63e88f80
 Hu Chunming
提交三方库
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/*!
@file
Defines a replacement for `std::decay`, which is sometimes too slow at
compile-time.
Copyright Louis Dionne 2013-2022
Distributed under the Boost Software License, Version 1.0.
(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
*/
#ifndef BOOST_HANA_DETAIL_DECAY_HPP
#define BOOST_HANA_DETAIL_DECAY_HPP
#include <boost/hana/config.hpp>
#include <type_traits>
namespace boost { namespace hana { namespace detail {
//! @ingroup group-details
//! Equivalent to `std::decay`, except faster.
//!
//! `std::decay` in libc++ is implemented in a suboptimal way. Since
//! this is used literally everywhere by the `make<...>` functions, it
//! is very important to keep this as efficient as possible.
//!
//! @note
//! `std::decay` is still being used in some places in the library.
//! Indeed, this is a peephole optimization and it would not be wise
//! to clutter the code with our own implementation of `std::decay`,
//! except when this actually makes a difference in compile-times.
template <typename T, typename U = typename std::remove_reference<T>::type>
struct decay {
using type = typename std::remove_cv<U>::type;
};
template <typename T, typename U>
struct decay<T, U[]> { using type = U*; };
template <typename T, typename U, std::size_t N>
struct decay<T, U[N]> { using type = U*; };
template <typename T, typename R, typename ...A>
struct decay<T, R(A...)> { using type = R(*)(A...); };
template <typename T, typename R, typename ...A>
struct decay<T, R(A..., ...)> { using type = R(*)(A..., ...); };
} }} // end namespace boost::hana
#endif // !BOOST_HANA_DETAIL_DECAY_HPP
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