Modern Cpp Practices

---
name: modern-cpp-practices
description: 'Use when C++ code is being written or reviewed and the compiler supports modern safe idioms. Returns a structured guidance report on supported features, anti-pattern replacements, and compiler-compatible hardening flags. Read-only; no source or remote-system changes.'
---

# Modern C++ practices

## Contract

| Field | Bound contract |
|---|---|
| Trigger | C++ code is being written, modernized, or reviewed and the selected compiler and standard permit newer safe idioms. |
| Authority | Read-only. No file, VCS, credential, paid, published, deployed, or remote mutation. Chat output only. |
| Side effect | Emits a structured guidance report to chat. No source files are modified. |
| Done | A guidance report is emitted listing applicable modern idioms, anti-pattern replacements, recommended hardening flags, and unavailable features with deferral notes. |

## Inputs

1. **C++ source code or diff** (required): the code being written, modernized, or reviewed.
2. **Compiler identification** (required if not inferrable from build files): compiler vendor and version (e.g., GCC 14, Clang 18, MSVC 19.40).
3. **C++ standard level** (required if not inferrable from build flags): the `-std=` value or project-agreed standard (e.g., C++20, C++23).
4. **Build system or compiler flags** (optional): existing warning and hardening flags.

## Procedure

1. Identify the compiler vendor, version, and active C++ standard level from build files (`CMakeLists.txt`, `Makefile`, `meson.build`, compiler command lines) or from the supplied inputs. If the compiler or standard level cannot be determined, report the ambiguity and stop. Done when: the compiler, version, and standard are identified, or the ambiguity is reported.
2. For each modern C++ idiom or feature the code uses or the review proposes, check the corresponding feature-test macro (e.g., `__cpp_concepts >= 202002L`, `__cpp_lib_format >= 202110L`, `__cpp_constexpr >= 202211L`, `__cpp_modules >= 202207L`, `__cpp_lib_expected >= 202211L`, `__cpp_lib_print >= 202207L`, `__cpp_lib_ranges >= 202110L`, `__cpp_lib_generator >= 202207L`, `__cpp_lib_mdspan >= 202207L`). A feature is available only when its macro is defined and meets the required value for the compiler and standard in use. Done when: every relevant feature-test macro is checked.
3. Catalog applicable C++20 features: concepts, ranges, coroutines (`co_await`/`co_yield`/`co_return`), `std::format`, three-way comparison (`<=>`), `consteval`, `constinit`, modules, designated initializers, `std::span`, `std::jthread`, `std::source_location`. Done when: the C++20 feature catalog is complete for the identified compiler.
4. Catalog applicable C++23 features: `std::expected`, `std::print`/`std::println`, `std::generator`, `std::mdspan`, `std::flat_map`/`std::flat_set`, `if consteval`, multidimensional subscript operator, `std::stacktrace`, deducing this. Done when: the C++23 feature catalog is complete for the identified compiler.
5. Catalog conditionally available C++26 features: `std::inplace_vector`, `std::contract` (contracts), reflection (`^`/`[:`/`:]`), pattern matching (`inspect`). Gate each on its feature-test macro; if the macro is absent or below threshold, mark the feature as unavailable for this compiler. Done when: the C++26 feature catalog is complete with availability gates.
6. Scan the code for cataloged anti-patterns and recommend each modern equivalent in the report. Do not modify source files. Done when: every identified anti-pattern has a recommended modern replacement in the report.
   - Raw `new`/`delete` -> smart pointers (`std::unique_ptr`, `std::shared_ptr`) or RAII wrappers.
   - C-style casts -> `static_cast`, `reinterpret_cast`, `std::bit_cast`.
   - Manual resource management in destructors -> RAII types or `std::scope_success`/`std::scope_fail` (C++26) / `std::unique_resource`.
   - `NULL`/`0` for null pointers -> `nullptr`.
   - Unscoped `enum` -> `enum class`.
   - `typedef` for type aliases -> `using`.
   - SFINAE metaprogramming -> concepts and `requires` clauses.
   - `std::bind` / `std::bind1st` / `std::bind2nd` -> lambdas.
   - `throw()` / dynamic exception specifications -> `noexcept`.
   - Implicit captures in complex lambdas -> explicit capture lists.
   - `#pragma once` in multi-TU builds -> include guards (or modules when available).
   - `std::auto_ptr` -> `std::unique_ptr`.
   - `volatile` for thread synchronization -> `std::atomic`.
7. Recommend compiler-compatible hardening flags in the report. Do not apply them to build files. Recommend the maximal set the compiler and standard support. Done when: the hardening flag recommendation is complete.
   - Warnings: `-Wall -Wextra -Wpedantic -Wshadow -Wconversion -Wsign-conversion -Wnull-dereference -Wimplicit-fallthrough`.
   - Treat warnings as errors in CI: `-Werror`.
   - Sanitizers (when available): `-fsanitize=address,undefined -fno-sanitize-recover=all`.
   - Stack protection: `-fstack-protector-strong`.
   - Fortification: `-D_FORTIFY_SOURCE=2` (GCC/libc) or `-D_FORTIFY_SOURCE=3` (glibc 2.38+).
   - Format security: `-Wformat -Wformat-security -Wformat=2`.
   - Control-flow integrity: `-fcf-protection` (GCC/Clang x86) or `-fsanitize=cfi` (Clang).
   - Position-independent code: `-fPIE -pie` for executables, `-fPIC` for shared libraries.
   - If a flag is unsupported by the compiler, omit it and note the omission in the report.
8. Verify that every recommended feature and flag is grounded in the compiler version and standard level identified in step 1. Do not recommend a feature whose feature-test macro is absent or below threshold. Do not invent unavailable features. Done when: every recommendation is verified against the identified compiler and standard.

## Failure and recovery

| Failure class | Behavior |
|---|---|
| Compiler or standard undetermined | Report the ambiguity with the evidence examined. Do not guess. Guidance is blocked until the compiler and standard are identified. |
| Feature-test macro absent or below threshold | Mark the feature as unavailable for this compiler. Recommend the closest available alternative or defer the feature. Do not assume availability from the standard level alone. |
| Hardening flag unsupported | Omit the flag from the recommendation. Note which compiler rejects it. Recommend the subset that compiles cleanly. |
| Code uses a feature not yet standardized | Report the feature, its current proposal status, and the compiler-specific extension if one exists. Do not recommend non-standard features as portable. |
| Anti-pattern has no direct modern replacement | Report the anti-pattern, explain why no drop-in replacement exists, and recommend the closest safe alternative or a refactor path. |

No partial result is claimed complete. If any step cannot finish, the guidance report states which steps succeeded and which are blocked.

## Output

A structured guidance report containing:
1. **Applicable modern idioms**: list of C++20/C++23/C++26 features available for the identified compiler and standard, each with its feature-test macro and required value.
2. **Anti-pattern replacements**: each identified anti-pattern with its recommended modern replacement. These are recommendations for the user to apply; this skill does not modify source files.
3. **Hardening flags**: the recommended compiler flag set, noting any unsupported flags. These are recommendations for the user to add to build configuration; this skill does not modify build files.
4. **Unavailable features**: features gated behind absent or insufficient feature-test macros, with deferral notes.