How to put ostringstream in codeblocks?

If you are using C++, then the following code

#include <iostream>
#include <sstream>
using namespace std;

int main()
{
  ostringstream oss;
  oss << "hello " << 12.5;
  cout << oss.str() << endl;
}

should work out of the box, since stringstream - C++ Reference[^] is part of the C++ standard library.
On the other hand, if you are using C (as the question is tagged) then you are out of luck.