What's the difference between a const member function and a non-const member function?

Question

I am very confused about the const version and non-const version member function like below:

value_type& top() { return this.item }
const value_type&         


        

Answer 1

value_type& top() { return this.item; } ensures that either the calling object's data members can be modified or the return value can be.

value_type& top() const { return this.item; } ensures that the calling object's data members can't be modified, but the return value can be. So, for example, if I perform value_type item_of_x = x.top();, item_of_x can be modified but x cannot. Otherwise, a compiler error occurs (such as having the code this.item = someValue; inside of the function body).

const value_type& top() { return this.item; } ensures that the calling object's data members are allowed to be modified, but the return value cannot be. It's the opposite of what is discussed above: if I perform const value_type item_of_x = x.top();, item_of_x can't be modified but x can. NOTE value_type item_of_x = x.top(); still allows for modification of item_of_x, as item_of_x is now non-const.

const value_type& top() const { return this.item; } ensures that neither the calling object's data members can be modified nor the return value can be.