cpp-idioms

C++ Idioms and Patterns

Modern C++ (17/20/23) rewards RAII, value semantics, and zero-cost abstractions. Lean into the type system and smart pointers. Idiomatic C++ = safe, deterministic, performant.

Scope: C++ coding idioms. Test naming: .agents/rules/testing-strategy.md.

Ownership and Memory

1. RAII — resources tied to object lifetime. No manual new/delete.

2. std::unique_ptr for exclusive ownership, std::shared_ptr only when sharing is necessary.

3. Move semantics — prefer moving over copying for expensive types.

4. Rule of Five/Zero:

   // ✅ Rule of Zero — let compiler-generated defaults work
   class TaskService {
       std::unique_ptr<TaskStorage> storage_;
       std::shared_ptr<Logger> logger_;
   public:
       // No destructor, copy/move constructors needed — smart pointers handle it
       explicit TaskService(std::unique_ptr<TaskStorage> storage, std::shared_ptr<Logger> logger)
           : storage_(std::move(storage)), logger_(std::move(logger)) {}
   };
   
   // ✅ Rule of Five — when managing raw resources (rare)
   class Buffer {
       char* data_;
       size_t size_;
   public:
       ~Buffer();                                   // Destructor
       Buffer(const Buffer&);                       // Copy constructor
       Buffer& operator=(const Buffer&);            // Copy assignment
       Buffer(Buffer&&) noexcept;                   // Move constructor
       Buffer& operator=(Buffer&&) noexcept;        // Move assignment
   };
   

5. Pass by value and move for sink parameters:

   // ✅ Sink parameter — takes ownership
   void addTask(Task task) {
       tasks_.push_back(std::move(task));
   }
   
   // ✅ Read-only — const reference
   void printTask(const Task& task);
   
   // ❌ Never pass smart pointers unless transferring ownership
   void processTask(std::shared_ptr<Task> task);  // Unnecessary ref-count bump
   
   // ✅ If function doesn't need ownership
   void processTask(const Task& task);
   

Error Handling

For universal error handling principles, see .agents/rules/error-handling-principles.md. Below: language-specific patterns only.

1. std::expected (C++23) or Result types for expected failures:

   // ✅ C++23 std::expected
   std::expected<Task, TaskError> getTask(std::string_view id) {
       auto it = tasks_.find(id);
       if (it == tasks_.end()) {
           return std::unexpected(TaskError::NotFound);
       }
       return it->second;
   }
   
   // Caller handles both paths
   auto result = getTask("123");
   if (!result) {
       handleError(result.error());
       return;
   }
   processTask(*result);
   

2. Exceptions for exceptional conditions only. Never for control flow.

3. noexcept on move constructors and destructors.

4. Domain error types:

   enum class TaskError {
       NotFound,
       ValidationFailed,
       StorageUnavailable,
   };
   
   // ✅ For exception-based paths
   class TaskNotFoundException : public std::runtime_error {
       std::string task_id_;
   public:
       explicit TaskNotFoundException(std::string id)
           : std::runtime_error("Task not found: " + id), task_id_(std::move(id)) {}
       const std::string& task_id() const noexcept { return task_id_; }
   };
   

Modern Features

1. std::string_view for read-only string parameters (no allocation):

   // ✅ Zero-copy string parameter
   void log(std::string_view message, std::string_view context);
   
   // ❌ Unnecessary copy
   void log(const std::string& message);
   

2. constexpr for compile-time evaluation:

   constexpr int maxRetries = 3;
   constexpr auto timeout = std::chrono::seconds(30);
   
   constexpr int factorial(int n) {
       return n <= 1 ? 1 : n * factorial(n - 1);
   }
   

3. Structured bindings: auto [id, title] = getTask();

4. std::optional for nullable values:

   // ✅ Explicit nullability
   std::optional<Task> findById(std::string_view id);
   
   // Caller checks explicitly
   if (auto task = findById("123"); task.has_value()) {
       process(*task);
   }
   

5. std::variant for type-safe unions (sum types):

   using TaskResult = std::variant<Task, TaskError>;
   
   TaskResult result = getTask(id);
   std::visit(overloaded{
       [](const Task& task) { process(task); },
       [](TaskError err) { handleError(err); },
   }, result);
   

6. Concepts (C++20) for constrained templates:

   template<typename T>
   concept Storable = requires(T t) {
       { t.id() } -> std::convertible_to<std::string_view>;
       { t.serialize() } -> std::same_as<std::vector<char>>;
   };
   
   template<Storable T>
   void save(const T& entity);
   

7. Ranges (C++20) for functional-style pipelines:

   auto active_titles = tasks
       | std::views::filter([](const Task& t) { return t.is_active(); })
       | std::views::transform(&Task::title)
       | std::views::take(10);
   

Interfaces and DI

// ✅ Interface (abstract class with pure virtuals)
class TaskStorage {
public:
    virtual ~TaskStorage() = default;
    virtual std::expected<Task, TaskError> getById(std::string_view id) = 0;
    virtual void save(const Task& task) = 0;
};

// ✅ Production implementation
class PostgresTaskStorage : public TaskStorage {
    // ...
};

// ✅ Test implementation
class MockTaskStorage : public TaskStorage {
    // MOCK_METHOD from Google Mock, or hand-rolled
};

// ✅ Constructor injection
class TaskService {
    std::unique_ptr<TaskStorage> storage_;
public:
    explicit TaskService(std::unique_ptr<TaskStorage> storage)
        : storage_(std::move(storage)) {}
};

Naming

1. PascalCase for types. camelCase or snake_case for functions (project-consistent).
2. _ suffix for private members: storage_, logger_.
3. No Hungarian notation.
4. ALL_CAPS only for macros — never for constants (use constexpr).

Anti-Patterns

• ❌ Raw new/delete — use std::make_unique / std::make_shared
• ❌ using namespace std; in headers — pollutes global namespace
• ❌ Catching ... without re-throwing — swallows all errors including crashes
• ❌ const_cast to remove constness — design flaw, fix the interface
• ❌ reinterpret_cast without safety justification — undefined behavior risk
• ❌ C-style casts (int)x — use static_cast<int>(x) for type safety
• ❌ std::shared_ptr when std::unique_ptr suffices — unnecessary overhead
• ❌ Copy-heavy loops without const auto&:

  // ❌ Copies every Task
  for (auto task : tasks) { process(task); }
  
  // ✅ Reference — no copy
  for (const auto& task : tasks) { process(task); }
  

Testing

Google Test or Catch2. Google Mock for interface mocking.

// Google Test + Google Mock
TEST(TaskServiceTest, ReturnsNotFoundForMissingTask) {
    auto storage = std::make_unique<MockTaskStorage>();
    EXPECT_CALL(*storage, getById("999"))
        .WillOnce(Return(std::unexpected(TaskError::NotFound)));

    TaskService service(std::move(storage));
    auto result = service.getTask("999");

    EXPECT_FALSE(result.has_value());
    EXPECT_EQ(result.error(), TaskError::NotFound);
}

Formatting and Static Analysis

Tool	Purpose	Command
clang-format	Formatting	clang-format -i src/**/*.cpp
clang-tidy	Static analysis	clang-tidy src/*.cpp --
cppcheck	Bug detection	cppcheck --enable=all src/
AddressSanitizer	Memory errors	-fsanitize=address
ThreadSanitizer	Race conditions	-fsanitize=thread
UBSanitizer	Undefined behavior	-fsanitize=undefined

Related

• Code Idioms and Conventions .agents/rules/code-idioms-and-conventions.md
• Resources and Memory Management @.agents/rules/resources-and-memory-management-principles.md
• Concurrency and Threading Principles @.agents/rules/concurrency-and-threading-principles.md
• Error Handling Principles @.agents/rules/error-handling-principles.md