TypeList 之TypeAt 通过索引查找TypeList的类型_{typelist[typeindex] ? typelist[typeindex] : '点击选择-优快云博客

本文链接：https://blog.youkuaiyun.com/zhangxiaonanwin/article/details/6076606

本文通过一个具体的例子展示了如何使用C++模板元编程实现类型列表，并基于该类型列表进行长度计算及指定索引处类型的检索。这一机制能够帮助开发者在编译阶段完成某些类型的操作。

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#include <cstdlib>
#include <iostream>
#include <typeinfo>
#include <string>
using namespace std;
class NullType;
namespace TL
{
template<class T,class U>
struct TypeList
{
    typedef T head;
    typedef U tail;
};
}
using namespace TL;
#define TYPELIST_1(T1) TypeList<T1,NullType>
#define TYPELIST_2(T1,T2) TypeList<T1,TYPELIST_1(T2)>
#define TYPELIST_3(T1,T2,T3) TypeList<T1,TYPELIST_2(T2,T3)>
typedef TypeList<char,TypeList<signed char,unsigned char> > CharList;
template<class Tlist>struct Length;
template<>struct Length<NullType>
{
enum{value = 0};
};
template<class T,class U>
struct Length<TypeList<T,U> >
{
enum{value = 1 + Length<U>::value};
};
//---------------------------------------------------------------
//利用索引查找对象
template<class T,int U>struct TypeAt;
template<class head,class tail>
struct TypeAt<TypeList<head,tail>,0>
{
typedef head Result;
};
template<class head,class tail,int i>
struct TypeAt<TypeList<head,tail>,i>
{
typedef typename TypeAt<tail,i - 1>::Result Result;
};
int main(int argc, char *argv[])
{
    cout<<"索引-----------------------------------------"<<endl;
    typedef TYPELIST_3(char,int,string) MyTypeList;
    TypeAt<MyTypeList,0>::Result gg[] = "char";
    cout<<"(0)应该是char:"<<gg<<endl;
    TypeAt<MyTypeList,1>::Result h = 11;
    cout<<"(1)应该是int:"<<h<<endl;
    TypeAt<MyTypeList,2>::Result ii = "string";
    cout<<"(2)应该是char:"<<ii<<endl;
    system("PAUSE");
    return EXIT_SUCCESS;
}