C++ 堆栈模板

#include<iostream>
#include<string>
#include<cassert>

using namespace std;

template<class T>
class Stack{
public:
	enum{ DefaultStack=50, EmptyStack=-1};
	Stack();
	Stack(int);
	~Stack();
	void push(const T&);
	T pop();
	T topNoPop() const;
	bool empty() const;
	bool full() const;
private:
	T* elements;
	int top;
	int size;
	void allocate(){
		elements = new T[size];
		top = EmptyStack;
	}
	void msg(const char* m)const{
		cout<<"*****"<<m<<"*****"<<endl;
	}
	friend ostream& operator<<(ostream&, const Stack<T>&);

};

template <class T>
Stack<T>::Stack(){
	size = defaultStack;
	allocate();
}

template<class T>
Stack<T>::Stack(int s){
	if(s < 0)
		s *= -1;
	else if(0 == s)
		s = DefaultStack;
	size = s;
	allocate();
}

template<class T>
Stack<T>::~Stack(){
	delete[] elements;
}

template<class T>
void Stack<T>::push(const T& e){
	assert(!full())
		elements[++top] = e;
	else
		msg("Stack full!");
}

template <class T>
T Stack<T>::pop(){
	assert(!empty());
	if(!empty())
		return elements[top--];
	else{
		msg("Stack empty!");
		T dummy_value;
		return dummy_value;
	}
}

template<class T>
T Stack<T>::topNoPop() const {
	assert(top > EmptyStack)
		if(!empty())
			return elements[top];
		else{
			msg("Stack empty!");
			T dummy_value;
			return dummy_value;
		}
}

template<class T>
bool Stack<T>::empty() const{
	return top <= EmptyStack;
}

template<class T>
bool Stack<T>::full() const{
	return top+1 >= size;
}

template<class T>
ostream& operator<<(ostream& os, const Stack<T>& s){
	s.msg("Stack contents:");
	int t=s.top;
	while(t>s.EmptyStack)
		cout<<s.elements[t--]<<endl;
	return os;
}