非类型的函数模板参数

#include <stdexcept>
template <typename T,int MAXSIZE>
class Stack
{
private:
    T elems[MAXSIZE];
    int numElems;
public:
    Stack();
    void push(T const&);
    void pop();
    T top() const;
    bool empty() const{return numElems==0;}
    bool full() const {return numElems==MAXSIZE;}
};

//构造函数
template <typename T,int MAXSIZE>
Stack<T,MAXSIZE>::Stack():numElems(0)
{    
}
template <typename T,int MAXSIZE>
void Stack<T,MAXSIZE>::push(T const& elem)
{
    if (numElems==MAXSIZE)
    {
        throw std::out_of_range("Stack<>::push(): Stack is full");
    }
    elems[numElems]=elem;
    ++numElems;
}
template <typename T,int MAXSIZE>
void Stack<T,MAXSIZE>::pop()
{
    if (numElems==0)
    {
        throw std::out_of_range("Stack<>::pop(): Stack is empty");
    }
    --numElems;
}
template <typename T,int MAXSIZE>
T Stack<T,MAXSIZE>::top() const
{
    if (numElems<=0)
    {
        throw std::out_of_range("Stack<>::pop(): Stack is empty");
    }
    return elems[numElems-1];
}

#include <iostream>
#include <string>
#include <cstdlib>
#include "1.h"
int main()
{
    try
    {
        Stack<int,20> int20Stack;  //可以存储20个int元素的栈
        Stack<int,40> int40Stack;  //可以存储40个int元素的栈
        Stack<std::string,40> stringStack; //可以存储40个string元素的栈
        //使用可存储20个int元素的栈
        int20Stack.push(7);
        std::cout<<int20Stack.top<<std::endl;
        int20Stack.pop();
        
        //使用可存储40个string元素的栈
        stringStack.push("123321");
        std::cout<<stringStack.top()<<std::endl;
        stringStack.pop();
        stringStack.pop();
        
    }
    catch(std::exception const& ex)
    {
        std::cerr<<"Exception: "<<ex.what()<<std::endl;
        return 1;
    }
    return 0;
}

非类型模板参数是有限制的,他们可以是常整数(包括枚举值)或者指向外部链接对象的指针, 但是浮点数,类对象和内部链接对象(string)是不允许作为非类型模板参数的