Question

For my compsci class, I am implementing a Stack template class, but have run into an odd error:

Stack.h: In member function ‘const T Stack<T>::top() const [with T = int]’:

Stack.cpp:10: error: passing ‘const Stack<int>’ as ‘this’ argument of ‘void Stack<T>::checkElements() [with T = int]’ discards qualifiers

Stack<T>::top() looks like this:

const T top() const {
    checkElements();
    return (const T)(first_->data);
}

Stack<T>::checkElements() looks like this:

void checkElements() {
    if (first_==NULL || size_==0)
        throw range_error("There are no elements in the stack.");
}

The stack uses linked nodes for storage, so first_ is a pointer to the first node.

Why am I getting this error?

Answer 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.