非类型模板参数

对于函数模板与类模板，模板参数并不局限于类型，普通值也可以作为模板参数。在基于类型参数的模板中，你定义了一些具体的细节来加以确定代码，直到代码被调用时这些细节才被真正的确定。但是在这里，我们面对的是这些细节是值，而不是类型，当要使用基于值的模板时，必须显式地指定这些值，才能够对模板进行实例化。

非类型的类模板参数

 

创建类的头文件

#include<stdexcept>
#include<iostream>
using namespace std;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;}
};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 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 out_of_range("Stack<>::pop(): stack is full");}--numElems;
}template<typename T, int MAXSIZE>
T Stack<T, MAXSIZE>::top() const
{if(numElems <= 0) {throw out_of_range("Stack<>::top(): stack is full");}return elems[numElems - 1];
}

#include<stdexcept>
#include<iostream>
using namespace std;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;}
};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 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 out_of_range("Stack<>::pop(): stack is full");}--numElems;
}template<typename T, int MAXSIZE>
T Stack<T, MAXSIZE>::top() const
{if(numElems <= 0) {throw out_of_range("Stack<>::top(): stack is full");}return elems[numElems - 1];
}

实现代码：

#include<iostream>
#include<vector>
#include<deque>
#include<stdexcept>
#include<string>
#include<cstdlib>
#include "stack4.h"
using namespace std;int main()
{try {Stack<int, 20> int20Stack;Stack<int, 40> int40Stack;Stack<string, 40> stringStack;int20Stack.push(7);cout<<int20Stack.top()<<endl;int20Stack.pop();stringStack.push("hello");cout<<stringStack.top()<<endl;stringStack.pop();stringStack.pop();}catch(exception const& ex) {cerr<<"Exception: "<<ex.what()<<endl;//return EXIT_FAILURE;}cin.get();  return 0;
}

#include<iostream>
#include<vector>
#include<deque>
#include<stdexcept>
#include<string>
#include<cstdlib>
#include "stack4.h"
using namespace std;int main()
{try {Stack<int, 20> int20Stack;Stack<int, 40> int40Stack;Stack<string, 40> stringStack;int20Stack.push(7);cout<<int20Stack.top()<<endl;int20Stack.pop();stringStack.push("hello");cout<<stringStack.top()<<endl;stringStack.pop();stringStack.pop();}catch(exception const& ex) {cerr<<"Exception: "<<ex.what()<<endl;//return EXIT_FAILURE;}cin.get();  return 0;
}

MAXSIZE是新加入的第二个模板参数，类型为int，它指定了数组最多可包含的栈元素的个数
同样，我们可以为模板参数指定缺省值：

template<typename T = int, int MAXSIZE = 100>
class Stack {...
};

　　非类型的函数模板参数

你也可以为函数模板定义非类型参数。例如:

template<typename T, int VAL>
T addValue(T const& x)
{return x + VAL:
}

借助于STL，可以传递这个函数模板的实例化给集中的每一个元素，让他们都增加一个整数值：

std::transform(source.begin(), source.end(), dest.begin(), (int(*)(int const&))addValue<int, 5>);

　　非类型模板参数的限制

非类型模板参数是有限制的，通常而言，它们可以是常整数（包括枚举值）或者指向外部链接对象的指针。

浮点数和类对象是不允许作为非类型模板参数的：

template<double VAT>
double process(double v) //error
{return V * VAT;
}template<string name>  //error
class MyClass {...
};

  另外，你也不能使用全局指针作为模板参数：

template<char const* name>
class MyClass{...
};char const* s = "hello";
MyClass<s> x;   //s是一个指向内部连接对象的指针

但是你可以这样使用：

template<char const* name>
class MyClass {...
};extern char const s[] = "hello";
MyClass<s> x;  //OK

全局字符数组s由"hello"初始化，是一个外部链接对象