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// The Art of C++ / Sequences
// Copyright (c) 2015 Daniel Frey
#ifndef TAOCPP_SEQUENCES_INCLUDE_TYPE_BY_INDEX_HPP
#define TAOCPP_SEQUENCES_INCLUDE_TYPE_BY_INDEX_HPP
#include <cstddef>
#include <type_traits>
#include "make_integer_sequence.hpp"
namespace tao
{
namespace seq
{
// based on http://stackoverflow.com/questions/18942322
namespace impl
{
template< std::size_t >
struct any
{
any( ... );
};
template< typename >
struct wrapper;
template< typename >
struct unwrap;
template< typename T >
struct unwrap< wrapper< T > >
{
using type = T;
};
template< typename >
struct get_nth;
template< std::size_t... Is >
struct get_nth< index_sequence< Is... > >
{
template< typename T >
static T deduce( any< Is & 0 >..., T*, ... );
};
}
template< std::size_t I, typename... Ts >
using type_by_index = impl::unwrap< decltype( impl::get_nth< make_index_sequence< I > >::deduce( std::declval< impl::wrapper< Ts >* >()... ) ) >;
template< std::size_t I, typename... Ts >
using type_by_index_t = typename type_by_index< I, Ts... >::type;
}
}
#endif // TAOCPP_SEQUENCES_INCLUDE_TYPE_BY_INDEX_HPP
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