C++ Systems & Libraries
Covers C++17 as the baseline, with C++20 features called out where a project's standard allows them. For plain C (manual lifetimes, status enums, native extensions), see the ia-c-systems skill.
Working rules
- Make resource ownership explicit and use RAII for release on every exit path.
- Keep borrowed views within backing-storage lifetimes and review copy/move behavior when special members change.
- Follow the project's error model and supported language standard.
- Preserve ABI and avoid invoking unknown callbacks while holding locks or invalidatable container iterators.
Repo conventions outrank this skill
Check CMakeLists.txt for CXX_STANDARD, read .clang-format and .clang-tidy, and read two adjacent translation units before writing. Where they conflict with the rules below, they win.
The conflicts that actually happen:
| Local constraint |
Consequence |
-fno-exceptions |
Error handling is codes or expected-alikes. Constructors cannot report recoverable failure, so use a fallible factory or construct a valid fallback state. new (std::nothrow) only where the project's OOM policy is to observe null and recover; plain new is fine where the policy is termination |
| Standard pinned below C++17 |
No std::optional/string_view/structured bindings/if constexpr; check before using any |
| Public header is ABI-stable |
No layout changes, no inline-function changes, no added virtuals: load the ABI reference |
| Embedded or freestanding target |
No RTTI, no dynamic allocation in hot paths, possibly no STL containers |
Ownership and RAII
Every resource has exactly one owner, and that owner is an object whose destructor releases it. A raw new or delete in application code is a defect.
std::unique_ptr<T> for sole ownership. It is the default; it costs nothing over a raw pointer.std::shared_ptr<T> only where lifetime is genuinely shared and cannot be expressed as "the owner outlives the users". Reach for it third, not first.std::weak_ptr<T> to break ownership cycles. LeakSanitizer does report a cycle that is unreachable from any root, but not one still reachable from a global or other registered root, and detection varies by platform and configuration. Do not rely on the sanitizer to find these.- Raw
T* and T& mean non-owning observation, and are correct in that role. A parameter taking unique_ptr by value is announcing that it consumes ownership; one taking T* is announcing it does not.
std::span<T> (C++20) or a pointer-plus-length pair for a borrowed contiguous range in a new public API, since const std::vector<T>& there refuses every other container. On a C++17 baseline, or for internal code whose callers all hold vectors anyway, const std::vector<T>& is fine and simpler.
Rule of zero: a class that owns nothing declares no destructor, no copy, and no move. Composing members that manage themselves gets all five special members correct for free. Rule of five: declaring any one of destructor, copy constructor, copy assignment, move constructor, or move assignment obliges the author to reason about all five. A user-declared destructor suppresses the implicit move operations, so a class that gained a destructor silently started deep-copying where it used to move.
Discipline
- Preserve behavior and API compatibility unless a break was requested. A public header change is a decision, not a cleanup.
- Do not introduce a template, an inheritance hierarchy, or a policy parameter for a single call site.
#include what the file uses; do not rely on transitive includes from another header.- No
using namespace at namespace scope in a header. Fully qualify instead, or scope the using to a function body.
- When a constraint forces a deviation, comment at the deviation site and state the constraint.
Verify
- Build clean with
-Wall -Wextra -Wpedantic -Wshadow -Wconversion -Werror
clang-tidy reports no new findings on the diff- Tests pass under
-fsanitize=address,undefined with zero reports
- Any threaded code touched by the change exercised under TSan with zero reports -- the warning bundle above does not catch lock misuse
clang-format --dry-run --Werror produces no diff- No new raw
new/delete, no new shared_ptr where unique_ptr suffices
- Any class that gained a destructor has its move operations reviewed
Task-specific references
Read the relevant reference before implementing or reviewing the matching behavior:
Existing specialized references, when the corresponding topic applies:
1---2name: ia-cpp-systems3description: Modern C++ patterns: RAII and ownership, rule of zero/five, exceptions and error handling, API and ABI boundaries, templates, and CMake tooling. Use when writing, reviewing, refactoring, or debugging C++, working with smart pointers, move semantics, memory leaks, template errors, or gtest. For plain C, see ia-c-systems.4---56# C++ Systems & Libraries78Covers C++17 as the baseline, with C++20 features called out where a project's standard allows them. For plain C (manual lifetimes, status enums, native extensions), see the `ia-c-systems` skill.910## Working rules1112- Make resource ownership explicit and use RAII for release on every exit path.13- Keep borrowed views within backing-storage lifetimes and review copy/move behavior when special members change.14- Follow the project's error model and supported language standard.15- Preserve ABI and avoid invoking unknown callbacks while holding locks or invalidatable container iterators.1617## Repo conventions outrank this skill1819Check `CMakeLists.txt` for `CXX_STANDARD`, read `.clang-format` and `.clang-tidy`, and read two adjacent translation units before writing. Where they conflict with the rules below, they win.2021The conflicts that actually happen:2223| Local constraint | Consequence |24|---|---|25| `-fno-exceptions` | Error handling is codes or `expected`-alikes. Constructors cannot report recoverable failure, so use a fallible factory or construct a valid fallback state. `new (std::nothrow)` only where the project's OOM policy is to observe null and recover; plain `new` is fine where the policy is termination |26| Standard pinned below C++17 | No `std::optional`/`string_view`/structured bindings/`if constexpr`; check before using any |27| Public header is ABI-stable | No layout changes, no inline-function changes, no added virtuals: load the ABI reference |28| Embedded or freestanding target | No RTTI, no dynamic allocation in hot paths, possibly no STL containers |293031## Ownership and RAII3233Every resource has exactly one owner, and that owner is an object whose destructor releases it. A raw `new` or `delete` in application code is a defect.3435- `std::unique_ptr<T>` for sole ownership. It is the default; it costs nothing over a raw pointer.36- `std::shared_ptr<T>` only where lifetime is genuinely shared and cannot be expressed as "the owner outlives the users". Reach for it third, not first.37- `std::weak_ptr<T>` to break ownership cycles. LeakSanitizer does report a cycle that is unreachable from any root, but not one still reachable from a global or other registered root, and detection varies by platform and configuration. Do not rely on the sanitizer to find these.38- Raw `T*` and `T&` mean **non-owning observation**, and are correct in that role. A parameter taking `unique_ptr` by value is announcing that it consumes ownership; one taking `T*` is announcing it does not.39- `std::span<T>` (C++20) or a pointer-plus-length pair for a borrowed contiguous range **in a new public API**, since `const std::vector<T>&` there refuses every other container. On a C++17 baseline, or for internal code whose callers all hold vectors anyway, `const std::vector<T>&` is fine and simpler.4041**Rule of zero**: a class that owns nothing declares no destructor, no copy, and no move. Composing members that manage themselves gets all five special members correct for free. **Rule of five**: declaring any one of destructor, copy constructor, copy assignment, move constructor, or move assignment obliges the author to reason about all five. A user-declared destructor suppresses the implicit move operations, so a class that gained a destructor silently started deep-copying where it used to move.424344## Discipline4546- Preserve behavior and API compatibility unless a break was requested. A public header change is a decision, not a cleanup.47- Do not introduce a template, an inheritance hierarchy, or a policy parameter for a single call site.48- `#include` what the file uses; do not rely on transitive includes from another header.49- No `using namespace` at namespace scope in a header. Fully qualify instead, or scope the `using` to a function body.50- When a constraint forces a deviation, comment at the deviation site and state the constraint.515253## Verify5455- Build clean with `-Wall -Wextra -Wpedantic -Wshadow -Wconversion -Werror`56- `clang-tidy` reports no new findings on the diff57- Tests pass under `-fsanitize=address,undefined` with zero reports58- Any threaded code touched by the change exercised under TSan with zero reports -- the warning bundle above does not catch lock misuse59- `clang-format --dry-run --Werror` produces no diff60- No new raw `new`/`delete`, no new `shared_ptr` where `unique_ptr` suffices61- Any class that gained a destructor has its move operations reviewed6263## Task-specific references6465Read the relevant reference before implementing or reviewing the matching behavior:6667- For constness, errors, API/ABI changes, templates, or standard-library behavior: [value-and-interface-design.md](./references/value-and-interface-design.md).68- For threading, compiler setup, tests, or build analysis: [concurrency-and-verification.md](./references/concurrency-and-verification.md).6970Existing specialized references, when the corresponding topic applies:7172- [api-and-abi.md](./references/api-and-abi.md).73- [cmake-and-tooling.md](./references/cmake-and-tooling.md).74- [legibility-standard.md](./references/legibility-standard.md).
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