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_CONST_METHOD0(GetSize, int());
  MOCK_METHOD1(Describe, string(const char* name));
  MOCK_METHOD1(Describe, string(int type));
  MOCK_METHOD2(Process, bool(Bar elem, int count));
};

To create a "nice" mock object which ignores all uninteresting calls, or a "strict" mock object, which treats them as failures:

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

Mocking a Class Template

To mock

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

(note that ~StackInterface() must be virtual) just append _T to the MOCK_* macros:

template <typename Elem>
class MockStack : public StackInterface<Elem> {
 public:
  ...
  MOCK_CONST_METHOD0_T(GetSize, int());
  MOCK_METHOD1_T(Push, void(const Elem& x));
};

Specifying Calling Conventions for Mock Functions

If your mock function doesn't use the default calling convention, you can specify it by appending _WITH_CALLTYPE to any of the macros described in the previous two sections and supplying the calling convention as the first argument to the macro. For example,

  MOCK_METHOD_1_WITH_CALLTYPE(STDMETHODCALLTYPE, Foo, bool(int n));
  MOCK_CONST_METHOD2_WITH_CALLTYPE(STDMETHODCALLTYPE, Bar, int(double x, double y));

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

Using Mocks in Tests

The typical flow is:

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

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

Setting Default Actions

Google Mock has a built-in default action for any function that returns void, bool, a numeric value, or a pointer.

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

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();

To customize the default action for a particular method, use ON_CALL():

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();        ?

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

A matcher matches a single argument. You can use it inside ON_CALL() or EXPECT_CALL(), or use it to validate a value directly:

Built-in matchers (where argument is the function argument) are divided into several categories:

Wildcard

Generic Comparison

Except Ref(), these matchers make a copy of value in case it's modified or destructed later. If the compiler complains that value doesn't have a public copy constructor, try wrap it in ByRef(), e.g. Eq(ByRef(non_copyable_value)). If you do that, make sure non_copyable_value is not changed afterwards, or the meaning of your matcher will be changed.

Floating-Point Matchers

The above matchers use ULP-based comparison (the same as used in Google Test). They automatically pick a reasonable error bound based on the absolute value of the expected value. DoubleEq() and FloatEq() conform to the IEEE standard, which requires comparing two NaNs for equality to return false. The NanSensitive* version instead treats two NaNs as equal, which is often what a user wants.

String Matchers

The argument can be either a C string or a C++ string object:

ContainsRegex() and MatchesRegex() use the regular expression syntax defined here. StrCaseEq(), StrCaseNe(), StrEq(), and StrNe() work for wide strings as well.

Container Matchers

Most STL-style containers support ==, so you can use Eq(expected_container) or simply expected_container to match a container exactly. If you want to write the elements in-line, match them more flexibly, or get more informative messages, you can use:

Notes:

• These matchers can also match:
  1. a native array passed by reference (e.g. in Foo(const int (&a)[5])), and
  2. an array passed as a pointer and a count (e.g. in Bar(const T* buffer, int len) -- see Multi-argument Matchers).
• The array being matched may be multi-dimensional (i.e. its elements can be arrays).
• m in Pointwise(m, ...) should be a matcher for ::testing::tuple<T, U> where T and U are the element type of the actual container and the expected container, respectively. For example, to compare two Foo containers where Foo doesn't support operator== but has an Equals() method, one might write:

using ::testing::get;
MATCHER(FooEq, "") {
  return get<0>(arg).Equals(get<1>(arg));
}
...
EXPECT_THAT(actual_foos, Pointwise(FooEq(), expected_foos));

Member Matchers

Matching the Result of a Function or Functor

Pointer Matchers

Multiargument Matchers

Technically, all matchers match a single value. A "multi-argument" matcher is just one that matches a tuple. The following matchers can be used to match a tuple (x, y):

You can use the following selectors to pick a subset of the arguments (or reorder them) to participate in the matching:

Composite Matchers

You can make a matcher from one or more other matchers:

Adapters for Matchers

Matchers as Predicates

Defining Matchers

Notes:

1. The MATCHER* macros cannot be used inside a function or class.
2. The matcher body 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).
3. You can use PrintToString(x) to convert a value x of any type to a string.

Matchers as Test Assertions

Actions

Actions specify what a mock function should do when invoked.

Returning a Value

Side Effects

Using a Function or a Functor as an Action

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

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

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

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

  InvokeArgument<2>(5, string("Hi"), ByRef(foo))

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

Default Action

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

Defining Actions

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:

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 given 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::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 dummy;

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

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

Verifying and Resetting a Mock

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

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

Mock::AllowLeak(&mock_obj);

Mock Classes

Google Mock defines a convenient mock class template

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

See this recipe for one application of it.

Flags