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std::unreachable(3)	      C++ Standard Libary	   std::unreachable(3)

NAME
       std::unreachable	- std::unreachable

Synopsis
	  Defined in header <utility>
	  [[noreturn]] void unreachable();  (since C++23)

	  Invokes  undefined behavior. An implementation may use this to opti-
       mize impossible
	  code branches	away (typically, in optimized builds) or to trap  them
       to prevent
	  further execution (typically,	in debug builds).

Notes
	   Feature-test	macro	 Value	  Std	    Feature
	  __cpp_lib_unreachable	202202L	(C++23)	std::unreachable

Possible implementation
	  [[noreturn]] inline void unreachable()
	  {
	      // Uses compiler specific	extensions if possible.
	      //  Even	if  no	extension is used, undefined behavior is still
       raised by
	      // an empty function body	and the	noreturn attribute.
	  #if defined(_MSC_VER)	&& !defined(__clang__) // MSVC
	      __assume(false);
	  #else	// GCC,	Clang
	      __builtin_unreachable();
	  #endif
	  }

Example
       // Run this code

	#include <cassert>
	#include <cstddef>
	#include <cstdint>
	#include <utility>
	#include <vector>

	struct Color { std::uint8_t r, g, b, a;	};

	// Assume that only restricted set of texture caps is supported.
	void generate_texture(std::vector<Color>& tex, std::size_t xy)
	{
	    switch (xy)
	    {
	    case 128: [[fallthrough]];
	    case 256: [[fallthrough]];
	    case 512: /* ... */
		tex.clear();
		tex.resize(xy *	xy, Color{0, 0,	0, 0});
		break;
	    default:
		std::unreachable();
	    }
	}

	int main()
	{
	    std::vector<Color> tex;
	    generate_texture(tex, 128);	// OK
	    assert(tex.size() == 128 * 128);
	    generate_texture(tex, 32);	// Results in undefined	behavior
	}

Possible output:
	Segmentation fault

See also
					specifies that the expression will al-
       ways evaluate to
	  [[assume(expression)]](C++23)	true at	a given	point
					(attribute specifier)
	  assume_aligned		informs	the compiler that a pointer is
       aligned
	  (C++20)			(function template)
	  C documentation for
	  unreachable

	  External Links

	  1.  GCC docs:	__builtin_unreachable
	  2.  Clang docs: __builtin_unreachable
	  3.  MSVC docs: __assume

http://cppreference.com		  2024.06.10		   std::unreachable(3)

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