C++ Testing (Agent Skill)

Agent-focused testing workflow for modern C++ (C++17/20) using GoogleTest/GoogleMock with CMake/CTest.

When to Use

• Writing new C++ tests or fixing existing tests
• Designing unit/integration test coverage for C++ components
• Adding test coverage, CI gating, or regression protection
• Configuring CMake/CTest workflows for consistent execution
• Investigating test failures or flaky behavior
• Enabling sanitizers for memory/race diagnostics

When NOT to Use

• Implementing new product features without test changes
• Large-scale refactors unrelated to test coverage or failures
• Performance tuning without test regressions to validate
• Non-C++ projects or non-test tasks

Core Concepts

• TDD loop: red → green → refactor (tests first, minimal fix, then cleanups).
• Isolation: prefer dependency injection and fakes over global state.
• Test layout: tests/unit, tests/integration, tests/testdata.
• Mocks vs fakes: mock for interactions, fake for stateful behavior.
• CTest discovery: use gtest_discover_tests() for stable test discovery.
• CI signal: run subset first, then full suite with --output-on-failure.

TDD Workflow

Follow the RED → GREEN → REFACTOR loop:

1. RED: write a failing test that captures the new behavior
2. GREEN: implement the smallest change to pass
3. REFACTOR: clean up while tests stay green

// tests/add_test.cpp
#include <gtest/gtest.h>

int Add(int a, int b); // Provided by production code.

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

// src/add.cpp
int Add(int a, int b) { // GREEN
  return a + b;
}

// REFACTOR: simplify/rename once tests pass

Code Examples

Basic Unit Test (gtest)

// tests/calculator_test.cpp
#include <gtest/gtest.h>

int Add(int a, int b); // Provided by production code.

TEST(CalculatorTest, AddsTwoNumbers) {
    EXPECT_EQ(Add(2, 3), 5);
}

Fixture (gtest)

// tests/user_store_test.cpp
// Pseudocode stub: replace UserStore/User with project types.
#include <gtest/gtest.h>
#include <memory>
#include <optional>
#include <string>

struct User { std::string name; };
class UserStore {
public:
    explicit UserStore(std::string /*path*/) {}
    void Seed(std::initializer_list<User> /*users*/) {}
    std::optional<User> Find(const std::string &/*name*/) { return User{"alice"}; }
};

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");
}

Mock (gmock)

// tests/notifier_test.cpp
#include <gmock/gmock.h>
#include <gtest/gtest.h>
#include <string>

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));
};

class Service {
public:
    explicit Service(Notifier &notifier) : notifier_(notifier) {}
    void Publish(const std::string &message) { notifier_.Send(message); }

private:
    Notifier &notifier_;
};

TEST(ServiceTest, SendsNotifications) {
    MockNotifier notifier;
    Service service(notifier);

    EXPECT_CALL(notifier, Send("hello")).Times(1);
    service.Publish("hello");
}

CMake/CTest Quickstart

# CMakeLists.txt (excerpt)
cmake_minimum_required(VERSION 3.20)
project(example LANGUAGES CXX)

set(CMAKE_CXX_STANDARD 20)
set(CMAKE_CXX_STANDARD_REQUIRED ON)

include(FetchContent)
# Prefer project-locked versions. If using a tag, use a pinned version per project policy.
set(GTEST_VERSION v1.17.0) # Adjust to project policy.
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

Running Tests

ctest --test-dir build --output-on-failure
ctest --test-dir build -R ClampTest
ctest --test-dir build -R "UserStoreTest.*" --output-on-failure

./build/example_tests --gtest_filter=ClampTest.*
./build/example_tests --gtest_filter=UserStoreTest.FindsExistingUser

Debugging Failures

1. Re-run the single failing test with gtest filter.
2. Add scoped logging around the failing assertion.
3. Re-run with sanitizers enabled.
4. Expand to full suite once the root cause is fixed.

Coverage

Prefer target-level settings instead of global flags.

option(ENABLE_COVERAGE "Enable coverage flags" 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 + gcov + 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 "Enable AddressSanitizer" OFF)
option(ENABLE_UBSAN "Enable UndefinedBehaviorSanitizer" OFF)
option(ENABLE_TSAN "Enable 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 Tests Guardrails

• Never use sleep for synchronization; use condition variables or latches.
• Make temp directories unique per test and always clean them.
• Avoid real time, network, or filesystem dependencies in unit tests.
• Use deterministic seeds for randomized inputs.

Best Practices

DO

• Keep tests deterministic and isolated
• Prefer dependency injection over globals
• Use ASSERT_* for preconditions, EXPECT_* for multiple checks
• Separate unit vs integration tests in CTest labels or directories
• Run sanitizers in CI for memory and race detection

DON'T

• Don't depend on real time or network in unit tests
• Don't use sleeps as synchronization when a condition variable can be used
• Don't over-mock simple value objects
• Don't use brittle string matching for non-critical logs

Common Pitfalls

• Using fixed temp paths → Generate unique temp directories per test and clean them.
• Relying on wall clock time → Inject a clock or use fake time sources.
• Flaky concurrency tests → Use condition variables/latches and bounded waits.
• Hidden global state → Reset global state in fixtures or remove globals.
• Over-mocking → Prefer fakes for stateful behavior and only mock interactions.
• Missing sanitizer runs → Add ASan/UBSan/TSan builds in CI.
• Coverage on debug-only builds → Ensure coverage targets use consistent flags.

Optional Appendix: Fuzzing / Property Testing

Only use if the project already supports LLVM/libFuzzer or a property-testing library.

• libFuzzer: best for pure functions with minimal I/O.
• RapidCheck: property-based tests to validate invariants.

Minimal libFuzzer harness (pseudocode: replace ParseConfig):

#include <cstddef>
#include <cstdint>
#include <string>

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

Alternatives to GoogleTest

• Catch2: header-only, expressive matchers
• doctest: lightweight, minimal compile overhead