bonobomock

Suppress Constructor Side Effects

Constructors and destructors cannot be mocked (see Limitations: Constructors and destructors). But the usual reason to want to mock a constructor is to stop it from running side effects — opening connections, doing I/O, registering globally. BonoboMock can suppress those with the BONOBO_DUMMY_* macros, which provide empty definitions that the linker uses in place of the real constructor and destructor.

The real object still links, so the class’s other methods stay intact and can be mocked with BONOBO_MOCK. This is what makes the pattern useful: dummy the side-effecting constructor, then mock the methods you care about.

Note: This works against statically linked dependencies and requires the ALLOW_CONSTRUCTOR_DUMMIES option (using a BONOBO_DUMMY_* macro without it is a compile error). See Limitations for the full constraints. Classes with reference or non-default-constructible members need the *_WITH_INIT macro variants, which take a member-initializer list.

The class under test

Suppose a dependency provides a class whose constructor talks to a backend:

namespace tsadmapi {
class FirmHdr {
public:
    explicit FirmHdr(int pricingNumber);  // side effects: fetches firm data
    bool isValid() const;
    int traddt() const;
};
}

and the code under test constructs it directly:

bsl::optional<int> getTradeDate(int pricingNumber) {
    tsadmapi::FirmHdr firmHeader(pricingNumber);
    if (firmHeader.isValid()) {
        return firmHeader.traddt();
    }
    return bsl::nullopt;
}

Write the unit test

Dummy the constructor and destructor at namespace scope (out-of-line member definitions cannot appear inside an anonymous namespace), then mock the methods:

// Namespace scope. Pass the leaf class name explicitly and the parameter list
// in parentheses.
BONOBO_DUMMY_CONSTRUCTOR_AND_DESTRUCTOR(tsadmapi::FirmHdr, FirmHdr, (int))

TEST(GetTradeDate, ReturnsTradeDateWhenValid) {
    auto validMock = BONOBO_MOCK(&tsadmapi::FirmHdr::isValid);
    EXPECT_CALL(*validMock, BONOBO_MOCK_FUNCTION(_)).WillRepeatedly(Return(true));

    auto traddtMock = BONOBO_MOCK(&tsadmapi::FirmHdr::traddt);
    EXPECT_CALL(*traddtMock, BONOBO_MOCK_FUNCTION(_)).WillRepeatedly(Return(20260101));

    EXPECT_EQ(getTradeDate(123), bsl::optional<int>(20260101));
}

TEST(GetTradeDate, ReturnsNulloptWhenInvalid) {
    auto validMock = BONOBO_MOCK(&tsadmapi::FirmHdr::isValid);
    EXPECT_CALL(*validMock, BONOBO_MOCK_FUNCTION(_)).WillRepeatedly(Return(false));

    EXPECT_EQ(getTradeDate(123), bsl::nullopt);
}

getTradeDate constructs FirmHdr via the dummied constructor, so no backend call happens; isValid() and traddt() are real methods redirected by BONOBO_MOCK to exercise both branches.

Enable the macros in CMake

The dummy definitions coexist with the real ones from the linked library, so the target must be built with ALLOW_CONSTRUCTOR_DUMMIES:

find_package(BonoboMockHelper REQUIRED)
add_bonobomock_executable(
    TARGET_NAME my_test.u.t
    TEST_FILES my_test.t.cpp
    ALLOW_CONSTRUCTOR_DUMMIES
)

target_link_libraries(my_test.u.t PRIVATE bonobomock the_dependency_library)

Example source code

You can find a working example in the unit test files under tests/unit/ (bonobomock_dummy_constructor.t.cpp and sample_side_effect_lib/).

---

← Back to Cookbook

← Documentation Home

This site is open source. Improve this page.