use the mocks ...
// Resets the default value.
DefaultValue<T>::Clear();

Example usage:

  // Sets the default action for return type std::unique_ptr<Buzz> to
  // creating a new Buzz every time.
  DefaultValue<std::unique_ptr<Buzz>>::SetFactory(
      [] { return MakeUnique<Buzz>(AccessLevel::kInternal); });

  // When this fires, the default action of MakeBuzz() will run, which
  // will return a new Buzz object.
  EXPECT_CALL(mock_buzzer_, MakeBuzz("hello")).Times(AnyNumber());

  auto buzz1 = mock_buzzer_.MakeBuzz("hello");
  auto buzz2 = mock_buzzer_.MakeBuzz("hello");
  EXPECT_NE(buzz1, nullptr);
  EXPECT_NE(buzz2, nullptr);
  EXPECT_NE(buzz1, buzz2);

  // Resets the default action for return type std::unique_ptr<Buzz>,
  // to avoid interfere with other tests.
  DefaultValue<std::unique_ptr<Buzz>>::Clear();

To customize the default action for a particular method of a specific mock object, use ON_CALL(). ON_CALL() has a similar syntax to EXPECT_CALL(), but it is used for setting default behaviors (when you do not require that the mock method is called). See here for a more detailed discussion.

ON_CALL(mock-object, method(matchers))
    .With(multi-argument-matcher)   ?
    .WillByDefault(action);

Setting Expectations

EXPECT_CALL() sets expectations on a mock method (How will it be called? What will it do?):

EXPECT_CALL(mock-object, method (matchers)?)
     .With(multi-argument-matcher)  ?
     .Times(cardinality)            ?
     .InSequence(sequences)         *
     .After(expectations)           *
     .WillOnce(action)              *
     .WillRepeatedly(action)        ?
     .RetiresOnSaturation();        ?

For each item above, ? means it can be used at most once, while * means it can be used any number of times.

In order to pass, EXPECT_CALL must be used before the calls are actually made.

The (matchers) is a comma-separated list of matchers that correspond to each of the arguments of method, and sets the expectation only for calls of method that matches all of the matchers.

If (matchers) is omitted, the expectation is the same as if the matchers were set to anything matchers (for example, (_, _, _, _) for a four-arg method).

If Times() is omitted, the cardinality is assumed to be:

Times(1) when there is neither WillOnce() nor WillRepeatedly();
Times(n) when there are n WillOnce()s but no WillRepeatedly(), where n >= 1; or
Times(AtLeast(n)) when there are n WillOnce()s and a WillRepeatedly(), where n >= 0.

A method with no EXPECT_CALL() is free to be invoked any number of times, and the default action will be taken each time.

Matchers

See the Matchers Reference.

Actions

Actions specify what a mock function should do when invoked.

Returning a Value


`Return()`	Return from a `void` mock function.
`Return(value)`	Return `value`. If the type of `value` is different to the mock function's return type, `value` is converted to the latter type at the time the expectation is set, not when the action is executed.
`ReturnArg<N>()`	Return the `N`-th (0-based) argument.
`ReturnNew<T>(a1, ..., ak)`	Return `new T(a1, ..., ak)`; a different object is created each time.
`ReturnNull()`	Return a null pointer.
`ReturnPointee(ptr)`	Return the value pointed to by `ptr`.
`ReturnRef(variable)`	Return a reference to `variable`.
`ReturnRefOfCopy(value)`	Return a reference to a copy of `value`; the copy lives as long as the action.
`ReturnRoundRobin({a1, ..., ak})`	Each call will return the next `ai` in the list, starting at the beginning when the end of the list is reached.

Side Effects


`Assign(&variable, value)`	Assign `value` to variable.
`DeleteArg<N>()`	Delete the `N`-th (0-based) argument, which must be a pointer.
`SaveArg<N>(pointer)`	Save the `N`-th (0-based) argument to `*pointer`.
`SaveArgPointee<N>(pointer)`	Save the value pointed to by the `N`-th (0-based) argument to `*pointer`.
`SetArgReferee<N>(value)`	Assign `value` to the variable referenced by the `N`-th (0-based) argument.
`SetArgPointee<N>(value)`	Assign `value` to the variable pointed by the `N`-th (0-based) argument.
`SetArgumentPointee<N>(value)`	Same as `SetArgPointee<N>(value)`. Deprecated. Will be removed in v1.7.0.
`SetArrayArgument<N>(first, last)`	Copies the elements in source range [`first`, `last`) to the array pointed to by the `N`-th (0-based) argument, which can be either a pointer or an iterator. The action does not take ownership of the elements in the source range.
`SetErrnoAndReturn(error, value)`	Set `errno` to `error` and return `value`.
`Throw(exception)`	Throws the given exception, which can be any copyable value. Available since v1.1.0.

Using a Function, Functor, or Lambda as an Action

In the following, by “callable” we mean a free function, std::function, functor, or lambda.


`f`	Invoke f with the arguments passed to the mock function, where f is a callable.
`Invoke(f)`	Invoke `f` with the arguments passed to the mock function, where `f` can be a global/static function or a functor.
`Invoke(object_pointer, &class::method)`	Invoke the method on the object with the arguments passed to the mock function.
`InvokeWithoutArgs(f)`	Invoke `f`, which can be a global/static function or a functor. `f` must take no arguments.
`InvokeWithoutArgs(object_pointer, &class::method)`	Invoke the method on the object, which takes no arguments.
`InvokeArgument<N>(arg1, arg2, ..., argk)`	Invoke the mock function's `N`-th (0-based) argument, which must be a function or a functor, with the `k` arguments.

The return value of the invoked function is used as the return value of the action.

When defining a callable to be used with Invoke*(), you can declare any unused parameters as Unused:

using ::testing::Invoke;
double Distance(Unused, double x, double y) { return sqrt(x*x + y*y); }
...
EXPECT_CALL(mock, Foo("Hi", _, _)).WillOnce(Invoke(Distance));

Invoke(callback) and InvokeWithoutArgs(callback) take ownership of callback, which must be permanent. The type of callback must be a base callback type instead of a derived one, e.g.

  BlockingClosure* done = new BlockingClosure;
  ... Invoke(done) ...;  // This won't compile!

  Closure* done2 = new BlockingClosure;
  ... Invoke(done2) ...;  // This works.

In InvokeArgument<N>(...), if an argument needs to be passed by reference, wrap it inside std::ref(). For example,

using ::testing::InvokeArgument;
...
InvokeArgument<2>(5, string("Hi"), std::ref(foo))

calls the mock function's #2 argument, passing to it 5 and string("Hi") by value, and foo by reference.

Default Action

Matcher	Description
`DoDefault()`	Do the default action (specified by `ON_CALL()` or the built-in one).

{: .callout .note} Note: due to technical reasons, DoDefault() cannot be used inside a composite action - trying to do so will result in a run-time error.

Composite Actions


`DoAll(a1, a2, ..., an)`	Do all actions `a1` to `an` and return the result of `an` in each invocation. The first `n - 1` sub-actions must return void and will receive a readonly view of the arguments.
`IgnoreResult(a)`	Perform action `a` and ignore its result. `a` must not return void.
`WithArg<N>(a)`	Pass the `N`-th (0-based) argument of the mock function to action `a` and perform it.
`WithArgs<N1, N2, ..., Nk>(a)`	Pass the selected (0-based) arguments of the mock function to action `a` and perform it.
`WithoutArgs(a)`	Perform action `a` without any arguments.

Defining Actions


`ACTION(Sum) { return arg0 + arg1; }`	Defines an action `Sum()` to return the sum of the mock function's argument #0 and #1.
`ACTION_P(Plus, n) { return arg0 + n; }`	Defines an action `Plus(n)` to return the sum of the mock function's argument #0 and `n`.
`ACTION_Pk(Foo, p1, ..., pk) { statements; }`	Defines a parameterized action `Foo(p1, ..., pk)` to execute the given `statements`.

The ACTION* macros cannot be used inside a function or class.

Cardinalities

These are used in Times() to specify how many times a mock function will be called:


`AnyNumber()`	The function can be called any number of times.
`AtLeast(n)`	The call is expected at least `n` times.
`AtMost(n)`	The call is expected at most `n` times.
`Between(m, n)`	The call is expected between `m` and `n` (inclusive) times.
`Exactly(n) or n`	The call is expected exactly `n` times. In particular, the call should never happen when `n` is 0.

Expectation Order

By default, the expectations can be matched in any order. If some or all expectations must be matched in a given order, there are two ways to specify it. They can be used either independently or together.

The After Clause

using ::testing::Expectation;
...
Expectation init_x = EXPECT_CALL(foo, InitX());
Expectation init_y = EXPECT_CALL(foo, InitY());
EXPECT_CALL(foo, Bar())
     .After(init_x, init_y);

says that Bar() can be called only after both InitX() and InitY() have been called.

If you don't know how many pre-requisites an expectation has when you write it, you can use an ExpectationSet to collect them:

using ::testing::ExpectationSet;
...
ExpectationSet all_inits;
for (int i = 0; i < element_count; i++) {
  all_inits += EXPECT_CALL(foo, InitElement(i));
}
EXPECT_CALL(foo, Bar())
     .After(all_inits);

says that Bar() can be called only after all elements have been initialized (but we don't care about which elements get initialized before the others).

Modifying an ExpectationSet after using it in an .After() doesn't affect the meaning of the .After().

Sequences

When you have a long chain of sequential expectations, it‘s easier to specify the order using sequences, which don’t require you to give each expectation in the chain a different name. All expected calls in the same sequence must occur in the order they are specified.

using ::testing::Return;
using ::testing::Sequence;
Sequence s1, s2;
...
EXPECT_CALL(foo, Reset())
    .InSequence(s1, s2)
    .WillOnce(Return(true));
EXPECT_CALL(foo, GetSize())
    .InSequence(s1)
    .WillOnce(Return(1));
EXPECT_CALL(foo, Describe(A<const char*>()))
    .InSequence(s2)
    .WillOnce(Return("dummy"));

says that Reset() must be called before both GetSize() and Describe(), and the latter two can occur in any order.

To put many expectations in a sequence conveniently:

using ::testing::InSequence;
{
  InSequence seq;

  EXPECT_CALL(...)...;
  EXPECT_CALL(...)...;
  ...
  EXPECT_CALL(...)...;
}

says that all expected calls in the scope of seq must occur in strict order. The name seq is irrelevant.

Verifying and Resetting a Mock

gMock will verify the expectations on a mock object when it is destructed, or you can do it earlier:

using ::testing::Mock;
...
// Verifies and removes the expectations on mock_obj;
// returns true if and only if successful.
Mock::VerifyAndClearExpectations(&mock_obj);
...
// Verifies and removes the expectations on mock_obj;
// also removes the default actions set by ON_CALL();
// returns true if and only if successful.
Mock::VerifyAndClear(&mock_obj);

You can also tell gMock that a mock object can be leaked and doesn't need to be verified:

Mock::AllowLeak(&mock_obj);

Mock Classes

gMock defines a convenient mock class template

class MockFunction<R(A1, ..., An)> {
 public:
  MOCK_METHOD(R, Call, (A1, ..., An));
};

See this recipe for one application of it.

Flags

Flag	Description
`--gmock_catch_leaked_mocks=0`	Don't report leaked mock objects as failures.
`--gmock_verbose=LEVEL`	Sets the default verbosity level (`info`, `warning`, or `error`) of Google Mock messages.