Question

Is streaming a stringstream a libstdc++ extension? This program compiles with gcc-4.2, gcc-4.7-2 (using -std=c++03), and clang 3.2 using -std=c++11 and libstdc++ (thanks to Andy Prowl, see comments). It does not compile with clang 3.2 using -std=c++11 and -stdlib=libc++.

#include<iostream>
#include<sstream>
int main() {

  std::stringstream s; s << "b";

  std::cout << "ss: " << s << std::endl;

  return 0;
}

By looking at the constructor of ofstream it can take a std::basic_streambuf<CharT, Traits>* or a basic_ostream& st. A stringstream is a std::basic_istream, however both are std::basic_ios<CharT, Traits> so I would guess it should work.

The following change makes the code compile under clang:

  std::cout << "ss: " << s.str() << std::endl;

What is the right way to do it? cout << s; or cout << s.str(); ?

Answer 1

No, it’s a difference between C++03 and C++11. All streams have a conversion operator that enables code using if (s) and while (s). In C++03 this is usually operator void*() or something similar.

In C++11 we have explicit operators, where an explicit operator bool() works for if (s), but not for cout << s.