How to enable the _Generic keyword

I have this test source:

#include <stdio.h>

    int main()
    {
            int x;

            printf("x=%d\n", _Generic('x', int: 1, default: 0));
            return 0;
    }

Compiling with c++ 4.9.2 fails:

t.cpp: In function ‘int main()’:
t.cpp:7:33: error: expected primary-expression before ‘int’
  printf("x=%d\n", _Generic('x', int: 1, default: 0));
                                 ^
t.cpp:7:41: error: expected primary-expression before ‘default’
  printf("x=%d\n", _Generic('x', int: 1, default: 0));
                                         ^
t.cpp:7:51: error: ‘_Generic’ was not declared in this scope
  printf("x=%d\n", _Generic('x', int: 1, default: 0));

The compiler arguments are:

c++ --std=c++11 t.cpp -o t

What am I doing wrong?