Define function / method if not defined before c++

I don't use C++11 yet, so I wrote the functions to_string(whatever) by myself. They should only be compiled if they don't exist. If I switch to C++11, they should be skipped. I have something like this:

#ifndef to_string

string to_string(int a){
    string ret;
    stringstream b;
    b << a;
    b >> ret;
    return ret;
}

string to_string(double a){
    string ret;
    stringstream b;
    b << a;
    b >> ret;
    return ret;
}

#endif

This doesn't work apparently. Is something like this possible and if yes, how?