|
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Program termination | |||||||||||||||||||||
| |||||||||||||||||||||
| Unreachable control flow | |||||||||||||||||||||
unreachable (C++23) | |||||||||||||||||||||
| Communicating with the environment | |||||||||||||||||||||
| Signals | |||||||||||||||||||||
| Signal types | |||||||||||||||||||||
| Non-local jumps | |||||||||||||||||||||
| Types | |||||||||||||||||||||
Defined in header <utility> | ||
[[noreturn]]void unreachable(); | (since C++23) | |
Invokesundefined 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).
Contents |
| Feature-test macro | Value | Std | Feature |
|---|---|---|---|
__cpp_lib_unreachable | 202202L | (C++23) | std::unreachable |
[[noreturn]]inlinevoid 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} |
#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){case128:[[fallthrough]];case256:[[fallthrough]];case512:/* ... */ 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);// OKassert(tex.size()==128*128); generate_texture(tex,32);// Results in undefined behavior}
Possible output:
Segmentation fault
[[assume(expression)]](C++23) | specifies that theexpression will always evaluate totrue at a given point (attribute specifier)[edit] |
(C++20) | informs the compiler that a pointer is aligned (function template)[edit] |
C documentation forunreachable | |
| 1. | GCC docs:__builtin_unreachable |
| 2. | Clang docs:__builtin_unreachable |
| 3. | MSVC docs:__assume |
