Why is it impossible to convert const X to X &?

Question

I'm trying to understand and achieve const correctness on a Tetris Project.

Here is a recurrent problem that i have when i'm trying to add const where I think it's necessary.

I have a (Piece) class, and one of it's class private member is

Point rotationCenter;

And I'm trying to write a getter like this:

inline Point & Piece::getRotationCenter() const
{
  return rotationCenter;
}

Before, I had the same getter, but not as a const function, and was working. Now, I got the C2240 error "impossible to convert const Point to Point &".

What should I do to correct this? Should I leave getRotationCenter without const ?

PS : I read https://isocpp.org/wiki/faq/const-correctness as tutorial.

Answer 1

Why is it impossible to convert const X to X &?

Because if it is allowed, the following dangerous code becomes valid:

const int x = 0;
int& rx = x;     // bind const variable to reference (to non-const)
rx = 99;         // oops, try to modify the const variable via the reference

What should I do to correct this? Should I leave getRotationCenter without const ?

It depends on your intent. If the returned object could be modified, then make the member function non-const and return Point&. If not, then leave the member function const and make the return type const Point&. A const member function means a promise that won't modify (or provide the possibility to modify) the object (and its members).

Answer 2

Inside const member functions all of the classes data members are const. You cannot bind a non-const reference to your const member data, so you get a compiler error.

If you don't want your caller to modify rotationCenter, then you can return by Point or const Point&.

inline const Point & Piece::getRotationCenter() const
{
  return rotationCenter;
}

If you do want the caller to modify rotationCenter (which I would generally not recommend), write two overloads: one which returns by Point& and one which returns by const Point& depending on the qualification of the object you call it on:

inline Point & Piece::getRotationCenter() //not const
{
  return rotationCenter;
}

inline const Point & Piece::getRotationCenter() const
{
  return rotationCenter;
}