Redirecting stdout from another program in C++

Question

I'm writing a unit test and therefore, cannot change the code within the file that I'm testing. The code that I'm testing has messages in cout that I am trying to redirect into a file to check to make sure that the program is outputting the right messages. Does anyone have a way to redirect stdout in another program that won't cause a lag? I have tried freopen() and that causes my program to hang for some reason.

Answer 1

You could create a filebuf then replace cout's streambuf with it:

{
  std::filebuf f;
  f.open("output.txt", std::ios::out);
  std::streambuf* o = std::cout.rdbuf(&f);
  std::cout << "hello" << std::endl;  // endl will flush the stream
  std::cout.rdbuf(o);
}

You need to restore cout's original streambuf again (or set it to a null pointer) or it will probably crash when the global streams are flushed and destroyed, because the filebuf will already have gone out of scope.