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

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;
    }
};

// constructor
template <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

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