Define function / method if not defined before c++

Question

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?

Answer 1

This is one of main purpose of namespace existence.

My suggest is to include your personal function in a proper namespace, something like:

namespace myns {
  std::string to_string(...) {
    // ...
  }
  // etc...
}

This is fundamental in order to avoid future conflict problems.

Afterwards, when you're going to use that function, you can simple select the proper function with a MACRO substitution.

Something like:

#if (__cplusplus >= 201103L) 
  #define my_tostring(X) std::to_string(X)
#else
  #define my_tostring(X) myns::to_string(X)
#endif

Note __cplusplus is a pre-defined macro which contains compiling information about standard version.

---

Edit:
Something less "violent", it will select the proper namespace for that specific function in accordance with the standard version:

#if (__cplusplus >= 201103L) 
  using std::to_string;
#else
  using myns::to_string;
#endif

// ... somewhere
to_string(/*...*/);  // it should use the proper namespace