bonobomock

Mock Lambda Function

In this example we are going to show you how to mock and test lambda functions using BonoboMock.

Note: In optimized builds (e.g., CMake build types RelWithDebInfo or Release), the compiler may inline functions, preventing BonoboMock from intercepting calls. See Function inlining for this limitation and the available mitigations using the optimization macros and CMake helper.

Define Lambda Function

Lambda functions are a C++11 language feature. BonoboMock supports lambda mocking on GCC and Clang for projects using C++11 or later. Lambda mocking is not supported on MSVC.

In order to mock the lambda function, we need to get the object of it first (we have to use auto here because we don’t know the exact type of a lambda function):

auto lambdaFunc = [](int a) { return a > 10; };

Write unittest using BonoboMock

Mock lambda function

The lambda function is basically implemented as a functor in C++ behind the scenes, so BonoboMock can mock it with existing functionalities. As we have the object of the lambda function, we can use decltype to extract the type of it. And because it is a functor, we just need to fetch the address of the corresponding operator() of the specific type. One thing worth noting is that the following code will only work when there is no compiler optimization.

TEST(Lambda, example_without_optimization) {
    // demonstrate existing behavior of our lambda
    EXPECT_FALSE(lambdaFunc(10));

    auto functor = &decltype(lambdaFunc)::operator();
    auto mock = BONOBO_MOCK(functor);

    EXPECT_CALL(*mock, BONOBO_MOCK_FUNCTION(_,_))
        .Times(1)
        .WillOnce(Return(true));

    // the behavior of the lambda is now overridden for our testing
    EXPECT_TRUE(lambdaFunc(10));

    mock->restore();

    // pre-existing behavior of the lambda is restored
    EXPECT_FALSE(lambdaFunc(10));
}

If we want to mock a lambda function with higher optimization, the only thing we can do is define a wrapper function that calls the lambda function inside, and add the bonobomock-optimization-macro around the wrapper function. This is because there is no place for us to add attributes to a lambda function.

MSVC limitation: Lambda mocking is not supported on MSVC. MSVC routes stateless lambda calls through an internal static invoke function rather than through operator(), bypassing the binary patch that BonoboMock installs. This affects both the optimized wrapper pattern and direct lambda mocking.

namespace {
auto lambdaFunc = [](int a) { return a > 10; };

BONOBO_MOCK_DISABLE_FUNCTION_OPTIMIZATION_START(lambda_wrapper)
bool lambda_wrapper(int a) {
    return lambdaFunc(a);
}
BONOBO_MOCK_DISABLE_FUNCTION_OPTIMIZATION_END(lambda_wrapper)
}

TEST(Lambda, example_with_optimization) {
    // calling lambda function directly won't trigger the mocking behavior due to compiler optimization
    EXPECT_FALSE(lambdaFunc(10));
    // demonstrate existing behavior of our lambda in the wrapper function
    EXPECT_FALSE(lambda_wrapper(10));
    
    auto functor = &decltype(lambdaFunc)::operator();
    auto mock = BONOBO_MOCK(functor);

    EXPECT_CALL(*mock, BONOBO_MOCK_FUNCTION(_,_))
        .Times(2)
        .WillOnce(Return(true))
        .WillOnce(Return(true));
    // the behavior of the lambda is now overridden for our testing by calling the wrapper function
    EXPECT_TRUE(lambda_wrapper(10));
    EXPECT_TRUE(lambda_wrapper(10));
    // calling lambda function directly won't trigger the mocking behavior due to compiler optimization
    EXPECT_FALSE(lambdaFunc(10));

    mock->restore();

    // calling lambda function directly won't trigger the mocking behavior due to compiler optimization
    EXPECT_FALSE(lambdaFunc(10));
    // pre-existing behavior of the lambda is restored when wrapper function is called
    EXPECT_FALSE(lambda_wrapper(10));
}

Example source code

You can find additional examples among the unit test files for the library under tests/unit/.

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