C++模板编程——typelist的实现
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雷同，只不过typelist存储的不是变量，而是类型。
typelist简朴来说就是一个类型容器，可以或许提供一系列的操作。
本文将展示使用元编程实现typelist。
2. 要实现的typelist的接口
在此罗列一下即将要编写的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>：向末端插入一个元素。
[*]replace_front<typelist, elem>：更换第一个元素。
[*]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。
[*]filter<typelist, Pre>:根据谓词Pre来过滤typelist中的类型。Pre应该是一个类模板，接受一个类型模板参数，并拥有一个bool类型的静态变量value，value为false时将该类型剔除typelist。
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<>;
}; 3.12 filter<typelist, Pre>
// 根据谓词过滤元素，谓词应该是一个类模板，接受一个类型参数，并且具有静态bool变量，false表示过滤掉该类型
template<typename TPLT, template<typename>typename Pre>
struct filter;
template<typename FirstElem, typename... OtherElems, template<typename>typename Pre>
struct filter<typelist<FirstElem, OtherElems...>, Pre>
{
private:
using RemainFilteredTL = typename filter<typelist<OtherElems...>, Pre>::type;
public:
using type = std::conditional_t< Pre<FirstElem>::value,
typename push_front<RemainFilteredTL, FirstElem>::type,
RemainFilteredTL >;
};
template<template<typename>typename Pre>
struct filter<typelist<>, Pre>
{
using type = typelist<>;
}; // 测试使用的类模板，用以剔除某个指定的类型
template<typename InputType, typename FilterType = double>
struct test_filter
{
static constexpr bool value = !std::is_same_v<InputType, FilterType>;//如果类型是int,就输出false,int不通过
};
4. 完备代码
#include <iostream>
#include <type_traits>
#include "typegetter.hpp"
using namespace std;
/*
*/
namespace myTypeList
{
template<typename... Elems>
struct typelist {};
template<typename TPLT>
struct front;
template<typename FirstElem, typename... OtherElems>
struct front < typelist<FirstElem, OtherElems...>>
{
using type = FirstElem;
};
template<typename TPLT>
struct size;
template<typename... Elems>
struct size<typelist<Elems...>>
{
static const inline size_t value = sizeof...(Elems);
};
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 FirstElem, typename... OtherElems>
struct pop_front< typelist<FirstElem, OtherElems...> >
{
using type = typelist<OtherElems...>;
};
template<typename TPLT, typename newElem>
struct push_front;
template<typename... Types, typename newElem>
struct push_front<typelist<Types...>, newElem>
{
using type = typelist<newElem, Types...>;
};
template<typename TPLT, typename newElem>
struct push_back;
template<typename... Types, typename newElem>
struct push_back<typelist<Types...>, newElem>
{
using type = typelist<Types..., newElem>;
};
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 FirstElem = typename front<TPLT>::type;
using RemainLT = typename pop_front<TPLT>::type;
using RemainMaxElem = typename get_maxsize_type<RemainLT>::type;
public:
using type = conditional_t < (sizeof(FirstElem) > sizeof(RemainMaxElem)),
FirstElem, RemainMaxElem >;
};
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<>;
};
// 根据谓词过滤元素，谓词应该是一个类模板，接受一个类型参数，并且具有静态bool变量，false表示过滤掉该类型
template<typename TPLT, template<typename>typename Pre>
struct filter;
template<typename FirstElem, typename... OtherElems, template<typename>typename Pre>
struct filter<typelist<FirstElem, OtherElems...>, Pre>
{
private:
using RemainFilteredTL = typename filter<typelist<OtherElems...>, Pre>::type;
public:
using type = std::conditional_t< Pre<FirstElem>::value,
typename push_front<RemainFilteredTL, FirstElem>::type,
RemainFilteredTL >;
};
template<template<typename>typename Pre>
struct filter<typelist<>, Pre>
{
using type = typelist<>;
};
}
class A {};
// 测试使用的类模板，用以剔除某个指定的类型
template<typename InputType, typename FilterType = double>
struct test_filter
{
static constexpr bool value = !is_same_v<InputType, FilterType>;//如果类型是int,就输出false,int不通过
};
int main()
{
using TPL_1 = myTypeList::typelist<char, short, char, int, double, int, long, A, double>;
using TPL_2 = myTypeList::typelist<>;
cout << "----------------------------------" << endl;
cout << "TPLT_1 为：" << TypeGetter<TPL_1>::name << endl;
cout << "TPLT_2 为：" << TypeGetter<TPL_2>::name << endl;
cout << "----------------------------------" << endl;
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;
cout << "TPL_1 经过int_filter过滤为：" << TypeGetter<myTypeList::filter<TPL_1, test_filter>::type>::name << endl;
cout << "TPL_2 经过int_filter过滤为：" << TypeGetter<myTypeList::filter<TPL_2, test_filter>::type>::name << endl;
return 0;
} typegetter.hpp的代码如下所示：
#pragma once
#include <string>
#include "boost/type_index.hpp"
template<typename T>
class TypeGetter
{
public:
static inline const std::string name = boost::typeindex::type_id_with_cvr<T>().pretty_name();
};
运行结果如下：
https://i-blog.csdnimg.cn/direct/dcd466d2320643ebbd04904b3d557319.png
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