std::unreachable
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specifies that the expression will always evaluate to true at a given point
(attribute specifier)[edit]
Type support (basic types, RTTI)
Library feature-test macros (C++20)
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General utilities
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(C++20)
Program support utilities
Defined in header
<utility>
[[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
| Feature-test macro | Value | Std | Feature |
|---|---|---|---|
__cpp_lib_unreachable |
202202L |
(C++23) | std::unreachable
|
[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
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
[edit] See also
[[assume(expression)]](C++23)
(attribute specifier)[edit]
C documentation for unreachable