libkw

Header-only C++26 library for Python-style keyword arguments.

#include "libkw.hpp"

using libkw::kw;

auto add3(int x, int y, int z) { return x + y + z; }

KW_CALL(add3, kw<"z">, 3, kw<"x">, 1, kw<"y">, 2); // == add3(1, 2, 3)

Zero cost: with mild optimization (-O2 is sufficient), generates identical assembly to a hand-written positional call. When the callee is inlinable, the two are byte-identical (*).

(*) Under [[gnu::noinline]] or [[gnu::noipa]], the assembly is not byte-identical — it matches only up to stack slot reordering.

Limitations

• KW_CALL/KW_METHOD_CALL cannot target an overloaded function or method. ^^f requires f to already be an unambiguous id-expression; there's no cast-based workaround (^^ rejects cast expressions outright, and reflecting a disambiguating function-reference variable reflects the variable, not the function). Give the target a unique name, or wrap it in a uniquely-named forwarding function.

Dependencies

• A C++26 compiler with -freflection (P2996) support — e.g. GCC 16+ trunk. Not yet supported by stable GCC/Clang releases.
• CMake >= 4.3, Ninja.
• Python 3, for the test suites.
• clang-format and ruff, for formatting (format.bash).

Build & test

./test.bash

Runs four suites:

• test_basic — functional correctness: argument matching/reordering, value-category (copy/move) preservation, free functions and methods.
• test_standalone_include — compiles libkw.hpp as the only #include, guarding against relying on a transitively-included header.
• expect_failure.py — compiles each test/expect_failure/*.cpp case and checks the compiler error matches the expected .oracle output.
• modal.py — the zero-cost assembly comparison described above.

License

BSD 3-Clause, see LICENSE.