What fixed the problem for me was adding this line:

C(const char *s) {
  std::locale::global(std::locale("C"));
  std::fstream f(s);
  double d;
  f >> d;
  std::cout << d;
  f >> d;
  std::cout << d;
}

I suspect that gtk might be modifiying locale itself after the application is launched so I have to correct it.