C++ Testing with GoogleTest

Source: affaan-m/everything-claude-code Original files: skills/cpp-testing/SKILL.md Curated: 2026-02-21

Core Concepts

• TDD loop: RED -> GREEN -> REFACTOR
• Isolation: Prefer dependency injection and fakes over globals
• Layout: tests/unit, tests/integration, tests/testdata
• Mocks vs fakes: Mock for interactions, fake for stateful behavior
• CTest: Use gtest_discover_tests() for stable test discovery

TDD Workflow

// RED: Failing test
TEST(AddTest, AddsTwoNumbers) {
  EXPECT_EQ(Add(2, 3), 5);
}

// GREEN: Minimal implementation
int Add(int a, int b) { return a + b; }

// REFACTOR: Clean up while green

Fixtures

class UserStoreTest : public ::testing::Test {
protected:
    void SetUp() override {
        store = std::make_unique<UserStore>(":memory:");
        store->Seed({{"alice"}, {"bob"}});
    }
    std::unique_ptr<UserStore> store;
};

TEST_F(UserStoreTest, FindsExistingUser) {
    auto user = store->Find("alice");
    ASSERT_TRUE(user.has_value());
    EXPECT_EQ(user->name, "alice");
}

Mocking with gmock

class Notifier {
public:
    virtual ~Notifier() = default;
    virtual void Send(const std::string &message) = 0;
};

class MockNotifier : public Notifier {
public:
    MOCK_METHOD(void, Send, (const std::string &message), (override));
};

TEST(ServiceTest, SendsNotifications) {
    MockNotifier notifier;
    Service service(notifier);
    EXPECT_CALL(notifier, Send("hello")).Times(1);
    service.Publish("hello");
}

CMake/CTest Setup

cmake_minimum_required(VERSION 3.20)
project(example LANGUAGES CXX)
set(CMAKE_CXX_STANDARD 20)

include(FetchContent)
set(GTEST_VERSION v1.17.0)
FetchContent_Declare(googletest
  URL https://github.com/google/googletest/archive/refs/tags/${GTEST_VERSION}.zip)
FetchContent_MakeAvailable(googletest)

add_executable(example_tests tests/calculator_test.cpp src/calculator.cpp)
target_link_libraries(example_tests GTest::gtest GTest::gmock GTest::gtest_main)

enable_testing()
include(GoogleTest)
gtest_discover_tests(example_tests)

cmake -S . -B build -DCMAKE_BUILD_TYPE=Debug
cmake --build build -j
ctest --test-dir build --output-on-failure

Coverage

CMake Configuration

option(ENABLE_COVERAGE "Enable coverage" OFF)
if(ENABLE_COVERAGE)
  if(CMAKE_CXX_COMPILER_ID MATCHES "GNU")
    target_compile_options(example_tests PRIVATE --coverage)
    target_link_options(example_tests PRIVATE --coverage)
  elseif(CMAKE_CXX_COMPILER_ID MATCHES "Clang")
    target_compile_options(example_tests PRIVATE -fprofile-instr-generate -fcoverage-mapping)
    target_link_options(example_tests PRIVATE -fprofile-instr-generate)
  endif()
endif()

GCC + lcov

cmake -S . -B build-cov -DENABLE_COVERAGE=ON
cmake --build build-cov -j && ctest --test-dir build-cov
lcov --capture --directory build-cov --output-file coverage.info
lcov --remove coverage.info '/usr/*' --output-file coverage.info
genhtml coverage.info --output-directory coverage

Clang + llvm-cov

cmake -S . -B build-llvm -DENABLE_COVERAGE=ON -DCMAKE_CXX_COMPILER=clang++
cmake --build build-llvm -j
LLVM_PROFILE_FILE="build-llvm/default.profraw" ctest --test-dir build-llvm
llvm-profdata merge -sparse build-llvm/default.profraw -o build-llvm/default.profdata
llvm-cov report build-llvm/example_tests -instr-profile=build-llvm/default.profdata

Sanitizers

option(ENABLE_ASAN "AddressSanitizer" OFF)
option(ENABLE_UBSAN "UndefinedBehaviorSanitizer" OFF)
option(ENABLE_TSAN "ThreadSanitizer" OFF)

if(ENABLE_ASAN)
  add_compile_options(-fsanitize=address -fno-omit-frame-pointer)
  add_link_options(-fsanitize=address)
endif()
if(ENABLE_UBSAN)
  add_compile_options(-fsanitize=undefined -fno-omit-frame-pointer)
  add_link_options(-fsanitize=undefined)
endif()
if(ENABLE_TSAN)
  add_compile_options(-fsanitize=thread)
  add_link_options(-fsanitize=thread)
endif()

Flaky Test Prevention

• Never use sleep for synchronization; use condition variables or latches
• Unique temp directories per test, always cleaned up
• No real time, network, or filesystem dependencies in unit tests
• Deterministic seeds for randomized inputs

Common Pitfalls

Best Practices

DO: Keep tests deterministic, prefer DI over globals, use ASSERT_* for preconditions and EXPECT_* for checks, separate unit/integration tests, run sanitizers in CI

DON'T: Depend on real time/network in unit tests, use sleeps for sync, over-mock value objects, use brittle string matching

Appendix: Fuzzing

extern "C" int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
    std::string input(reinterpret_cast<const char *>(data), size);
    // ParseConfig(input);
    return 0;
}

Alternatives to GoogleTest: Catch2 (header-only, expressive), doctest (lightweight).