use “const” as parameter in operator overloading

Question

I'm trying to understand operators overloading, in the tutorial i use there is an example of overloading "+" operator for adding two objects.

  Box operator+(const Box& b)
  {
     Box box;
     box.length = this->length + b.length;
     box.breadth = this->breadth + b.breadth;
     box.height = this->height + b.height;
     return box;
  }

why does the parameter needs to be const reference to object?

Answer 1

The parameter is const because you don't need to modify the Box that was passed as argument.
The method itself should also be marked const , as it does not modify *this either.

Answer 2

To build on Quentin's answer, the most efficient way to pass in a "read-only" argument is to pass it in by reference or by means of a pointer (which are basically the same thing).

However, this may bring up a problem, because if the argument is modified within the function (it shouldn't, because you should be using it as "read-only", but if it is), then the original variable that was passed into the function is also modified. To prevent this, the parameter is marked as const .

This would not matter nearly as much if the "read-only" argument were not passed by reference or by pointer, but as I said before, it is much more efficient to do it this way.

Summary: The best way to pass in "read-only" paremeters is by const reference ( & ).

I hope this helps.