std::unreachable

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

Execution support library (C++26)

Technical specifications

Symbols index

External libraries

 

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 is_constant_evaluated (C++20) is_within_lifetime (C++26) initializer_list (C++11) source_location (C++20) Three-way comparison three_way_comparablethree_way_comparable_with (C++20)(C++20) strong_ordering (C++20) weak_ordering (C++20) partial_ordering (C++20) common_comparison_category (C++20) compare_three_way_result (C++20) compare_three_way (C++20) strong_order (C++20) weak_order (C++20) partial_order (C++20) compare_strong_order_fallback (C++20) compare_weak_order_fallback (C++20) compare_partial_order_fallback (C++20)   is_eqis_ltis_lteq (C++20)(C++20)(C++20)   is_neqis_gtis_gteq (C++20)(C++20)(C++20)

General utilities Date and time Function objects Formatting library (C++20)   bitset hash (C++11) Relational operators (deprecated in C++20) rel_ops::operator!=rel_ops::operator>    rel_ops::operator<=rel_ops::operator>= Integer comparison functions cmp_equalcmp_lesscmp_less_than (C++20)(C++20)(C++20)   cmp_not_equalcmp_greatercmp_greater_than (C++20)(C++20)(C++20) in_range (C++20) Swap and type operations swap ranges::swap (C++20) exchange (C++14) declval (C++11) to_underlying (C++23) forward (C++11) forward_like (C++23) move (C++11) move_if_noexcept (C++11) as_const (C++17) Common vocabulary types pair tuple (C++11) optional (C++17) any (C++17) variant (C++17) tuple_size (C++11) tuple_element (C++11) apply (C++17) make_from_tuple (C++17) expected (C++23) Elementary string conversions to_chars (C++17) from_chars (C++17) chars_format (C++17) to_chars_result (C++17) from_chars_result (C++17)

 

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).

Notes

Feature-test macro	Value	Std	Feature
__cpp_lib_unreachable

Possible implementation

First version
[[noreturn]] inline void unreachable() {}
Second version
[[noreturn]] inline void unreachable() { __builtin_unreachable(); // Compiler specific: GCC, Clang, ICC. // On MSVC could be: __assume(false); }

Example

Run this code

#include <vector>
#include <cassert>
#include <cstddef>
#include <cstdint>
#include <utility>
 
struct Color { std::uint8_t r, g, b, a; };
 
// Assume that only restricted set of the textures 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

External Links

• GCC docs: __builtin_unreachable
• Clang docs: __builtin_unreachable
• MSVC docs: __assume