Question

ostream& operator<<(ostream& os, const PT& p)
{
   os << "(" << p.x << "," << p.y << ")";
}

PT is a structure and x , y are its members.
Can someone please explain what exactly the above line does. Can’t the desired text be printed using cout?

I came across this snippet of code from this site.

Answer 1

This provides a method of outputting the PT. Now, you can use this:

PT p;
std::cout << p;

This gets translated into a call of

operator<< (std::cout, p);

That matches your overload, so it works, printing the x and y values in brackets with less effort on the user’s part. In fact, it doesn’t have to be cout. It can be anything that “is” a std::ostream. There are quite a few things that inherit from it, meaning they are std::ostreams as well, and so this works with them too. std::ofstream, for file I/O, is one example.

One thing that the sample you found doesn’t do, but should, though, is return os;. Without doing that, you can’t say std::cout << p << '\n'; because the result of printing p will not return cout for you to use to print the newline.