Syntax and Sample Usage of _Generic in C11

Question

I heard C11 added generics. I've googled a bit, looked at some articles, understood there's a new keyword ( _Generic ) and all. But I can't seem to grasp it all.

Is it something like the generics in C# or templates in C++? Can anyone give me a brief explanation of the C11 definition of generics, its syntax and a simple sample usage example?

Answer 1

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' 

Answer 2

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() .

Answer 3

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);
};