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Author updated Aug 25, 2026, 07:09 AM
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Interpretation is structured for decision-making; original keeps the upstream SKILL.md unchanged.

Decide Fit First

Core job: Guides C++ code toward modern idioms (C++20/23/26). Use when writing new C++ code, modernizing legacy patterns, or working on se…
Best fit: Use it when the task has reusable inputs, steps, and validation criteria rather than a one-off answer.
Avoid forcing it: If the source lacks commands, platform support, or external-service evidence, keep those fields unknown instead of guessing.

Design Intent

Structure: The skill is organized around “When to Use This Skill”, “When NOT to Use This Skill”, “Anti-Patterns to Avoid”, “Decision Tree”, showing how the author expects the agent to judge fit, collect context, and produce verifiable output.
Trigger evidence: Prioritize the author’s wording around when to use it, what context to collect, and what output shape to produce.
Evidence boundary: Author text states facts, repository files prove commands and paths, and Fluxly only adds fit, limits, and usage judgment.

How To Use It

Inputs: Provide target material, scope, expected result, forbidden changes, and validation method.
Invocation: Name modern-cpp directly; if the source includes slash commands, start with the command and then add task context.
Validation: Start small and check whether the result follows “When to Use This Skill / When NOT to Use This Skill / Anti-Patterns to Avoid” before expanding.

Boundaries And Review

Dependencies: It usually needs no extra API key, so start with a small validation task.
Permissions: Declared permissions include read / write / shell-exec; ask the agent to state file, command, and rollback boundaries before acting.
Quality bar: A useful result names the deliverable, evidence, and next action. Generic prose means the task needs tighter context.

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

Avoid	Use Instead	Why
`new`/`delete`	`std::make_unique`, `std::make_shared`	Eliminates leaks, double-free
Raw owning pointers	`std::unique_ptr`, `std::shared_ptr`	RAII ownership semantics
C arrays (`int arr[N]`)	`std::array<int, N>`	Bounds-aware, value semantics
Pointer + length params	`std::span<T>`	Non-owning, bounds-checkable
`printf` / `sprintf`	`std::format`, `std::print`	Type-safe, no buffer overflow
C-style casts `(int)x`	`static_cast<int>(x)`	Explicit intent, auditable
`#define` constants	`constexpr` variables	Scoped, typed, debuggable
SFINAE / `enable_if`	Concepts + `requires`	Readable constraints and errors
Error codes + out params	`std::expected<T, E>`	Composable, type-safe errors
`union`	`std::variant`	Type-safe, no silent UB
Raw `mutex.lock()/unlock()`	`std::scoped_lock`	Exception-safe, no deadlocks
`std::thread`	`std::jthread`	Auto-join, stop token support
`assert()` macro	`contract_assert` (C++26)	Visible to tooling, configurable
Manual CRTP	Deducing `this` (C++23)	Simpler, no template boilerplate
Macro code generation	Reflection (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)

Feature	Replaces	Standard
Concepts + `requires`	SFINAE, `enable_if`	C++20
Ranges + views	Raw iterator loops	C++20
`std::span<T>`	Pointer + length	C++20
`std::format`	`sprintf`, iostream chains	C++20
Three-way comparison `<=>`	Manual comparison operators	C++20
`std::jthread`	`std::thread` + manual join	C++20
Designated initializers	Positional struct init	C++20
`std::expected<T,E>`	Error codes, exceptions at boundaries	C++23
`std::print` / `std::println`	`printf`, `std::cout <<`	C++23
Deducing `this`	CRTP, const/non-const duplication	C++23
`std::flat_map`	`std::map` for read-heavy use	C++23
Monadic `std::optional`	Nested if-checks on optionals	C++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

Rationalization	Why 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

anti-patterns.md — Full legacy-to-modern migration table (30+ patterns)
cpp20-features.md — Concepts, ranges, span, format, coroutines
cpp23-features.md — expected, print, deducing this, flat_map
cpp26-features.md — Reflection, contracts, memory safety improvements
compiler-hardening.md — Flags, sanitizers, hardened libc++
safe-idioms.md — Security patterns by vulnerability class

Fluxly profile Author and license come from source; runtime, permissions, and network are Fluxly detections or estimates

Fluxly category: Documentation
Author-declared agents: No explicit declaration found; this is not inferred or tested compatibility
Static check: 88 / 100 · heuristic scan, not runtime safety proof
Author / version / license: @trailofbits · no license declared
Fluxly token estimate: Lean
Fluxly setup estimate: Guided setup
External API key: No requirement detected
Detected OS requirements: Unspecified
Runtime requirements: Unspecified
Detected file/system behavior: Read-only
Write / modify
Shell exec
Detected network behavior: Local-only
Install commands: None (reference only)

Profile is derived at build time from SKILL.md and install vectors. Subject to drift from author intent.

Heads up: 未限定 allowed-tools，默认拥有全部工具权限。

Output preview modern-cpp.preview

The current SKILL.md does not define a fixed output example.

SKILL.md SKILL.md

# 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

| Avoid | Use Instead | Why |
|-------|-------------|-----|
| `new`/`delete` | `std::make_unique`, `std::make_shared` | Eliminates leaks, double-free |
| Raw owning pointers | `std::unique_ptr`, `std::shared_ptr` | RAII ownership semantics |
| C arrays (`int arr[N]`) | `std::array<int, N>` | Bounds-aware, value semantics |
| Pointer + length params | `std::span<T>` | Non-owning, bounds-checkable |
| `printf` / `sprintf` | `std::format`, `std::print` | Type-safe, no buffer overflow |
| C-style casts `(int)x` | `static_cast<int>(x)` | Explicit intent, auditable |
| `#define` constants | `constexpr` variables | Scoped, typed, debuggable |
| SFINAE / `enable_if` | Concepts + `requires` | Readable constraints and errors |
| Error codes + out params | `std::expected<T, E>` | Composable, type-safe errors |
| `union` | `std::variant` | Type-safe, no silent UB |
| Raw `mutex.lock()/unlock()` | `std::scoped_lock` | Exception-safe, no deadlocks |

…

Evidence boundary and execution chain

Author text anchors workflow facts; Fluxly only indexes current sections, terms, files, and commands.

sections -> When to Use This Skill → When NOT to Use This Skill → Anti-Patterns to Avoid → Decision Tree → Feature Tiers → Tier 1: Use Today (C++20/23, solid compiler support)
terms    -> User explicitly requires older standard · Pure C code · Build system questions · Non-C++ projects · practical usability today · Compiler hardening flags · Hardened libc++ · Sanitizers in CI
files/cmd -> new · delete · std::makeunique · std::makeshared · std::uniqueptr · std::sharedptr · int arr[N] · std::array<int, N>
body sha256 -> 378699403a8b