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Modern Cpp

Trail of BitsOfficialPublished by Trail of Bits

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From the author

Guides C++ code toward modern idioms (C++20/23/26). Use when writing new C++ code, modernizing legacy patterns, or working on security-critical C++. Replaces raw pointers with smart pointers, SFINAE with concepts, printf with std::print, error codes with std::expected.
— the description by Trail of Bits

AutoGineer summary

Written by AutoGineer from the skill’s own files, not by the author

This skill provides a comprehensive framework for transitioning C++ codebases toward modern standards. It emphasizes patterns that eliminate common vulnerability classes, such as memory leaks and buffer overflows, by replacing legacy C-style constructs with modern, type-safe alternatives. The content includes migration tables for anti-patterns, compiler hardening configurations, and feature tiers based on practical usability.

What it does

  • Provides migration paths from legacy C++ patterns to modern C++20/23/26 idioms.
  • Lists specific anti-patterns and their safer, modern replacements.
  • Details compiler hardening flags for GCC and Clang to improve security.
  • Explains the use of hardened libc++ modes for bounds-checking.
  • Categorizes C++ features by their current practical deployment readiness.
  • Offers a checklist for adopting safe coding practices in new and existing projects.

When to use it

  • When writing new C++ functions, classes, or libraries.
  • When modernizing legacy C++ codebases to improve safety and maintainability.
  • When working on security-critical or safety-sensitive C++ applications.
  • When reviewing C++ code to ensure adoption of modern idioms.
  • When configuring compiler and linker settings for enhanced security.

Advantages

  • Reduces vulnerability classes like memory leaks and buffer overflows.
  • Improves code readability and reduces boilerplate through modern language features.
  • Provides actionable, security-focused compiler and linker configurations.
  • Offers clear, evidence-based guidance on replacing legacy C-style constructs.
  • Helps align code with current C++ standards for better long-term maintainability.

Usage examples

“How should I replace raw pointers in my C++ code?”
The skill recommends using std::unique_ptr for exclusive ownership or std::shared_ptr for shared ownership to ensure RAII semantics and eliminate manual memory management.
“What are the recommended compiler flags for hardening a C++ project?”
The skill provides a list of essential flags including -D_FORTIFY_SOURCE=3, -fstack-protector-strong, and -ftrivial-auto-var-init=zero, along with linker settings like -Wl,-z,relro,-z,now.
“How can I replace printf in C++23?”
The skill suggests using std::print or std::println, which are type-safe and avoid the format string vulnerabilities associated with printf.
“What is the modern alternative to SFINAE?”
The skill advises using C++20 Concepts and the requires clause to constrain templates, which provides more readable code and clearer error messages.

Install

  1. Download the skill as a ZIP:

    https://mp-staging.autogineer.com/agent-skills/trailofbits/modern-cpp/download

  2. Unzip it into the cross-client skills folder of your project:

    mkdir -p .agents/skills && unzip modern-cpp.zip -d .agents/skills

Documentation by Trail of Bits

Modern C++

Guide for writing modern C++ using C++20, C++23, and C++26 idioms. Focuses on patterns that eliminate vulnerability classes and reduce boilerplate, with a security emphasis from Trail of Bits.

When to Use This Skill

  • Writing new C++ functions, classes, or libraries

  • Modernizing existing C++ code (pre-C++20 patterns)

  • Choosing between legacy and modern approaches

  • Working on security-critical or safety-sensitive C++

  • Reviewing C++ code for modern idiom adoption

When NOT to Use This Skill

  • User explicitly requires older standard: Respect constraints (embedded, legacy ABI)

  • Pure C code: This skill is C++-specific

  • Build system questions: CMake, Meson, Bazel configuration is out of scope

  • Non-C++ projects: Mixed codebases where C++ isn't primary

Anti-Patterns to Avoid

AvoidUse InsteadWhy
new/deletestd::make_unique, std::make_sharedEliminates leaks, double-free
Raw owning pointersstd::unique_ptr, std::shared_ptrRAII ownership semantics
C arrays (int arr[N])std::array<int, N>Bounds-aware, value semantics
Pointer + length paramsstd::span<T>Non-owning, bounds-checkable
printf / sprintfstd::format, std::printType-safe, no buffer overflow
C-style casts (int)xstatic_cast<int>(x)Explicit intent, auditable
#define constantsconstexpr variablesScoped, typed, debuggable
SFINAE / enable_ifConcepts + requiresReadable constraints and errors
Error codes + out paramsstd::expected<T, E>Composable, type-safe errors
unionstd::variantType-safe, no silent UB
Raw mutex.lock()/unlock()std::scoped_lockException-safe, no deadlocks
std::threadstd::jthreadAuto-join, stop token support
assert() macrocontract_assert (C++26)Visible to tooling, configurable
Manual CRTPDeducing this (C++23)Simpler, no template boilerplate
Macro code generationReflection (C++26)Zero-overhead, composable

See anti-patterns.md for the full table (30+ patterns).

Decision Tree

What are you doing?
|
+-- Writing new C++ code?
|   +-- Use modern idioms by default (C++20/23)
|   +-- Choose the newest standard your compiler supports
|   +-- See Feature Tiers below
|
+-- Modernizing existing code?
|   +-- Start with Tier 1 (C++20/23) replacements
|   +-- Prioritize by security impact (memory > types > style)
|   +-- See anti-patterns.md for the migration table
|
+-- Security-critical code?
|   +-- Enable compiler hardening flags (see below)
|   +-- Enable hardened libc++ mode
|   +-- Run sanitizers in CI
|   +-- See safe-idioms.md and compiler-hardening.md
|
+-- Using C++26 features?
    +-- Reflection: YES, plan for it (GCC 16+)
    +-- Contracts: cautiously, for new API boundaries
    +-- std::execution: wait for ecosystem maturity
    +-- See cpp26-features.md

Feature Tiers

Features are ranked by practical usability today, not by standard version.

Tier 1: Use Today (C++20/23, solid compiler support)
FeatureReplacesStandard
Concepts + requiresSFINAE, enable_ifC++20
Ranges + viewsRaw iterator loopsC++20
std::span<T>Pointer + lengthC++20
std::formatsprintf, iostream chainsC++20
Three-way comparison <=>Manual comparison operatorsC++20
std::jthreadstd::thread + manual joinC++20
Designated initializersPositional struct initC++20
std::expected<T,E>Error codes, exceptions at boundariesC++23
std::print / std::printlnprintf, std::cout <<C++23
Deducing thisCRTP, const/non-const duplicationC++23
std::flat_mapstd::map for read-heavy useC++23
Monadic std::optionalNested if-checks on optionalsC++23

See cpp20-features.md and cpp23-features.md.

Tier 2: Deploy Now (no standard bump needed)

These improve safety without changing your C++ standard version:

  • Compiler hardening flags — -D_FORTIFY_SOURCE=3, -fstack-protector-strong, -ftrivial-auto-var-init=zero

  • Hardened libc++ — -D_LIBCPP_HARDENING_MODE=_LIBCPP_HARDENING_MODE_FAST for ~0.3% overhead bounds-checking

  • Sanitizers in CI — ASan + UBSan as minimum; TSan for concurrent code

  • Warning flags — -Wall -Wextra -Wpedantic -Werror

See compiler-hardening.md.

Tier 3: Plan For (C++26, worth restructuring around)

Reflection is the single most transformative C++26 feature. It eliminates:

  • Serialization boilerplate (one generic function replaces per-struct to_json)

  • Code generators (protobuf codegen, Qt MOC)

  • Macro-based registration and enum-to-string hacks

GCC 16 (April 2026) has reflection merged. Plan new code to benefit from it.

Tier 4: Watch (C++26, needs maturation)

  • Contracts (pre/post/contract_assert) — Better than assert(), but no virtual function support and limited compiler support. Adopt cautiously for new API boundaries.

  • std::execution (senders/receivers) — Powerful async framework, but steep learning curve, no scheduler ships with it, and poor documentation. Wait for ecosystem maturity.

See cpp26-features.md.

Compiler Hardening Quick Reference

Essential Flags (GCC + Clang)
-Wall -Wextra -Wpedantic -Werror
-D_FORTIFY_SOURCE=3
-fstack-protector-strong
-fstack-clash-protection
-ftrivial-auto-var-init=zero
-fPIE -pie
-Wl,-z,relro,-z,now
Clang-Specific
-Wunsafe-buffer-usage
Hardened libc++ (Clang/libc++ only)
-D_LIBCPP_HARDENING_MODE=_LIBCPP_HARDENING_MODE_FAST

Google deployed this across Chrome and their server fleet: ~0.3% overhead, 1000+ bugs found, 30% reduction in production segfaults.

See compiler-hardening.md for the full guide.

Rationalizations to Reject

RationalizationWhy It's Wrong
"It compiles without warnings"Warnings depend on which flags you enable. Add -Wall -Wextra -Wpedantic.
"ASan is too slow for production"Use GWP-ASan for sampling-based production detection (~0% overhead).
"We only use safe containers"Iterator invalidation and unchecked optional access are still exploitable.
"Smart pointers are slower"std::unique_ptr has zero overhead vs raw pointers. Measure before claiming.
"Our code doesn't have memory bugs"Google found 1000+ bugs when enabling hardened libc++. So did everyone else.
"C++26 features aren't available yet"C++20/23 features are. Hardening flags work on any standard. Start there.
"Modern C++ is harder to read"std::expected is more readable than checking error codes across 5 out-params.

Best Practices Checklist

  • [ ] Use smart pointers for ownership, raw pointers only for non-owning observation

  • [ ] Prefer std::span over pointer + length for function parameters

  • [ ] Use std::expected for functions that can fail with typed errors

  • [ ] Constrain templates with concepts, not SFINAE

  • [ ] Enable compiler hardening flags and hardened libc++ in all builds

  • [ ] Run ASan + UBSan in CI; add TSan for concurrent code

  • [ ] Use constexpr / consteval where possible (UB-free by design)

  • [ ] Mark functions [[nodiscard]] when ignoring the return value is likely a bug

  • [ ] Prefer value semantics; use std::variant over union, enum class over enum

  • [ ] Initialize all variables at declaration

Read Next

Files

Requirements and compatibility

Works with: Claude Code, Claude apps, OpenAI Codex, GitHub Copilot, Cursor, Gemini CLI, Kiro, OpenCode, Goose, Amp, Roo Code, JetBrains Junie, Factory Droid, Devin (ex-Windsurf), Mistral Vibe, OpenHands, Letta

“Works with” means the tool reads the open Agent Skills format; a skill written for one tool may use features others lack.

Security

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Attribution and licence

Published by
Trail of Bits
Publisher
Trail of Bits (Official)
Source
trailofbits/skills · plugins/modern-cpp/skills/modern-cpp · commit e4dd0bb
Licence
Creative Commons Attribution-ShareAlike 4.0 International (redistribution allowed)
  • LICENSE (added by AutoGineer from LICENSE)
  • ATTRIBUTION.md (written by AutoGineer)
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