C++ template inhertitance

Question

Why the warning show "Use of undeclared identifier 'Isempty'"and "Use of undeclared identifier 'top'"? It seems to be fail to inherit.

template <class T>
void Stack<T>::Pop(){
    if(Isempty())                   //warning
        throw"Stack is empty.";
    top--;                          //warning
}

The following is class declaration.

#include <iostream>
using  namespace::std;

template <class T>
class Bag
{
public:
    Bag(int bagcapacity=10);
    virtual ~Bag();
    virtual int Size()const;
    virtual bool Isempty()const;
    virtual T& Element()const;
    virtual void Push(const T&);
    virtual void Pop();
    
protected:
    T *array;
    int capacity;
    int top;
};

template <class T>
class Stack:public Bag<T>
{
public:
    Stack(int stackcapacity=10);
    ~Stack();
    T& Top()const;
    void Pop();
};