c++ template笔记(3)非类型模板参数nontype template parameters

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算是模板使用的一种技巧

用模板当数组元素参数维度

#include <stdexcept>template <typename T, int MAXSIZE>class Stack {  private:    T elems[MAXSIZE];        // elements    int numElems;            // current number of elements  public:    Stack();                  // constructor    void push(T const&);      // push element    void pop();               // pop element    T top() const;            // return top element    bool empty() const {      // return whether the stack is empty        return numElems == 0;    }    bool full() const {       // return whether the stack is full        return numElems == MAXSIZE;    }};// constructortemplate <typename T, int MAXSIZE>Stack<T,MAXSIZE>::Stack ()  : numElems(0)               // start with no elements{    // nothing else to do}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;   // append element    ++numElems;               // increment number of elements}template<typename T, int MAXSIZE>void Stack<T,MAXSIZE>::pop (){    if (numElems <= 0) {        throw std::out_of_range("Stack<>::pop(): empty stack");    }    --numElems;               // decrement number of elements}template <typename T, int MAXSIZE>T Stack<T,MAXSIZE>::top () const{    if (numElems <= 0) {        throw std::out_of_range("Stack<>::top(): empty stack");    }    return elems[numElems-1];  // return last element}

使用时就可以根据需要指定最大的元素个数,不用造成资源的浪费

#include <iostream>#include <string>#include <cstdlib>#include "stack4.hpp"int main(){    try {        Stack<int,20>         int20Stack;     // stack of up to 20 ints        Stack<int,40>         int40Stack;     // stack of up to 40 ints        Stack<std::string,40> stringStack;    // stack of up to 40 strings        // manipulate stack of up to 20 ints        int20Stack.push(7);        std::cout << int20Stack.top() << std::endl;        int20Stack.pop();        // manipulate stack of up to 40 strings        stringStack.push("hello");        std::cout << stringStack.top() << std::endl;         stringStack.pop();        stringStack.pop();    }    catch (std::exception const& ex) {        std::cerr << "Exception: " << ex.what() << std::endl;        return EXIT_FAILURE;  // exit program with ERROR status    }}

注意:被实例化出来的int20Stack和int40Stack不可以隐式转换.

模板成员参数

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

特化的参数，用于不在于类型,直接适用于函数中

注意点:浮点类型不可以作为nontype template parameters

以下定义是错误的

//errortemplate <double T>double process(double v){    return v*T;}

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