cpp-expert

name	cpp-expert
description	Expert modern C++ (17/20/23): safe, performant code, RAII, move semantics, templates/concepts, and UB diagnosis. Trigger keywords: C++, C++17, C++20, C++23, smart pointer, unique_ptr, RAII, move semantics, rvalue, template, concept, STL, undefined behavior, constexpr, memory, dangling. Use for C++ implementation, optimization, generic programming, or UB/memory issues.

C++ Expert

RAII owns every resource; values move, don't copy. Make ownership explicit, lean on the STL, and treat undefined behavior as a bug to prevent — not a surprise to debug.

When to Use

• Writing or reviewing C++.
• Modern features (C++17/20/23), templates/metaprogramming, STL.
• Performance/memory tuning; smart pointers and ownership.
• Compile errors, crashes, leaks, or undefined behavior (UB).

When NOT to Use

• Another language → relevant language skill.
• Contest algorithms (language incidental) → competitive-programming-expert.
• System/architecture design → software-architect.

Core Principles

1. Modern by default

• Target C++17/20 unless constrained. Prefer auto, range-based for, structured bindings, std::optional/variant/string_view/span.
• Constrain templates with Concepts (C++20) or static_assert so errors surface at the call site, not deep in instantiation.

2. Ownership & RAII

• No owning raw pointers. std::unique_ptr for sole ownership, shared_ptr only when ownership is genuinely shared; create with make_unique/make_shared. Raw pointers/references are non-owning observers.
• Every resource (memory, file, lock, socket) is owned by an object that releases it in its destructor. Locks via std::scoped_lock/lock_guard.
• Follow the Rule of 0 (no manual special members) — or Rule of 5 if you manage a resource directly.

3. Value semantics & performance

• Pass const& for read-only, by value + std::move to sink, string_view/span for non-owning views. Rely on (N)RVO — return by value.
• Avoid premature shared_ptr (atomic refcount cost) and needless copies; emplace_back, reserve. Choose containers deliberately (vector by default). constexpr/consteval for compile-time work. Profile before micro-optimizing.

4. Avoid UB proactively

• Dangling refs/views, use-after-move, out-of-bounds, signed overflow, uninitialized reads, iterator invalidation, data races. Build with warnings (-Wall -Wextra), sanitizers (ASan/UBSan/TSan), and tools (clang-tidy).

Common Mistakes

• Owning raw new/delete → leaks/double-free; use smart pointers + RAII.
• Returning string_view/reference to a local/temporary → dangling.
• Using a moved-from object beyond a valid-but-unspecified state.
• shared_ptr everywhere → atomic overhead + cycles (use weak_ptr to break).
• C-style casts / reinterpret_cast → use static_cast/dynamic_cast.
• Iterator/reference invalidation after container growth/erase.
• std::endl in loops → forced flush; use '\n'.

Examples

RAII + move-only ownership (C++17)

#include <memory>
#include <string>
#include <utility>

class Connection {
public:
    explicit Connection(std::string dsn) : dsn_(std::move(dsn)) { /* open */ }
    ~Connection() noexcept { /* close */ }
    Connection(const Connection&) = delete;            // non-copyable
    Connection& operator=(const Connection&) = delete;
    Connection(Connection&&) noexcept = default;        // movable
    Connection& operator=(Connection&&) noexcept = default;
private:
    std::string dsn_;
};

auto make_conn(std::string dsn) {
    return std::make_unique<Connection>(std::move(dsn)); // ownership is explicit
}

Concept-constrained template (clear errors, no overflow)

#include <concepts>
template <std::integral T>
constexpr T midpoint(T a, T b) noexcept { return a + (b - a) / 2; }

See Also

• competitive-programming-expert — STL-heavy solutions and constant-factor tuning.
• performance-expert — profiling and allocation reduction.
• rust-expert — comparing systems-language ownership models.
• rules/cpp-style-guide.md — enforceable style rules.