std::unreachable - cppreference.com (original) (raw)

| | | | | ------------------------------------ | | ------------- | | [[noreturn]] void unreachable(); | | (since C++23) |

Invokes undefined behavior at a given point.

An implementation may use this to optimize impossible code branches away (typically, in optimized builds) or to trap them to prevent further execution (typically, in debug builds).

[edit] Notes

[edit] 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 }

[edit] Example

#include #include #include #include #include   struct Color { std::uint8_t r, g, b, a; };   // Assume that only restricted set of texture caps is supported. void generate_texture(std::vector& 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 tex; generate_texture(tex, 128); // OK assert(tex.size() == 128 * 128); generate_texture(tex, 32); // Results in undefined behavior }

Possible output:

[edit] See also

| | specifies that the expression will always evaluate to true at a given point(attribute specifier)[edit] | | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | | informs the compiler that a pointer is aligned (function template) [edit] | | |

[edit] External Links