Syntax and Sample Usage of _Generic in C11
The best example I have seen inspired the following (runnable) example, which unlocks all sorts of freaky possibilities for cracked-out introspection...
#include <stdio.h>#include <stddef.h>#include <stdint.h>#define typename(x) _Generic((x), /* Get the name of a type */ \ \ _Bool: "_Bool", unsigned char: "unsigned char", \ char: "char", signed char: "signed char", \ short int: "short int", unsigned short int: "unsigned short int", \ int: "int", unsigned int: "unsigned int", \ long int: "long int", unsigned long int: "unsigned long int", \long long int: "long long int", unsigned long long int: "unsigned long long int", \ float: "float", double: "double", \ long double: "long double", char *: "pointer to char", \ void *: "pointer to void", int *: "pointer to int", \ default: "other")#define fmt "%20s is '%s'\n"int main() { size_t s; ptrdiff_t p; intmax_t i; int ai[3] = {0}; return printf( fmt fmt fmt fmt fmt fmt fmt fmt, "size_t", typename(s), "ptrdiff_t", typename(p), "intmax_t", typename(i), "character constant", typename('0'), "0x7FFFFFFF", typename(0x7FFFFFFF), "0xFFFFFFFF", typename(0xFFFFFFFF),"0x7FFFFFFFU", typename(0x7FFFFFFFU), "array of int", typename(ai));}
╔═══════════════╗ ═════════════════╣ Amazeballs... ╠═════════════════════════════════════ ╚═══════════════╝ size_t is 'unsigned long int' ptrdiff_t is 'long int' intmax_t is 'long int'character constant is 'int' 0x7FFFFFFF is 'int' 0xFFFFFFFF is 'unsigned int' 0x7FFFFFFFU is 'unsigned int' array of int is 'other'
This is a pretty good introduction. Here's the Overview:
Generic selection is implemented with a new keyword: _Generic. The syntax is similar to a simple switch statement for types:
_Generic( 'a', char: 1, int: 2, long: 3, default: 0)
evaluates to 2 (character constants are ints in C).
Basically it works like a kind of switch
, where the labels are type names which are tested against the type of the first expression (the 'a'
above). The result becomes the result of evaluating the _Generic()
.
I use clion 1.2.4, and clion doesn't support c11 now, so I use following code in c99 instead of _Generic
#include <stdio.h>int main(int argc, char **argv) { char *s; if (__builtin_types_compatible_p(__typeof__(s), long)) { puts("long"); } else if (__builtin_types_compatible_p(__typeof__(s), char*)) { puts("str"); } return (0);};