problem with const member function and returning *this

I'm reading a book and doing an exercise where I just basically add some members, one of which is a const member function. It returns *this and I have it just like it has in the book, but it's giving me an error

Code:

class Screen
{
	public:
		typedef std::string::size_type index;
		
		Screen(index h, index w, std::string c) : 
					height(h), width(w), contents(c){}
								
		Screen &move(index, index);
		Screen &set(char);
		Screen &set(index, index, char);
		
		const Screen &display(std::ostream &os) const //error
                  { do_display(os); return *this; }                     //
		Screen &display(std::ostream out)
				{ do_display(out); return *this; }
		
	private:
		void do_display(std::ostream &out) {out << contents;}
		
		std::string contents;
		index cursor;
		index height, width;
};

The error is as follows
[code]
Screen.h:21: error: passing `const Screen' as `this' argument of `void Screen::do_display(std::ostream&)' discards qualifiers
[code]

Make do_display const too. You can only call const functions from other const functions.

so basically if a const member function also calls a non_const member function, I get the same error. Which is basically the point of const members to begin with, gotcha.

Yup. You're on to it.

Yup. And a very powerful language functionality.

I have caught many errors at compile time by being very generous in my usage of the const keyword. Basically, const is your friend.