Yeah, it looks like you did a lot of guesswork in that comment, and a lot of those guesses were inaccurate. Not really trying to be hostile here, but you did acknowledge that you were unfamiliar with std::format.

The part that fmtlib / std::format has, which is printf-like, is the idea of having a format string and arguments, rather than having a bunch of separate, piecemeal strings.

  // Old printf code, works ok for most people
  std::printf("failed to clone %s from %s", target, src);
  // <iostream>
  std::cout << "failed to clone " << target << " from " << src;
  // New std::format / fmtlib
  std::print("failed to clone {} from {}", target, src);

The std::print interface can work equally well with FILE or std::ofstream, or whatever you want. This is C++, and so you can just use a templated output iterator—or one of the overloads that creates one automatically.

There are a lot of problems with <iostream>. I think it’s telling that lots of languages have copied printf, but nobody (or almost nobody) thought <iostream> was good enough to copy. There are just too many serious design flaws with <iostream>. It would be one thing if <iostream> were just annoying to use, but it poses problems for localization, thread-safety, accidental misuse through its statefulness, and its operator overloading syntax is bad.