C++ TDD Command

This command enforces test-driven development methodology for C++ code using GoogleTest/GoogleMock with CMake/CTest.

What This Command Does

1. Define Interfaces: Scaffold class/function signatures first
2. Write Tests: Create comprehensive GoogleTest test cases (RED)
3. Run Tests: Verify tests fail for the right reason
4. Implement Code: Write minimal code to pass (GREEN)
5. Refactor: Improve while keeping tests green
6. Check Coverage: Ensure 80%+ coverage

When to Use

Use /cpp-test when:

• Implementing new C++ functions or classes
• Adding test coverage to existing code
• Fixing bugs (write failing test first)
• Building critical business logic
• Learning TDD workflow in C++

TDD Cycle

RED     → Write failing GoogleTest test
GREEN   → Implement minimal code to pass
REFACTOR → Improve code, tests stay green
REPEAT  → Next test case

Example Session

User: /cpp-test I need a function to validate email addresses

Agent:
# TDD Session: Email Validator

## Step 1: Define Interface

```cpp
// validator/email.hpp
#pragma once
#include <string>
#include <expected>

enum class EmailError {
    Empty,
    InvalidFormat
};

std::expected<void, EmailError> validate_email(const std::string& email);
```

## Step 2: Write Tests (RED)

```cpp
// validator/email_test.cpp
#include <gtest/gtest.h>
#include "email.hpp"

TEST(ValidateEmail, AcceptsSimpleEmail) {
    auto result = validate_email("user@example.com");
    EXPECT_TRUE(result.has_value());
}

TEST(ValidateEmail, AcceptsSubdomain) {
    EXPECT_TRUE(validate_email("user@mail.example.com").has_value());
}

TEST(ValidateEmail, AcceptsPlus) {
    EXPECT_TRUE(validate_email("user+tag@example.com").has_value());
}

TEST(ValidateEmail, RejectsEmpty) {
    auto result = validate_email("");
    ASSERT_FALSE(result.has_value());
    EXPECT_EQ(result.error(), EmailError::Empty);
}

TEST(ValidateEmail, RejectsNoAtSign) {
    EXPECT_FALSE(validate_email("userexample.com").has_value());
}

TEST(ValidateEmail, RejectsNoDomain) {
    EXPECT_FALSE(validate_email("user@").has_value());
}

TEST(ValidateEmail, RejectsNoLocalPart) {
    EXPECT_FALSE(validate_email("@example.com").has_value());
}
```

## Step 3: Run Tests - Verify FAIL

```bash
$ cmake --build build && ctest --test-dir build --output-on-failure

1/1 Test #1: email_validator_test .....***Failed
    --- undefined reference to `validate_email`

FAIL
```

✓ Tests fail as expected (unimplemented).

## Step 4: Implement Minimal Code (GREEN)

```cpp
// validator/email.cpp
#include "email.hpp"
#include <regex>

std::expected<void, EmailError> validate_email(const std::string& email) {
    if (email.empty()) {
        return std::unexpected(EmailError::Empty);
    }
    static const std::regex pattern(R"([a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,})");
    if (!std::regex_match(email, pattern)) {
        return std::unexpected(EmailError::InvalidFormat);
    }
    return {};
}
```

## Step 5: Run Tests - Verify PASS

```bash
$ cmake --build build && ctest --test-dir build --output-on-failure

1/1 Test #1: email_validator_test .....   Passed    0.01 sec

100% tests passed.
```

✓ All tests passing!

## Step 6: Check Coverage

```bash
$ cmake -DCMAKE_CXX_FLAGS="--coverage" -B build && cmake --build build
$ ctest --test-dir build
$ lcov --capture --directory build --output-file coverage.info
$ lcov --list coverage.info

validator/email.cpp     | 100%
```

✓ Coverage: 100%

## TDD Complete!

Test Patterns

Basic Tests

TEST(SuiteName, TestName) {
    EXPECT_EQ(add(2, 3), 5);
    EXPECT_NE(result, nullptr);
    EXPECT_TRUE(is_valid);
    EXPECT_THROW(func(), std::invalid_argument);
}

Fixtures

class DatabaseTest : public ::testing::Test {
protected:
    void SetUp() override { db_ = create_test_db(); }
    void TearDown() override { db_.reset(); }
    std::unique_ptr<Database> db_;
};

TEST_F(DatabaseTest, InsertsRecord) {
    db_->insert("key", "value");
    EXPECT_EQ(db_->get("key"), "value");
}

Parameterized Tests

class PrimeTest : public ::testing::TestWithParam<std::pair<int, bool>> {};

TEST_P(PrimeTest, ChecksPrimality) {
    auto [input, expected] = GetParam();
    EXPECT_EQ(is_prime(input), expected);
}

INSTANTIATE_TEST_SUITE_P(Primes, PrimeTest, ::testing::Values(
    std::make_pair(2, true),
    std::make_pair(4, false),
    std::make_pair(7, true)
));

Coverage Commands

# Build with coverage
cmake -DCMAKE_CXX_FLAGS="--coverage" -DCMAKE_EXE_LINKER_FLAGS="--coverage" -B build

# Run tests
cmake --build build && ctest --test-dir build

# Generate coverage report
lcov --capture --directory build --output-file coverage.info
lcov --remove coverage.info '/usr/*' --output-file coverage.info
genhtml coverage.info --output-directory coverage_html

Coverage Targets

Code Type	Target
Critical business logic	100%
Public APIs	90%+
General code	80%+
Generated code	Exclude

TDD Best Practices

DO:

• Write test FIRST, before any implementation
• Run tests after each change
• Use EXPECT_* (continues) over ASSERT_* (stops) when appropriate
• Test behavior, not implementation details
• Include edge cases (empty, null, max values, boundary conditions)

DON'T:

• Write implementation before tests
• Skip the RED phase
• Test private methods directly (test through public API)
• Use sleep in tests
• Ignore flaky tests

Related Commands

• /cpp-build - Fix build errors
• /cpp-review - Review code after implementation
• /verify - Run full verification loop

Related

• Skill: skills/cpp-testing/
• Skill: skills/tdd-workflow/