C++ Exercise for the Day

C++ exercise for the day. Write the header file (say "util.h") needed to define (not implement) operator<< for a vector of integers. Basically, the following code should compile and work in all situations:

#include "util.h"

// ... Some time later in a function or method ...
{
    std::vector<int> v;
    v.push_back(1);
    v.push_back(2);
    v.push_back(3);
    std::cout << "My vector: " << v << std::endl;
}

Maybe I'm just dumb, but it took me the better part of a whole day to figure out how to do this correctly. My guess is that if you aren't 100% sure of the answer, you will be surprised by it. Answer to come in a day or two.