kotatsu C++ Coding Style

Redundancy Is a Defect

This is the convention we care most about. Redundant code is not a style nit — it actively misleads: every guard implies the guarded state can occur, every branch implies it can be taken, every parameter implies a caller needs it. When that implication is false, the reader wastes time defending against ghosts.

Every branch must be reachable. Before adding a guard or fallback, prove the state can actually occur — construct the input that hits it. If you can't, don't write it.
No speculative generality. No parameters, options, hooks, or abstraction layers for hypothetical future callers. Add them when the second real caller arrives.
One way to do each thing. Don't leave an old path alive next to its replacement "just in case" — migrate all callers and delete it in the same change.
Re-read after every change. Edits leave residue: conditions that became constant, variables read once, branches that now collapse, else after return, a helper with one remaining caller. Fold them before you're done — simplification that removes a concept beats one that merely shortens lines.
Delete, don't comment out. Git history is the archive.

Files & Organization

Headers are .h with #pragma once — never include guards. Public headers live under include/kotatsu/<module>/.
Source extensions are mixed across modules (.cpp in some, .cc in others) — match the module you are editing.
File names are snake_case.
File-local helpers: a single one is static; a cluster of them goes in one anonymous namespace.

Module Boundaries & macro.h

Libraries with user-facing declaration macros keep them in a dedicated macro.h (today: codec, zest) instead of exporting them from regular headers. Iron rule: such a macro.h contains zero #include directives — not even standard headers. It must stay a pure macro sheet a consumer can include from anywhere with no transitive cost.
This rule covers only those dedicated macro sheets. Configuration and implementation macros that ordinary headers legitimately define — e.g. KOTATSU_THROW and the platform feature macros in kotatsu/support/config.h — stay where they are.

Data & Types

struct by default, even for types with methods. class only when there is a real invariant that private access protects.
enum class with an explicit underlying type (e.g. : std::uint8_t); document each enumerator with /// when its meaning is not obvious.
Prefer designated initializers ({.field = value}) for aggregate construction.
West const: const T&, never T const&.

Errors, Exceptions & Defense

kotatsu is a library that builds both with and without exceptions (KOTATSU_ENABLE_EXCEPTIONS). Library code never writes bare throw/try/catch — use the KOTATSU_THROW / KOTATSU_TRY / KOTATSU_CATCH_ALL / KOTATSU_RETHROW macros from kotatsu/support/config.h, which degrade to std::abort() in the no-exceptions build.
Fallible synchronous operations return std::expected<T, E>; async code uses kotatsu's outcome types.
assert pins preconditions; impossible branches end in std::unreachable().
Defend only against states that can occur (the redundancy rule) — graceful degradation is not scattered null checks. The ASan/TSan build trees are the safety net.

Error Handling (control flow)

Prefer if with init-statements to tightly scope error variables, but avoid them when they compromise code readability or flatten control flow.
Omit redundant conditions: If the error type provides an operator bool or evaluates implicitly (e.g., standard error codes, custom error wrappers), omit the redundant condition check.
Avoid forced else branches: If scoping the variable inside the if requires you to introduce an else block for the success path (especially when returning early on error), declare the variable in the local scope instead to keep the control flow flat.

// Good: Omit redundant condition when the type has operator bool
if (auto err = foo()) {
    /* handle error */
}

// Bad: Redundant condition check
if (auto err = foo(); err) {
    /* handle error */
}

// Good: Use init-statement when a custom condition is required,
// AND the variable isn't needed outside the if-statement
if (auto result = foo(); !result.has_value()) {
    /* handle error */
}

// Bad: Using init-statement forces an 'else' block because 'result'
// goes out of scope, leading to nested/redundant code.
if (auto result = get_data(); !result.has_value()) {
    return result.error();
} else {
    process(result.value()); // Success path is forced into a nested block
}

// Good: Declare as a regular local variable to allow early exit
// and keep the success path un-nested (flat control flow).
auto result = get_data();
if (!result.has_value()) {
    return result.error();
}
process(result.value());

Concurrency & Async

Async code is coroutine-style (kotatsu::async::task, co_await) — an operation that produces a single asynchronous result returns an awaitable, never takes a completion callback; completion callbacks appear only at the libuv boundary inside the implementation.
Event handlers and customization hooks are a different animal: public callback registration APIs such as Peer::on_request, Peer::on_notification, or set_logger are legitimate extension points — do not replace them with coroutine APIs.

Naming Conventions

Variables, member fields, function names: snake_case. Class member fields do NOT use any special suffix/prefix (no trailing _, no m_ prefix).
Class names, template parameter names, enum names: PascalCase. Exception: some class names also use snake_case — follow the existing style in the module — and so do types modelled on the standard library's views and references: string_ref, function_ref, buffer_ref, string_list_ref, components_view, table_view.
Enum values: PascalCase.
Doc comments on declarations use ///.

Template & Type Traits

Do NOT blindly add std::remove_cvref_t on every template parameter. Understand C++ template argument deduction rules:
- template<typename T> void f(T x) — T is always deduced as a non-reference, non-cv-qualified type. No need for remove_cvref_t.
- template<typename T> void f(T& x) — T is deduced as the referred-to type (possibly cv-qualified, but never a reference). No need for remove_cvref_t to strip references.
- template<typename T> void f(const T& x) — T is deduced as a non-const, non-reference type. No need for remove_cvref_t.
- template<typename T> void f(T&& x) — forwarding reference: T CAN be deduced as an lvalue reference (e.g., int&). This is the ONLY case where std::remove_cvref_t<T> is needed to get the bare type.
- The same reasoning, not a blanket rule, applies to other deduction contexts: strip cv/ref only where the context can actually produce a cv/ref-qualified type. Plain auto return deduction and by-value CTAD guides never yield references, but decltype(auto) and explicit deduction guides can carry references and cv-qualifiers.
- Separately from deduction: explicitly supplied template arguments bypass deduction entirely, so T is whatever the caller wrote — f<int&>(x) makes T = int& even for a by-value f(T x). Only defend with remove_cvref_t if the API actually invites explicit reference arguments.

Type Traits & Concepts (C++20/23)

This project targets C++20/23. Use variable templates directly for type traits — do NOT use the old pattern of wrapping a class template static member in a variable template. Prefer:

// Good: directly specialize a variable template
template<typename T>
inline constexpr bool is_my_type_v = false;

template<>
inline constexpr bool is_my_type_v<MyType> = true;

// Bad: unnecessary class template wrapper
template<typename T>
struct is_my_type : std::false_type {};

template<>
struct is_my_type<MyType> : std::true_type {};

template<typename T>
inline constexpr bool is_my_type_v = is_my_type<T>::value;

When defining a concept that checks a type trait, do NOT add std::remove_cvref_t unless you specifically intend the concept to see through references/cv-qualifiers. If the concept is meant for a bare type, just use T directly — the caller is responsible for passing the right type.
```
// Good
template<typename T>
concept MyTrait = is_my_type_v<T>;

// Bad: unnecessary remove_cvref_t
template<typename T>
concept MyTrait = is_my_type_v<std::remove_cvref_t<T>>;
```

String Literals

Prefer C++11 raw string literals R"(...)" over escaped strings. Avoid \", \\, \n in string literals when a raw literal is cleaner.

Style

Prefer [[maybe_unused]] over (void) for intentionally unused variables or parameters.
No banner comments (a section title between // ---- rules). A file too long to read without them splits by aspect; a comment says what the names do not.

Modern C++ Usage

Use C++20/23 APIs whenever possible.
Library code does not print — it returns. Do NOT use <iostream> facilities (std::cout, std::cin, std::cerr, etc.) or C-style I/O (printf, fprintf, etc.) in library components. Exempt are components whose purpose is producing output: CLI presentation (e.g. cli usage printing), test reporting in zest, and fatal-diagnostic paths that must emit before aborting.
Prefer std::format for string building.
A cleanup a scope owes on its way out, or undoes unless the scope succeeds, is a kotatsu::ScopeExit (support/scope_exit.h), released once nothing is left to undo, not a local struct with a destructor; a type with state of its own beyond that stays a named type. What must happen even when a failed ZASSERT ends a test's process is a zest::FatalHook.
Prefer std::ranges / std::views APIs over raw loops and traditional <algorithm> calls.

Parameter Passing Preferences

For string parameters, prefer std::string_view > const std::string&.
For array/span parameters, prefer std::span > const std::vector&.
A function that produces text, such as a path or what the system reports, writes it into a buffer_ref parameter (support/buffer_ref.h) rather than returning std::string: the caller picks the buffer (std::string, small_string<N>, llvm::SmallString<N>) and may reuse it across calls. A function that reads a list of strings takes a string_list_ref (support/string_list_ref.h). A sequence drawn from an input, such as a path's components, is a lazy view of it; lists of records (directory entries, environment variables) stay std::vector. fs and sys follow this.

cpp-style

SKILL.md.claude/skills/cpp-style/SKILL.md