从参数包创建索引和类型对

Question

我正在尝试创建一个元组或一对索引参数包。

下面是代码想要实现的示例：

  {
    using Indices =
        std::tuple< IndexTypePair< 0, int >, IndexTypePair< 1, int >, IndexTypePair< 2, float > >;
    test2< Indices, 0 >();
    test2< Indices, 1 >();
    test2< Indices, 2 >();
  }

这里是概括：

  {
    template < typename... T >
    using make_index_tuple = tuple< IndexTypePair< Index_v< T, T... >, T >... >;
    using Indices = make_index_tuple< int, int, float >;
    test2< Indices, 0 >();
    test2< Indices, 1 >();
    test2< Indices, 2 >();
  }

此解决方案基于此问题的答案： How to get the index of a type in a variadic type pack?

我遇到的一个问题是创建可变参数模板别名，前面的代码给出了以下错误：

$ make index-sequence clang++-3.8 -Wall -Werror -Wextra -std=c++14
-pedantic -Wconversion -stdlib=libc++ -O3 -pthread    index-sequence.cpp   -o index-sequence index-sequence.cpp:50:5: error: expected expression
    template < typename... T >
    ^ index-sequence.cpp:52:21: error: unknown type name 'make_index_tuple'
    using Indices = make_index_tuple< int, int, float >;
                    ^ ...

---

这里有一些其他的点点滴滴，使这个代码工作：

#include <array>
#include <cstddef>
#include <iostream>
#include <type_traits>
#include <utility>

template < typename T, typename... Ts >
struct Index;

template < typename T, typename... Ts >
struct Index< T, T, Ts... > : std::integral_constant< std::size_t, 0 >
{
};

template < typename T, typename U, typename... Ts >
struct Index< T, U, Ts... > : std::integral_constant< std::size_t, 1 + Index< T, Ts... >::value >
{
};

template < typename T, typename... Ts >
constexpr std::size_t Index_v = Index< T, Ts... >::value;

template < size_t i, typename T >
struct IndexTypePair
{
  static constexpr size_t index{i};
  using Type = T;
};

template < typename Idx, size_t i >
void test2()
{
  using ITP = typename std::tuple_element< i, Idx >::type;
  typename ITP::Type v{};
  //....
  std::cout << ITP::index << ' ' << v << std::endl;
}

Answer 1

使用嵌套结构设置以及标准make_index_sequence 来帮助您：

template < typename ... T >
struct make_index_type_tuple_helper
{    
    template< typename V >
    struct idx;

    template< size_t ... Indices >
    struct idx<std::index_sequence<Indices...>>
    {
        using tuple_type = std::tuple<IndexTypePair<Indices, T>...>;
    };

    using tuple_type = typename idx<std::make_index_sequence<sizeof...(T)>>::tuple_type;
};

template < typename ... T>
using make_index_type_tuple = typename make_index_type_tuple_helper<T...>::tuple_type;

然后你可以完全按照你的意愿使用它（live demo）：

using Indices = make_index_type_tuple< int, int, float >;
test2< Indices, 0 >();
test2< Indices, 1 >();
test2< Indices, 2 >();