cpp-testing

SKILL.md

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:

RED: write a failing test that captures the new behavior

GREEN: implement the smallest change to pass

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
  # Google Test framework (official repository)
  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

Re-run the single failing test with gtest filter.

Add scoped logging around the failing assertion.

Re-run with sanitizers enabled.

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