如何从结构中提取最高索引的专业化？ [英] How to extract the highest-indexed specialization from a structure?

问题描述

我想做一些模板元编程，我发现需要提取某种类型的某些结构的专业化的最高索引。

I'm trying to do some template metaprogramming and I'm finding the need to "extract" the highest index of a specialization of some structure in some type.

例如，如果我有一些类型：

For example, if I have some types:

struct A
{
    template<unsigned int> struct D;
    template<> struct D<0> { };
};

struct B
{
    template<unsigned int> struct D;
    template<> struct D<0> { };
    template<> struct D<1> { };
};

struct C
{
    template<unsigned int> struct D;
    template<> struct D<0> { };
    template<> struct D<1> { };
    template<> struct D<2> { };
};

然后我可以这样写一个元功能：

How can I then write a metafunction like this:

template<class T>
struct highest_index
{
    typedef ??? type;
    // could also be:   static size_t const index = ???;
};

给我最高索引 D

推荐答案

这是第一个版本，它为您提供了定义特殊化的最大索引。

This is the first version which gets you the maximum index for which specialization is defined. From this, you will get the corresponding type!

template<class T>
struct highest_index
{
  private:
     template<int i>
     struct is_defined {};

     template<int i>
     static char f(is_defined<sizeof(typename T::template D<i>)> *);

     template<int i>
     static int f(...);

     template<int i>
     struct get_index;

     template<bool b, int j>
     struct next
     {
        static const int value = get_index<j>::value;
     };
     template<int j>
     struct next<false, j>
     {
        static const int value = j-2;
     };
     template<int i>
     struct get_index
     {
        static const bool exists = sizeof(f<i>(0)) == sizeof(char);
        static const int value = next<exists, i+1>::value;
     };

    public:
     static const int index = get_index<0>::value; 
};

测试代码：

Test code:

#include <iostream>

struct A
{
    template<unsigned int> struct D;
};
template<> struct A::D<0> { };
template<> struct A::D<1> { };

struct B
{
    template<unsigned int> struct D;
};
template<> struct B::D<0> { };
template<> struct B::D<1> { };
template<> struct B::D<2> { };


int main()
{
    std::cout << highest_index<A>::index << std::endl;
    std::cout << highest_index<B>::index << std::endl;
}

输出：

1
2

Live demo 。 ： - ）

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