文章最后给出了汇总的代码，可直接运行
1. typelist是什么
typelist是一种用来操作类型的容器。和我们所熟知的vector、list、deque类似，只不过typelist存储的不是变量，而是类型。
typelist简单来说就是一个类型容器，能够提供一系列的操作。
本文将展示使用元编程实现typelist。
2. 要实现的typelist的接口
在此列举一下即将要编写的typelist的所有接口：
- typelist：要实现的容器类型。
- front<typelist>：获取容器中的第一个元素。
- size<typelist>：获取容器的元素数量。
- pop_front<typelist, elem>：移出第一个元素。
- push_front<typelist, elem>：向开头插入一个元素。
- push_back<typelist, elem>：向结尾插入一个元素。
- replace_front<typelist, elem>：替换第一个元素。
- is_empty<typelist>：判断是否为空。
- find<typelist, index>：查找下标为index的元素。
- get_maxsize_type<typelist>：容器中尺寸（sizeof）最大的元素。
- reverse<typelist>：翻转容器中的元素。
3. 接口实现
3.1 容器
template<typename... Elems>
struct typelist{};3.2 front<typelist>
template<typename TPLT>// typelist的简写
struct front;
struct front<typelist<FirstElem, OtherElems...>>
{
using type = FirstElem;
};3.3 size<typelist>
template<typename TPLT>
struct size;
template<typename... Elems>
struct size<typelist<Elems...>>
{
static inline const value = sizeof...(Elems);
};3.4 pop_front<typelist, elem>
template<typename TPLT>
struct pop_front;
template<typename FirstElem, typename... OtherElems>
struct pop_front<typelist<FirstElem, OtherElems...>
{
using type = typelist<OtherrElems...>;
};3.5 push_front<typelist, elem>
template<typename TPLT, typename newElem>
struct push_front;
template<typename... Elems, typename newElem>
struct push_front<typelist<Elems...>, newElem>
{
using type = typelist<newElem, Elems...>;
};3.6 push_back<typelist, elem>
template<typename TPLT, typename newElem>
struct push_back;
template<typename... Elems, typename newElem>
struct push_back<typelist<Elems...>, newElem>
{
using type = typelist<Elems..., newElem>;
};3.7 replace_front<TPLT, elem>
template<typename TPLT, typename Elem>
struct replace_front;
template<typename FirstElem, typename... OtherElems, typename Elem>
struct replace_front<typelist<FirstElem, OtherElems...>, Elem>
{
using type = typelist<Elem, OtherElems...>;
};3.8 is_empty<TPLT>
template<typename TPLT>
struct is_empty;
template<typename... Elems>
struct is_empty
{
static inline const bool value = sizeof...(Elems) == 0;
};3.9 find<typelist, index>
template<typename TPLT, size_t index>
struct find : find<typename pop_front<TPLT>::type, index - 1>
{
};
template<typename TPLT>
struct find<TPLT, 0> :front<TPLT>
{
};3.10 get_maxsize_type<typelist>
template<typename TPLT>
struct get_maxsize_type
{
private:
using FirstType = typename front<TPLT>::type;
using RemainLT = typename pop_front<TPLT>::type;
using RemainMaxType = typename get_maxsize_type<RemainLT>::type;
public:
using type = conditional_t < (sizeof(FirstType) > sizeof(RemainMaxType)),
FirstType, RemainMaxType >;
};
template<typename Elem>
struct get_maxsize_type<typelist<Elem>>
{
using type = Elem;
};
template<>
struct get_maxsize_type<typelist<>>;3.11 reverse<typelist>
template<typename TPLT>
struct reverse
{
private:
using FirstElem = typename front<TPLT>::type;
using RemainTL = typename pop_front<TPLT>::type;
using ReversedRemainTL = typename reverse<RemainTL>::type;
public:
using type = typename push_back<ReversedRemainTL, FirstElem>::type;
};
template<>
struct reverse<typelist<>>
{
using type = typelist<>;
};或
template<typename TPLT, bool = is_empty<TPLT>::value>
struct reverse;
template<typename TPLT>
struct reverse<TPLT, false>
{
private:
using FirstElem = typename front<TPLT>::type;
using RemainTL = typename pop_front<TPLT>::type;
using ReversedRemainTL = typename reverse<RemainTL>::type;
public:
using type = typename push_back<ReversedRemainTL, FirstElem>::type;
};
template<typename TPLT>
struct reverse<TPLT, true>
{
using type = typelist<>;
};4. 完整代码
#include <iostream>
#include <string>
#include "boost/type_index.hpp"
using namespace std;
template<typename T>
class TypeGetter
{
public:
static inline const std::string name = boost::typeindex::type_id_with_cvr<T>().pretty_name();
};
namespace myTypeList
{
template<typename... T>
struct typelist {};
template<typename TPLT>
struct front;
template<typename FirstType, typename... Others>
struct front < typelist<FirstType, Others...>>
{
using type = FirstType;
};
template<typename TPLT>
struct size;
template<typename... Args>
struct size<typelist<Args...>>
{
static const inline size_t value = sizeof...(Args);
};
template<typename TPLT>
struct is_empty;
template<typename... Elems>
struct is_empty<typelist<Elems...>>
{
static constexpr bool value = sizeof...(Elems) == 0;
};
template<typename TPLT>
struct pop_front;
template<typename FirstType, typename... Others>
struct pop_front< typelist<FirstType, Others...> >
{
using type = typelist<Others...>;
};
template<typename TPLT, typename T>
struct push_front;
template<typename... Types, typename T>
struct push_front<typelist<Types...>, T>
{
using type = typelist<T, Types...>;
};
template<typename TPLT, typename T>
struct push_back;
template<typename... Types, typename T>
struct push_back<typelist<Types...>, T>
{
using type = typelist<Types..., T>;
};
template<typename TPLT, typename newElem>
struct replace_front;
template<typename FirstElem, typename... OtherElems, typename newElem>
struct replace_front<typelist<FirstElem, OtherElems...>, newElem>
{
using type = typelist<newElem, OtherElems...>;
};
template<typename TPLT, size_t index>
struct find : find<typename pop_front<TPLT>::type, index - 1>
{
};
/*
和下面的写法是等价的
template<typename TPLT, size_t index>
struct find
{
using type = typename find<typename pop_front<TPLT>::type, index - 1>::type;
};
*/
template<typename TPLT>
struct find<TPLT, 0> :front<TPLT>
{
};
/*
get_maxsize_type: 获取typelist中尺寸最大的类型
*/
template<typename TPLT>
struct get_maxsize_type
{
private:
using FirstType = typename front<TPLT>::type;
using RemainLT = typename pop_front<TPLT>::type;
using RemainMaxType = typename get_maxsize_type<RemainLT>::type;
public:
using type = conditional_t < (sizeof(FirstType) > sizeof(RemainMaxType)),
FirstType, RemainMaxType >;
};
template<typename Elem>
struct get_maxsize_type<typelist<Elem>>
{
using type = Elem;
};
template<>
struct get_maxsize_type<typelist<>>;
/*
reverse: 翻转typelist
*/
/*
// 版本一
template<typename TPLT>
struct reverse
{
private:
using FirstElem = typename front<TPLT>::type;
using RemainTL = typename pop_front<TPLT>::type;
using ReversedRemainTL = typename reverse<RemainTL>::type;
public:
using type = typename push_back<ReversedRemainTL, FirstElem>::type;
};
template<>
struct reverse<typelist<>>
{
using type = typelist<>;
};
*/
template<typename TPLT, bool = is_empty<TPLT>::value>
struct reverse;
template<typename TPLT>
struct reverse<TPLT, false>
{
private:
using FirstElem = typename front<TPLT>::type;
using RemainTL = typename pop_front<TPLT>::type;
using ReversedRemainTL = typename reverse<RemainTL>::type;
public:
using type = typename push_back<ReversedRemainTL, FirstElem>::type;
};
template<typename TPLT>
struct reverse<TPLT, true>
{
using type = typelist<>;
};
}
class A {};
int main()
{
using TPL_1 = myTypeList::typelist<char, short, int, A, double>;
using TPL_2 = myTypeList::typelist<>;
cout << "TPL_1 的第一个类型为" << TypeGetter< myTypeList::front<TPL_1>::type >::name << endl;
//cout << "TPL_2 的第一个类型为" << TypeGetter< myTypeList::front<TPL_2>::type >::name << endl;
cout << "TPL_1 的size为：" << myTypeList::size<TPL_1>::value << endl;
cout << "TPL_2 的size为：" << myTypeList::size<TPL_2>::value << endl;
cout << "TPL_1 的pop_front为：" << TypeGetter< myTypeList::pop_front< TPL_1 >::type >::name << endl;
//cout << "TPL_2 的pop_front为：" << TypeGetter< myTypeList::pop_front< TPL_2 >::type >::name << endl;
cout << "TPL_1 push_front bool 为：" << TypeGetter< myTypeList::push_front<TPL_1, bool>::type>::name << endl;
cout << "TPL_2 push_front bool 为：" << TypeGetter< myTypeList::push_front<TPL_2, bool>::type>::name << endl;
cout << "TPL_1 push_back bool 为：" << TypeGetter< myTypeList::push_back<TPL_1, bool>::type>::name << endl;
cout << "TPL_2 push_back bool 为：" << TypeGetter< myTypeList::push_back<TPL_2, bool>::type>::name << endl;
cout << "TPL_1 replace_front with char 为：" << TypeGetter < myTypeList::replace_front< TPL_1, char >::type>::name << endl;
cout << "TPL_1 index 2 type 为：" << TypeGetter< myTypeList::find<TPL_1, 2>::type > ::name << endl;
cout << "TPL_1 max size type 为：" << TypeGetter<myTypeList::get_maxsize_type<TPL_1>::type>::name << endl;
cout << "TPL_1 为：" << TypeGetter<TPL_1>::name << endl;
cout << "TPL_1 reverse 为：" << TypeGetter<myTypeList::reverse<TPL_1>::type>::name << endl;
return 0;
}
运行结果如下：
