非类型模板参数(参考《C++ Templates 英文版第二版》)

Chapter 3

3.1 非类型类模板参数

与前几章的简单例子不同,你也可以通过std::array实例化一个固定大小的栈,这样做的优点在于内存管理,

#include <array>
#include <cassert>template<typename T, std::size_t Maxsize>
class Stack {private:std::array<T,Maxsize> elems; // elementsstd::size_t numElems;        // current number of elementspublic:Stack();                   // constructorvoid push(T const& elem);  // push elementvoid pop();                // pop elementT const& top() const;      // return top elementbool empty() const {       // return whether the stack is emptyreturn numElems == 0;}std::size_t size() const { // return current number of elementsreturn numElems;}
};template<typename T, std::size_t Maxsize>
Stack<T,Maxsize>::Stack (): numElems(0)                 // start with no elements
{// nothing else to do
}template<typename T, std::size_t Maxsize>
void Stack<T,Maxsize>::push (T const& elem)
{assert(numElems < Maxsize);elems[numElems] = elem;    // append element++numElems;                // increment number of elements
}template<typename T, std::size_t Maxsize>
void Stack<T,Maxsize>::pop ()
{assert(!elems.empty());--numElems;                // decrement number of elements
}template<typename T, std::size_t Maxsize>
T const& Stack<T,Maxsize>::top () const
{assert(!elems.empty());return elems[numElems-1];  // return last element
}

运行一下:

#include "stacknontype.hpp"
#include <iostream>
#include <string>int main()
{Stack<int, 20>         int20Stack;     // stack of up to 20 intsStack<int, 40>         int40Stack;     // stack of up to 40 intsStack<std::string, 40> stringStack;    // stack of up to 40 strings// manipulate stack of up to 20 intsint20Stack.push(7);std::cout << int20Stack.top() << '\n';int20Stack.pop();// manipulate stack of up to 40 stringsstringStack.push("hello");std::cout << stringStack.top() << '\n';stringStack.pop();
}

你也可以设置默认参数

template<typename T, std::size_t Maxsize = 100>,但最好不要这样做,因为栈大小最好还是有程序员自己控制

3.2 非类型函数模板参数

你也可以为函数定义非类型模板参数

template<int val,typename  T>
int addval(T a)
{return a + val;
}
int main()
{std::cout << addval<10>(10) << std::endl;
}

这种函数是很有用的,可以将它作为参数.

例如,如果你可以使用STL,你可以传入一个函数模板的实例,使集合中的每个数加一个值

	std::vector<int> source{ 1,4,5 };std::vector<int> dest{0,0,0};std::transform(source.begin(), source.end(), dest.begin(), addval<10,int>);for (auto value : dest){std::cout << value << std::endl;}

如果你制定非类型函数模板参数是int那么就无法使用其他类型,是有什么方法可以做到自动推导其他类型呢?

其实,你可以指定一个根据之前的模板参数推断出来的模板参数

template<auto val,typename  T = decltype(val)>
T addval(T a)
{return a + val;
}

3.3 非类型模板参数的限制

非类型模板参数有一些限制,例如,它们只能是整数,(对象,函数,成员)指针,(对象,函数,成员)引用,std::nullptr

浮点数指针和类对象不可以作为非类型模板参数

3.4 模板参数类型auto

C++17 ,你可以定义一个非类型模板模板参数去普遍的接收任何类型,使用这个特性,你可以定义一个更加一般的固定大小栈

#include <array>
#include <cassert>template<typename T, auto Maxsize>
class Stack {public:using size_type = decltype(Maxsize);private:std::array<T,Maxsize> elems; // elementssize_type numElems;          // current number of elementspublic:Stack();                   // constructorvoid push(T const& elem);  // push elementvoid pop();                // pop element T const& top() const;      // return top elementbool empty() const {       // return whether the stack is emptyreturn numElems == 0;}size_type size() const {   // return current number of elementsreturn numElems;}
};// constructor
template<typename T, auto Maxsize>
Stack<T,Maxsize>::Stack (): numElems(0)                 // start with no elements
{// nothing else to do
}template<typename T, auto Maxsize>
void Stack<T,Maxsize>::push (T const& elem)
{assert(numElems < Maxsize);elems[numElems] = elem;    // append element++numElems;                // increment number of elements
}template<typename T, auto Maxsize>
void Stack<T,Maxsize>::pop ()
{assert(!elems.empty());--numElems;                // decrement number of elements
}template<typename T, auto Maxsize>
T const& Stack<T,Maxsize>::top () const
{assert(!elems.empty());return elems[numElems-1];  // return last element
}

运行:

Stack<int, 20u>        int20Stack;     // stack of up to 20 intsStack<std::string, 40> stringStack;    // stack of up to 40 strings// manipulate stack of up to 20 intsint20Stack.push(7);std::cout << int20Stack.top() << '\n';auto size1 = int20Stack.size();// manipulate stack of up to 40 stringsstringStack.push("hello");std::cout << stringStack.top() << '\n';auto size2 = stringStack.size();if (!std::is_same_v<decltype(size1), decltype(size2)>) {std::cout << "size types differ" << '\n';}