49 lines
1.7 KiB
C++
49 lines
1.7 KiB
C++
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/*!
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@file
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Defines a replacement for `std::decay`, which is sometimes too slow at
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compile-time.
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@copyright Louis Dionne 2013-2016
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Distributed under the Boost Software License, Version 1.0.
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(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
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*/
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#ifndef BOOST_HANA_DETAIL_DECAY_HPP
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#define BOOST_HANA_DETAIL_DECAY_HPP
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#include <boost/hana/config.hpp>
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#include <type_traits>
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BOOST_HANA_NAMESPACE_BEGIN namespace detail {
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//! @ingroup group-details
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//! Equivalent to `std::decay`, except faster.
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//!
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//! `std::decay` in libc++ is implemented in a suboptimal way. Since
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//! this is used literally everywhere by the `make<...>` functions, it
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//! is very important to keep this as efficient as possible.
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//!
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//! @note
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//! `std::decay` is still being used in some places in the library.
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//! Indeed, this is a peephole optimization and it would not be wise
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//! to clutter the code with our own implementation of `std::decay`,
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//! except when this actually makes a difference in compile-times.
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template <typename T, typename U = typename std::remove_reference<T>::type>
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struct decay {
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using type = typename std::remove_cv<U>::type;
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};
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template <typename T, typename U>
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struct decay<T, U[]> { using type = U*; };
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template <typename T, typename U, std::size_t N>
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struct decay<T, U[N]> { using type = U*; };
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template <typename T, typename R, typename ...A>
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struct decay<T, R(A...)> { using type = R(*)(A...); };
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template <typename T, typename R, typename ...A>
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struct decay<T, R(A..., ...)> { using type = R(*)(A..., ...); };
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} BOOST_HANA_NAMESPACE_END
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#endif // !BOOST_HANA_DETAIL_DECAY_HPP
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