< cpp‎ | utility
Utilities library
Language support
Type support (basic types, RTTI)
Library feature-test macros (C++20)
Dynamic memory management
Program utilities
Coroutine support (C++20)
Variadic functions
Debugging support
Three-way comparison
General utilities
Date and time
Function objects
Formatting library (C++20)
Relational operators (deprecated in C++20)
Integer comparison functions
Swap and type operations
Common vocabulary types
Elementary string conversions

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

Invokes undefined behavior. 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
#else // GCC, Clang

[edit] Example

#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.resize(xy * xy, Color{0, 0, 0, 0});
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) 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]
C documentation for unreachable

[edit] External Links

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