标签：c++
C++模板变参实现Loki中的Length和TypeAt
一，原Loki中的Length和TypeAt模拟实现如下
1，模板文件
/**********************************
*
* Author : szyu
*
* Date : 2017.1.7
*
**************************************/
#ifndef __SZYU_LOKI__
#define __SZYU_LOKI__
#include <iostream>
#include <vector>
template <typename T, typename U>
class TypeList
{
public:
typedef T Head;
typedef U Tail;
};
class NullType;
#define TYPELIST_1(type1) TypeList<type1, NullType>
#define TYPELIST_2(type1, type2) TypeList<type1, TYPELIST_1(type2)>
#define TYPELIST_3(type1, type2, type3) TypeList<type1, TYPELIST_2(type2, type3)>
/*******************************************
*
* Loki Length
*
**********************************************/
template <typename T> class Length;
template <typename T, typename U>
class Length<TypeList<T, U> >
{
public:
enum { value = 1 + Length<U>::value };
};
template <>
class Length<NullType>
{
public:
enum { value = 0 };
};
/*******************************************
*
* Loki TypeAt
*
**********************************************/
template <typename T, unsigned int index> class TypeAt;
template <typename T, typename U, unsigned int index>
class TypeAt<TypeList<T, U>, index>
{
public:
typedef typename TypeAt<U, index - 1>::Result Result;
};
template <typename T, typename U>
class TypeAt<TypeList<T, U>, 0>
{
public:
typedef T Result;
};
#endif2，测试用例
/**********************************
*
* Author : szyu
*
* Date : 2017.1.7
*
**************************************/
#include "loki.h"
void
test1()
{
std::cout << "Length<TYPELIST_1(int)>..." << Length<TYPELIST_1(int)>::value << std::endl;
std::cout << "Length<TYPELIST_2(int, int)>..." << Length<TYPELIST_2(int, int)>::value << std::endl;
std::cout << "Length<TYPELIST_3(int, int, int)>..." << Length<TYPELIST_3(int, int, int)>::value << std::endl;
std::cout << "************************************" << std::endl;
std::vector<int> IntVector;
std::cout << typeid(TypeAt<TYPELIST_3( double, std::string, std::vector<int>), 0>::Result).name() << std::endl;
std::cout << typeid(TypeAt<TYPELIST_3( double, std::string, std::vector<int>), 1>::Result).name() << std::endl;
std::cout << typeid(TypeAt<TYPELIST_3( double, std::string, std::vector<int>), 2>::Result).name() << std::endl;
TypeAt<TYPELIST_3( double, std::string, std::vector<int>), 0>::Result var1;
var1 = 10.10;
TypeAt<TYPELIST_3( double, std::string, std::vector<int>), 1>::Result var2;
var2 = "abc";
TypeAt<TYPELIST_3( double, std::string, std::vector<int>), 2>::Result var3;
var3 = IntVector;
}
int
main( int argc, char *argv[] )
{
test1();
return 0;
}3，执行结果如下：
二，使用C++11模板变参特性改进Length和TypeAt
在不使用模板变参会出现繁杂的TypeList嵌套问题，虽然使用宏定义能增加代码可读性，但是也因此需要定义大量的宏来支持。当出现几百个参数（极端）情况下，那需要定义几百个宏。显然比较麻烦。C++11提供了模板的变参的支持，故变参实现Length和TypeAt如下：
1，头文件
/**********************************
*
* Author : szyu
*
* Date : 2017.1.6
*
***********************************/
#ifndef __SZYU_LOKI__
#define __SZYU_LOKI__
#include <iostream>
#include <vector>
/***********************************
*
* Loki length
*
***************************************/
template <typename T, typename... Args>
class Length
{
public:
enum { value = 1 + Length<Args...>::value };
};
template <typename T>
class Length<T>
{
public:
enum { value = 1 };
};
/***********************************
*
* Loki TypeAt
*
***************************************/
template <unsigned int index, typename T, typename... Args>
class TypeAt
{
public:
typedef typename TypeAt<index - 1, Args...>::Result Result;
};
template <typename T, typename... Args>
class TypeAt<1, T, Args...>
{
public:
typedef T Result;
};
#endif2，测试用例
/**********************************
*
* Author : szyu
*
* Date : 2017.1.6
*
***********************************/
#include "TypeList.h"
void
test1()
{
std::cout << "Length<int>..." << Length<int>::value << std::endl;
std::cout << "Length<int, int>..." << Length<int, int>::value << std::endl;
std::cout << "Length<int, int, char>..." << Length<int, int, char>::value << std::endl;
std::cout << "********************************" << std::endl;
std::vector<int> IntVector;
std::cout << typeid(TypeAt<1, int, std::string, std::vector<int> >::Result).name() << std::endl;
std::cout << typeid(TypeAt<2, int, std::string, std::vector<int> >::Result).name() << std::endl;
std::cout << typeid(TypeAt<3, int, std::string, std::vector<int> >::Result).name() << std::endl;
TypeAt<1, int, std::string, std::vector<int> >::Result var1;
var1 = 10;
TypeAt<2, int, std::string, std::vector<int> >::Result var2;
var2 = "ssss";
TypeAt<3, int, std::string, std::vector<int> >::Result var3;
var3 = IntVector;
}
int
main( int argc, char *argv[] )
{
test1();
return 0;
}3，执行结果
TypeAt的实现中，由于变参要位于最后一个参数，故把位置参数移到了模板第一个参数。
住：
Loki中Length和TypeAt的具体说明参见《c++设计新思维》
本文出自 “11988200” 博客，请务必保留此出处http://11998200.blog.51cto.com/11988200/1889957
[C++模板]Loki中的Length和TypeAt使用C++11特性改进
标签：c++
原文地址：http://11998200.blog.51cto.com/11988200/1889957
