When do you define the ostream operator << for a class?

Question

The question could be subjective, so the syntax of

std::ostream& operator << (std::ostream & o, const SomeClass &a) {
    return o << a.accessor().. ; 
}

When do you normally define this for the classes that you write, when do you avoid writing this friend function for your class.

Answer 1

IF I want to stream a class I normally write this:

std::ostream& operator << (std::ostream& o, const SomeClass& a)
{
    a.print(o);
    return o; 
}

Then make print a const method on SomeClass that knows how to serialize the class to a stream.

Answer 2

I would only overload operator<< when that has anything to do with streaming, or with shifting and the class is purely numeral. For writing something into an ostream, as in your code, i think it's fine. Anything else, i think, will cause confusion and i would better use member functions for those other purposes. One other application, which i think i would still do as an exception to the rule, is doing something like this:

StringList list;
list << "foo" << "bar" << "baz";

It is how Qt does it with its string list, and which i find quite nice.