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

The const-qualified member function will be called if the member function is called on an object that is const-qualified.

The non-const-qualified member function will be called if the member function is called on an object that is not const-qualified.

For example:

MyStack s;
s.top(); // calls non-const member function

const MyStack t;
t.top(); // calls const member function

Note that the same rules apply when calling a member function on a reference to an object or through a pointer to an object: if the pointer or reference is to a const object, the const member function will be called; otherwise the non-const member function will be called.