#ifndef _STACKNODE_H_
#define _STACKNODE_H_
class StackNode {
    public:
        StackNode(void * const payload);
        /* This const annotation on the StackNode type for the argument 'next' promises that we will
         * not use this argument to modify the StackNode the argument points to.
         */
        StackNode(void * const payload, const StackNode * const next);
        void *getPayload() const;
        const StackNode *getNext() const;
    private:
        void *payload_;
        /* Note that because our constructor takes a pointer to a const StackNode and we save it,
         * we must make next_ a const StackNode*, and consequently getNext(), which returns the
         * value of next_, must return a const StackNode*; any time you keep a reference to a const
         * T in a field, the const keyword tends to propagate far and wide through your code. 
         *
         * This propagation isn't necessary if you don't hold onto a const T argument, because you
         * just read, return and forget.*/
        const StackNode *next_;
        StackNode(const StackNode &n);
        StackNode &operator=(const StackNode &rhs);

};
#endif // _STACKNODE_H_