类型访问者在c ++中的类型链接 [英] type visitor over typelist in c++

问题描述

我正在寻找一种方法来实现类型访问者超过c ++类型串。

I'm looking for a way to implement 'type' visitor over c++ typelist. Here, I meant type visitor as to execute particular operator (such as sizeof) over types in typelist.

从概念上讲，我想要做的是：

Conceptually what I want to do is:

typedef TYPELIST_3(bool, int, double) tl;
size_t tl_size     = TL_sum_size<tl>();  // 13 = 1+4+8
size_t tl_min_size = TL_min_size<tl>();  // 1
size_t tl_max_size = TL_max_size<tl>();  // 8
vector<size_t> tl_sizes = TL_list_size<tl>();  // {1, 4, 8}
TL_AddCounter<tl>(3); // Call AddCounter(3) for each type in typelist

当然，每个函数应该类型串。
示例在类型串中使用sizeof和 static void T :: addCounter（int x）（以跟踪类型使用次数）。
通常，我想对任意参数执行任何关于类型的任何'静态'操作。

Of course, each function should be templetized over typelist. The example uses sizeof, and static void T::addCounter(int x) in the typelist (to track how many time that type is used). Generically, I want to execute any arbitrary 'static' operation about type with arbitrary parameters.

首先，以上的任何函数是否可能？如果是，我该怎么办？

Well, first of all, is any of functions above possible? if yes, how can I do? I am not sure how to iterate through typelist.

推荐答案

这样可能会有效：

#include <type_traits>

template <typename T>
struct SizeVisitor : std::integral_constant<unsigned int, sizeof(T)> { };

template <template <typename> class Visitor, typename ...Args> struct Visit;

template <template <typename> class Visitor, typename T, typename ...Rest>
struct Visit<Visitor, T, Rest...> : std::integral_constant<unsigned int,
    Visitor<T>::value + Vist<Visitor, Rest...>::value> { };

template <template <typename> class Visitor>
struct Visit<Visitor, T> : std::integral_constant<unsigned int, 0U> { };

现在您可以说访问< SizeVisitor，double，char，int> ;: ：value 。

您可以修改此方法以接受一个单独的 tuple ）类，而不是类型列表的裸体类型，你还可以将 SizeVisitor 概括为一些任意的二进制函数，类似于 std :: accumulate （而不是 0u 你会有蓄电池的中性元件）。

You can modify this approach to take a single (say tuple) class instead of the naked types for the type list, and you can also generalize the SizeVisitor into some arbitrary binary functor, akin to std::accumulate (and instead of 0u you'd have the accumulator's neutral element).

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