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

cpp
1// tests/add_test.cpp
2#include <gtest/gtest.h>
3
4int Add(int a, int b); // Provided by production code.
5
6TEST(AddTest, AddsTwoNumbers) { // RED
7  EXPECT_EQ(Add(2, 3), 5);
8}
9
10// src/add.cpp
11int Add(int a, int b) { // GREEN
12  return a + b;
13}
14
15// REFACTOR: simplify/rename once tests pass

Code Examples

Basic Unit Test (gtest)

cpp
1// tests/calculator_test.cpp
2#include <gtest/gtest.h>
3
4int Add(int a, int b); // Provided by production code.
5
6TEST(CalculatorTest, AddsTwoNumbers) {
7    EXPECT_EQ(Add(2, 3), 5);
8}

Fixture (gtest)

cpp
1// tests/user_store_test.cpp
2// Pseudocode stub: replace UserStore/User with project types.
3#include <gtest/gtest.h>
4#include <memory>
5#include <optional>
6#include <string>
7
8struct User { std::string name; };
9class UserStore {
10public:
11    explicit UserStore(std::string /*path*/) {}
12    void Seed(std::initializer_list<User> /*users*/) {}
13    std::optional<User> Find(const std::string &/*name*/) { return User{"alice"}; }
14};
15
16class UserStoreTest : public ::testing::Test {
17protected:
18    void SetUp() override {
19        store = std::make_unique<UserStore>(":memory:");
20        store->Seed({{"alice"}, {"bob"}});
21    }
22
23    std::unique_ptr<UserStore> store;
24};
25
26TEST_F(UserStoreTest, FindsExistingUser) {
27    auto user = store->Find("alice");
28    ASSERT_TRUE(user.has_value());
29    EXPECT_EQ(user->name, "alice");
30}

Mock (gmock)

cpp
1// tests/notifier_test.cpp
2#include <gmock/gmock.h>
3#include <gtest/gtest.h>
4#include <string>
5
6class Notifier {
7public:
8    virtual ~Notifier() = default;
9    virtual void Send(const std::string &message) = 0;
10};
11
12class MockNotifier : public Notifier {
13public:
14    MOCK_METHOD(void, Send, (const std::string &message), (override));
15};
16
17class Service {
18public:
19    explicit Service(Notifier &notifier) : notifier_(notifier) {}
20    void Publish(const std::string &message) { notifier_.Send(message); }
21
22private:
23    Notifier &notifier_;
24};
25
26TEST(ServiceTest, SendsNotifications) {
27    MockNotifier notifier;
28    Service service(notifier);
29
30    EXPECT_CALL(notifier, Send("hello")).Times(1);
31    service.Publish("hello");
32}

CMake/CTest Quickstart

cmake
1# CMakeLists.txt (excerpt)
2cmake_minimum_required(VERSION 3.20)
3project(example LANGUAGES CXX)
4
5set(CMAKE_CXX_STANDARD 20)
6set(CMAKE_CXX_STANDARD_REQUIRED ON)
7
8include(FetchContent)
9# Prefer project-locked versions. If using a tag, use a pinned version per project policy.
10set(GTEST_VERSION v1.17.0) # Adjust to project policy.
11FetchContent_Declare(
12  googletest
13  # Google Test framework (official repository)
14  URL https://github.com/google/googletest/archive/refs/tags/${GTEST_VERSION}.zip
15)
16FetchContent_MakeAvailable(googletest)
17
18add_executable(example_tests
19  tests/calculator_test.cpp
20  src/calculator.cpp
21)
22target_link_libraries(example_tests GTest::gtest GTest::gmock GTest::gtest_main)
23
24enable_testing()
25include(GoogleTest)
26gtest_discover_tests(example_tests)

bash
1cmake -S . -B build -DCMAKE_BUILD_TYPE=Debug
2cmake --build build -j
3ctest --test-dir build --output-on-failure

Running Tests

bash
1ctest --test-dir build --output-on-failure
2ctest --test-dir build -R ClampTest
3ctest --test-dir build -R "UserStoreTest.*" --output-on-failure

bash
1./build/example_tests --gtest_filter=ClampTest.*
2./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.

cmake
1option(ENABLE_COVERAGE "Enable coverage flags" OFF)
2
3if(ENABLE_COVERAGE)
4  if(CMAKE_CXX_COMPILER_ID MATCHES "GNU")
5    target_compile_options(example_tests PRIVATE --coverage)
6    target_link_options(example_tests PRIVATE --coverage)
7  elseif(CMAKE_CXX_COMPILER_ID MATCHES "Clang")
8    target_compile_options(example_tests PRIVATE -fprofile-instr-generate -fcoverage-mapping)
9    target_link_options(example_tests PRIVATE -fprofile-instr-generate)
10  endif()
11endif()

GCC + gcov + lcov:

bash
1cmake -S . -B build-cov -DENABLE_COVERAGE=ON
2cmake --build build-cov -j
3ctest --test-dir build-cov
4lcov --capture --directory build-cov --output-file coverage.info
5lcov --remove coverage.info '/usr/*' --output-file coverage.info
6genhtml coverage.info --output-directory coverage

Clang + llvm-cov:

bash
1cmake -S . -B build-llvm -DENABLE_COVERAGE=ON -DCMAKE_CXX_COMPILER=clang++
2cmake --build build-llvm -j
3LLVM_PROFILE_FILE="build-llvm/default.profraw" ctest --test-dir build-llvm
4llvm-profdata merge -sparse build-llvm/default.profraw -o build-llvm/default.profdata
5llvm-cov report build-llvm/example_tests -instr-profile=build-llvm/default.profdata

Sanitizers

cmake
1option(ENABLE_ASAN "Enable AddressSanitizer" OFF)
2option(ENABLE_UBSAN "Enable UndefinedBehaviorSanitizer" OFF)
3option(ENABLE_TSAN "Enable ThreadSanitizer" OFF)
4
5if(ENABLE_ASAN)
6  add_compile_options(-fsanitize=address -fno-omit-frame-pointer)
7  add_link_options(-fsanitize=address)
8endif()
9if(ENABLE_UBSAN)
10  add_compile_options(-fsanitize=undefined -fno-omit-frame-pointer)
11  add_link_options(-fsanitize=undefined)
12endif()
13if(ENABLE_TSAN)
14  add_compile_options(-fsanitize=thread)
15  add_link_options(-fsanitize=thread)
16endif()

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):

cpp
1#include <cstddef>
2#include <cstdint>
3#include <string>
4
5extern "C" int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
6    std::string input(reinterpret_cast<const char *>(data), size);
7    // ParseConfig(input); // project function
8    return 0;
9}

Alternatives to GoogleTest

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