Question

I want to put a text description of a class instance onto an ostream, as in

ostream << myInstance;

I know how to declare an ostream inserter;

ostream& operator<<(ostream&, myClass&);

I want to be able to put different levels of detail to the ostream. I could do this if I could define two or more ostream inserters, or pass an extra argument to the inserter, or pass a method to the inserter, or perhaps call a method that returns a stringstream (can I do that?)

Has anyone solved this problem?

Answer 1

You can add “flags” into the stream by actually encaspulating the stream itself.

struct Verbose {
  explicit Verbose(std::ostream* s = 0): _stream(s) {}
  std::ostream* _stream;
};

Verbose operator<<(std::ostream& out, Verbose) {
  return Verbose(&out);
}

And then, you define a new operator for verbose output:

std::ostream& operator<<(Verbose v, MyClass const& mc) {
  assert(v._stream);
  std::ostream& out = *v._stream;

  // ...
  return out;
}

Usage:

std::cout << Verbose() << myInstance << "\n";

You’ll note that Verbose is not tied to the class, at all, and can be reused accross classes easily enough.

Note: if you want to, you could add an extra parameter to Verbose to actually control the level of verbosity directly there.