BaseStack

template <typename ItemType>
class BaseStack
{
public:
    BaseStack() {}
    virtual BaseStack<ItemType>& push(const ItemType& item) = 0;
    virtual BaseStack<ItemType>& pop() = 0;
    virtual ItemType& peek() = 0;
    virtual const ItemType& peek() const = 0;
    virtual int length() const = 0;
    virtual bool isEmpty() const = 0;
    virtual ~BaseStack() {}
};