Discards qualifiers error

Solution 1:

Your checkElements() function is not marked as const so you can't call it on const qualified objects.

top(), however is const qualified so in top(), this is a pointer to a const Stack (even if the Stack instance on which top() was called happens to be non-const), so you can't call checkElements() which always requires a non-const instance.

Solution 2:

You cannot call a non-const method from a const method. That would 'discard' the const qualifier.

Basically it means that if it allowed you to call the method, then it could change the object, and that would break the promise of not modifying the object that the const at the end of the method signature offers.

Solution 3:

You're calling a non-const method from a const method.