unreachable
From cppreference.com
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Defined in header
<stddef.h>
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||
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#define unreachable() /* see below */
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(since C23) | |
The function-like macro
unreachable
expands to a
void
expression. Executing
unreachable
(
)
results in
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).
Possible implementation
// Uses compiler specific extensions if possible. #ifdef __GNUC__ // GCC, Clang, ICC #define unreachable() (__builtin_unreachable()) #elifdef _MSC_VER // MSVC #define unreachable() (__assume(false)) #else // Even if no extension is used, undefined behavior is still raised by // the empty function body and the noreturn attribute. // The external definition of unreachable_impl must be emitted in a separated TU // due to the rule for inline functions in C. [[noreturn]] inline void unreachable_impl() {} #define unreachable() (unreachable_impl()) #endif |
Example
Run this code
#include <assert.h> #include <stddef.h> #include <stdint.h> #include <stdlib.h> struct Color { uint8_t r, g, b, a; }; struct ColorSpan { struct Color* data; size_t size; }; // Assume that only restricted set of texture caps is supported. struct ColorSpan allocate_texture(size_t xy) { switch (xy) { case 128: [[fallthrough]]; case 256: [[fallthrough]]; case 512: { /* ... */ struct ColorSpan result = { .data = malloc(xy * xy * sizeof(struct Color)), .size = xy * xy }; if (!result.data) result.size = 0; return result; } default: unreachable(); } } int main(void) { struct ColorSpan tex = allocate_texture(128); // OK assert(tex.size == 128 * 128); struct ColorSpan badtex = allocate_texture(32); // Undefined behavior free(badtex.data); free(tex.data); }
Possible output:
Segmentation fault
See also
|
C++ documentation
for
unreachable
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External Links
| 1. |
GCC docs:
__builtin_unreachable
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| 2. |
Clang docs:
__builtin_unreachable
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| 3. |
MSVC docs:
__assume
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