googletest/docs/gmock_cheat_sheet.md

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.

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:

1. Import the gMock names you need to use. All gMock symbols 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 will they do?).
5. Exercise code that uses the mock objects; if necessary, check the result using googletest assertions.
6. 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("good", MyProductionFunction(&foo));  // #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(nullptr, buzz1);
  EXPECT_NE(nullptr, buzz2);
  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

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

Macro	Description
EXPECT_THAT(actual_value, matcher)	Asserts that actual_value matches matcher.
ASSERT_THAT(actual_value, matcher)	The same as EXPECT_THAT(actual_value, matcher), except that it generates a fatal failure.

Note: Although equality matching via EXPECT_THAT(actual_value, expected_value) is supported, prefer to make the comparison explicit via EXPECT_THAT(actual_value, Eq(expected_value)) or EXPECT_EQ(actual_value, expected_value).

Built-in matchers (where argument is the function argument, e.g. actual_value in the example above, or when used in the context of EXPECT_CALL(mock_object, method(matchers)), the arguments of method) are divided into several categories:

Wildcard

Matcher	Description
_	argument can be any value of the correct type.
A<type>() or An<type>()	argument can be any value of type type.

Generic Comparison

Matcher	Description
Eq(value) or value	argument == value
Ge(value)	argument >= value
Gt(value)	argument > value
Le(value)	argument <= value
Lt(value)	argument < value
Ne(value)	argument != value
IsFalse()	argument evaluates to false in a Boolean context.
IsTrue()	argument evaluates to true in a Boolean context.
IsNull()	argument is a NULL pointer (raw or smart).
NotNull()	argument is a non-null pointer (raw or smart).
Optional(m)	argument is optional<> that contains a value matching m. (For testing whether an optional<> is set, check for equality with nullopt. You may need to use Eq(nullopt) if the inner type doesn't have ==.)
VariantWith<T>(m)	argument is variant<> that holds the alternative of type T with a value matching m.
Ref(variable)	argument is a reference to variable.
TypedEq<type>(value)	argument has type type and is equal to value. You may need to use this instead of Eq(value) when the mock function is overloaded.

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 std::ref(), e.g. Eq(std::ref(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.

IsTrue and IsFalse are useful when you need to use a matcher, or for types that can be explicitly converted to Boolean, but are not implicitly converted to Boolean. In other cases, you can use the basic EXPECT_TRUE and EXPECT_FALSE assertions.

Floating-Point Matchers

Matcher	Description
DoubleEq(a_double)	argument is a double value approximately equal to a_double, treating two NaNs as unequal.
FloatEq(a_float)	argument is a float value approximately equal to a_float, treating two NaNs as unequal.
NanSensitiveDoubleEq(a_double)	argument is a double value approximately equal to a_double, treating two NaNs as equal.
NanSensitiveFloatEq(a_float)	argument is a float value approximately equal to a_float, treating two NaNs as equal.
IsNan()	argument is any floating-point type with a NaN value.

The above matchers use ULP-based comparison (the same as used in googletest). 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.

Matcher	Description
DoubleNear(a_double, max_abs_error)	argument is a double value close to a_double (absolute error <= max_abs_error), treating two NaNs as unequal.
FloatNear(a_float, max_abs_error)	argument is a float value close to a_float (absolute error <= max_abs_error), treating two NaNs as unequal.
NanSensitiveDoubleNear(a_double, max_abs_error)	argument is a double value close to a_double (absolute error <= max_abs_error), treating two NaNs as equal.
NanSensitiveFloatNear(a_float, max_abs_error)	argument is a float value close to a_float (absolute error <= max_abs_error), treating two NaNs as equal.

String Matchers

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

Matcher	Description
ContainsRegex(string)	argument matches the given regular expression.
EndsWith(suffix)	argument ends with string suffix.
HasSubstr(string)	argument contains string as a sub-string.
IsEmpty()	argument is an empty string.
MatchesRegex(string)	argument matches the given regular expression with the match starting at the first character and ending at the last character.
StartsWith(prefix)	argument starts with string prefix.
StrCaseEq(string)	argument is equal to string, ignoring case.
StrCaseNe(string)	argument is not equal to string, ignoring case.
StrEq(string)	argument is equal to string.
StrNe(string)	argument is not equal to string.

ContainsRegex() and MatchesRegex() take ownership of the RE object. They use the regular expression syntax defined here. All of these matchers, except ContainsRegex() and MatchesRegex() 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:

Matcher	Description
BeginEndDistanceIs(m)	argument is a container whose begin() and end() iterators are separated by a number of increments matching m. E.g. BeginEndDistanceIs(2) or BeginEndDistanceIs(Lt(2)). For containers that define a size() method, SizeIs(m) may be more efficient.
ContainerEq(container)	The same as Eq(container) except that the failure message also includes which elements are in one container but not the other.
Contains(e)	argument contains an element that matches e, which can be either a value or a matcher.
Each(e)	argument is a container where every element matches e, which can be either a value or a matcher.
ElementsAre(e0, e1, ..., en)	argument has n + 1 elements, where the i-th element matches ei, which can be a value or a matcher.
ElementsAreArray({e0, e1, ..., en}), ElementsAreArray(a_container), ElementsAreArray(begin, end), ElementsAreArray(array), or ElementsAreArray(array, count)	The same as ElementsAre() except that the expected element values/matchers come from an initializer list, STL-style container, iterator range, or C-style array.
IsEmpty()	argument is an empty container (container.empty()).
IsSubsetOf({e0, e1, ..., en}), IsSubsetOf(a_container), IsSubsetOf(begin, end), IsSubsetOf(array), or IsSubsetOf(array, count)	argument matches UnorderedElementsAre(x0, x1, ..., xk) for some subset {x0, x1, ..., xk} of the expected matchers.
IsSupersetOf({e0, e1, ..., en}), IsSupersetOf(a_container), IsSupersetOf(begin, end), IsSupersetOf(array), or IsSupersetOf(array, count)	Some subset of argument matches UnorderedElementsAre(expected matchers).
Pointwise(m, container), Pointwise(m, {e0, e1, ..., en})	argument contains the same number of elements as in container, and for all i, (the i-th element in argument, the i-th element in container) match m, which is a matcher on 2-tuples. E.g. Pointwise(Le(), upper_bounds) verifies that each element in argument doesn't exceed the corresponding element in upper_bounds. See more detail below.
SizeIs(m)	argument is a container whose size matches m. E.g. SizeIs(2) or SizeIs(Lt(2)).
UnorderedElementsAre(e0, e1, ..., en)	argument has n + 1 elements, and under some permutation of the elements, each element matches an ei (for a different i), which can be a value or a matcher.
UnorderedElementsAreArray({e0, e1, ..., en}), UnorderedElementsAreArray(a_container), UnorderedElementsAreArray(begin, end), UnorderedElementsAreArray(array), or UnorderedElementsAreArray(array, count)	The same as UnorderedElementsAre() except that the expected element values/matchers come from an initializer list, STL-style container, iterator range, or C-style array.
UnorderedPointwise(m, container), UnorderedPointwise(m, {e0, e1, ..., en})	Like Pointwise(m, container), but ignores the order of elements.
WhenSorted(m)	When argument is sorted using the < operator, it matches container matcher m. E.g. WhenSorted(ElementsAre(1, 2, 3)) verifies that argument contains elements 1, 2, and 3, ignoring order.
WhenSortedBy(comparator, m)	The same as WhenSorted(m), except that the given comparator instead of < is used to sort argument. E.g. WhenSortedBy(std::greater(), ElementsAre(3, 2, 1)).

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 ::std::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==, one might write:

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

Member Matchers

Matcher	Description
Field(&class::field, m)	argument.field (or argument->field when argument is a plain pointer) matches matcher m, where argument is an object of type class.
Field(field_name, &class::field, m)	The same as the two-parameter version, but provides a better error message.
Key(e)	argument.first matches e, which can be either a value or a matcher. E.g. Contains(Key(Le(5))) can verify that a map contains a key <= 5.
Pair(m1, m2)	argument is an std::pair whose first field matches m1 and second field matches m2.
FieldsAre(m...)	argument is a compatible object where each field matches piecewise with m.... A compatible object is any that supports the std::tuple_size<Obj>+get<I>(obj) protocol. In C++17 and up this also supports types compatible with structured bindings, like aggregates.
Property(&class::property, m)	argument.property() (or argument->property() when argument is a plain pointer) matches matcher m, where argument is an object of type class. The method property() must take no argument and be declared as const.
Property(property_name, &class::property, m)	The same as the two-parameter version, but provides a better error message.

Matching the Result of a Function, Functor, or Callback

Matcher	Description
ResultOf(f, m)	f(argument) matches matcher m, where f is a function or functor.

Pointer Matchers

Matcher	Description
Address(m)	the result of std::addressof(argument) matches m.
Pointee(m)	argument (either a smart pointer or a raw pointer) points to a value that matches matcher m.
Pointer(m)	argument (either a smart pointer or a raw pointer) contains a pointer that matches m. m will match against the raw pointer regardless of the type of argument.
WhenDynamicCastTo<T>(m)	when argument is passed through dynamic_cast<T>(), it matches matcher m.

Multi-argument 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):

Matcher	Description
Eq()	x == y
Ge()	x >= y
Gt()	x > y
Le()	x <= y
Lt()	x < y
Ne()	x != y

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

Matcher	Description
AllArgs(m)	Equivalent to m. Useful as syntactic sugar in .With(AllArgs(m)).
Args<N1, N2, ..., Nk>(m)	The tuple of the k selected (using 0-based indices) arguments matches m, e.g. Args<1, 2>(Eq()).

Composite Matchers

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

Matcher	Description
AllOf(m1, m2, ..., mn)	argument matches all of the matchers m1 to mn.
AllOfArray({m0, m1, ..., mn}), AllOfArray(a_container), AllOfArray(begin, end), AllOfArray(array), or AllOfArray(array, count)	The same as AllOf() except that the matchers come from an initializer list, STL-style container, iterator range, or C-style array.
AnyOf(m1, m2, ..., mn)	argument matches at least one of the matchers m1 to mn.
AnyOfArray({m0, m1, ..., mn}), AnyOfArray(a_container), AnyOfArray(begin, end), AnyOfArray(array), or AnyOfArray(array, count)	The same as AnyOf() except that the matchers come from an initializer list, STL-style container, iterator range, or C-style array.
Not(m)	argument doesn't match matcher m.

Adapters for Matchers

Matcher	Description
MatcherCast<T>(m)	casts matcher m to type Matcher<T>.
SafeMatcherCast<T>(m)	safely casts matcher m to type Matcher<T>.
Truly(predicate)	predicate(argument) returns something considered by C++ to be true, where predicate is a function or functor.

AddressSatisfies(callback) and Truly(callback) take ownership of callback, which must be a permanent callback.

Using Matchers as Predicates

Matcher	Description
Matches(m)(value)	evaluates to true if value matches m. You can use Matches(m) alone as a unary functor.
ExplainMatchResult(m, value, result_listener)	evaluates to true if value matches m, explaining the result to result_listener.
Value(value, m)	evaluates to true if value matches m.

Defining Matchers

Matcher	Description
MATCHER(IsEven, "") { return (arg % 2) == 0; }	Defines a matcher IsEven() to match an even number.
MATCHER_P(IsDivisibleBy, n, "") { *result_listener << "where the remainder is " << (arg % n); return (arg % n) == 0; }	Defines a matcher IsDivisibleBy(n) to match a number divisible by n.
MATCHER_P2(IsBetween, a, b, absl::StrCat(negation ? "isn't" : "is", " between ", PrintToString(a), " and ", PrintToString(b))) { return a <= arg && arg <= b; }	Defines a matcher IsBetween(a, b) to match a value in the range [a, b].

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.

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).

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 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::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.