The result of int c=0; cout<<c++<<c;

Question

I think it should be 01 but someone says its \"undefined\", any reason for that?

Answer 1

As I see it, f(c++); is equivalent to: f(c); c += 1;

And f(c++,c++); is equivalent to: f(c,c); c += 1; c += 1;

But it may be the case that f(c++,c++); becomes f(c,c+1); c+= 2;

An experiment with gcc and clang, first in C

#include 

void f(int a, int b) {
    printf("%d %d\n",a,b);
}

int main(int argc, char **argv) {
    int c = 0;
    f(c++,c++);

    return 0;
}

and in C++

#include 

int main(int argc, char **argv) {
    int c = 0;
    std::cout << c++ << " " << c++ << std::endl;
    return 0;
}

Is interesting, as gcc and g++ compiled results in 1 0 whereas clang compiled results in 0 1