adl_barrier.h File Reference

Detailed Description

Keeps fl's generic templates out of argument-dependent lookup (ADL).

Every fl type (CRGB, CHSV, palettes, containers, ...) lives in namespace fl, so an unqualified call on one of them searches fl. If fl declared template <class T> void swap(T&, T&) or void* memcpy(void*, const void*, / size_t) directly, those would join the overload set next to std::swap or ::memcpy and tie. That made legacy code like memset(leds, 0, n), free(leds) and even std::reverse(leds, leds + n) (libstdc++ calls swap unqualified) fail to compile (FastLED#4827).

Such functions are declared in fl::adl_barrier instead, and fl imports them with a using-directive. ADL ignores using-directives, so the barrier is invisible to it. Ordinary lookup does follow the directive, so fl::swap(a, / b), fl::memcpy(...) and unqualified calls inside fl work as before.

Rules:
- Put every namespace-scope overload of a barrier name in the barrier.
  Qualified `fl::name` lookup stops at fl and skips the directive if fl
  declares any `name` itself.
  Lint enforces this (AdlBarrierChecker, FastLED#4851), and also requires
  each barrier function to be called as `fl::name` in
  tests/fl/stl/adl_barrier.cpp.
- The C library mirrors (fl::memcpy, fl::free, fl::strlen, ...) use a
  different fix, see fl/stl/cstring.h: they are templates with an unused
  defaulted parameter, so the non-template ::memcpy wins any tie, including
  under a using-directive for fl, which a using-directive cannot help with.
- swap and clamp are exceptions: it stays directly in fl so `swap(a, b)` on fl
  types still works via ADL; their generic overloads are written to be
  less specialized than std::swap / std::clamp so std wins ties (see
  type_traits.h and fl/math/math.h).
- Before 3.10.4 CRGB was a global type, so none of these names were
  reachable through ADL on a pixel; the barrier restores that behaviour. 

Definition in file adl_barrier.h.

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Namespaces
namespace	fl
	Base definition for an LED controller.
namespace	fl::adl_barrier