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| 1 | +#pragma once |
| 2 | + |
| 3 | +/* |
| 4 | + synopsis |
| 5 | +
|
| 6 | + template<class> |
| 7 | + struct tag; // not defined |
| 8 | +
|
| 9 | + template<std::size_t I, class T> |
| 10 | + struct type_to_index; |
| 11 | +
|
| 12 | + template<class T, class, class...> |
| 13 | + struct meta_index_of_impl; // for internal use |
| 14 | +
|
| 15 | + template<class T, class... Args> |
| 16 | + struct meta_index_of; |
| 17 | +*/ |
| 18 | +#include <utility> |
| 19 | + |
| 20 | +template<class> struct tag; |
| 21 | + |
| 22 | +template<std::size_t I, class T> |
| 23 | +struct type_to_index { |
| 24 | + std::integral_constant<std::size_t, I> operator()(T); |
| 25 | +}; |
| 26 | + |
| 27 | +template<class, class, class...> struct meta_index_of_impl; |
| 28 | + |
| 29 | +template<class T, std::size_t... Idx, class... Args> |
| 30 | +struct meta_index_of_impl<T, std::index_sequence<Idx...>, Args...> |
| 31 | + : type_to_index<Idx, tag<Args>**>... |
| 32 | +{ |
| 33 | + using type_to_index<Idx, tag<Args>**>:: operator()...; |
| 34 | + static const std::size_t value; |
| 35 | +}; |
| 36 | +template<class T, std::size_t... Idx, class... Args> |
| 37 | +inline constexpr std::size_t |
| 38 | + meta_index_of_impl<T, std::index_sequence<Idx...>, Args...>::value = |
| 39 | + decltype(meta_index_of_impl{}(static_cast<tag<T>**>(nullptr)))::value; |
| 40 | + |
| 41 | +// Given a type T followed by a sequence of types, the trait meta_index_of |
| 42 | +// computes the 0-based index of T in the sequence. It is required that |
| 43 | +// T is in the sequence and the sequence does not contain duplicated types. |
| 44 | +template<class T, class... Args> |
| 45 | +struct meta_index_of |
| 46 | + : std::integral_constant<std::size_t, |
| 47 | + meta_index_of_impl<T, std::index_sequence_for<Args...>, Args...>::value |
| 48 | +> {}; |
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