Defined in header <stddef.h> | ||
---|---|---|
#define unreachable() /* see below */ | (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).
#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:
C++ documentation for unreachable |
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