Difficulty in ostream

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.

Answer 2

It's a custom overload for operator<<.

It means you can do this:

PT p = ...;
std::cout << p << "\n";

or this:

PT p = ...;
std::stringstream ss;
ss << p << "\n";
std::cout << ss;

or lots of other useful stuff.

However, it should be noted that the code you quoted won't work properly. It needs to return os.

Answer 3

It allows the << operator to append the PT object to the stream. It seems the object has elements x and y that are added with a comma separator.

Answer 4

This operator << overloading for outputing object of PT class .

Here:

ostream& operator<<(ostream& os, const PT& p)

First param is for output stream where p will be appended. It returns reference to os for chaining like this:

cout << pt << " it was pt" << endl;