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cpp-testing

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Use only when writing/updating/fixing C++ tests, configuring GoogleTest/CTest, diagnosing failing or flaky tests, or adding coverage/sanitizers.

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

Running Tests

Terminal window
ctest --test-dir build --output-on-failure
ctest --test-dir build -R ClampTest
ctest --test-dir build -R "UserStoreTest.*" --output-on-failure
Terminal window
./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:

Terminal window
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:

Terminal window
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
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