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115 lines
2.4 KiB
C++
115 lines
2.4 KiB
C++
//
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// Copyright (c) 2019 Vinnie Falco (vinnie.falco@gmail.com)
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//
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// Distributed under the Boost Software License, Version 1.0. (See accompanying
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// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
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//
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// Official repository: https://github.com/vinniefalco/json
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//
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#ifndef BOOST_PILFER_HPP
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#define BOOST_PILFER_HPP
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#include <memory>
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#include <new>
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#include <type_traits>
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/*
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Implements "pilfering" from P0308R0
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"Valueless Variants Considered Harmful"
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http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0308r0.html
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Author: Peter Dimov
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*/
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namespace boost {
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template<class T>
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class pilfered
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{
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T& t_;
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public:
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explicit
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constexpr
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pilfered(T&& t) noexcept
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: t_(t)
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{
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}
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constexpr T&
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get() const noexcept
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{
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return t_;
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}
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constexpr T*
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operator->() const noexcept
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{
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return std::addressof(t_);
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}
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};
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namespace detail {
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template<class>
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struct not_pilfered
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{
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};
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} // detail
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template<class T>
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struct is_pilfer_constructible :
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std::integral_constant<bool,
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std::is_nothrow_move_constructible<T>::value ||
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(
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std::is_nothrow_constructible<
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T, pilfered<T> >::value &&
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! std::is_nothrow_constructible<
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T, detail::not_pilfered<T> >::value
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)>
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{
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};
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template<class T>
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auto
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pilfer(T&& t) noexcept ->
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typename std::conditional<
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std::is_nothrow_constructible<
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typename std::remove_reference<T>::type,
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pilfered<typename std::remove_reference<T>::type> >::value &&
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! std::is_nothrow_constructible<
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typename std::remove_reference<T>::type,
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detail::not_pilfered<typename std::remove_reference<T>::type> >::value,
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pilfered<typename std::remove_reference<T>::type>,
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typename std::remove_reference<T>::type&&
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>::type
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{
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using U =
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typename std::remove_reference<T>::type;
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BOOST_JSON_STATIC_ASSERT(
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is_pilfer_constructible<U>::value);
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return typename std::conditional<
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std::is_nothrow_constructible<
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U, pilfered<U> >::value &&
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! std::is_nothrow_constructible<
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U, detail::not_pilfered<U> >::value,
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pilfered<U>, U&&
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>::type(std::move(t));
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}
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template<class T>
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void
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relocate(T* dest, T& src) noexcept
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{
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BOOST_JSON_STATIC_ASSERT(
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is_pilfer_constructible<T>::value);
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::new(dest) T(pilfer(src));
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src.~T();
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}
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} // boost
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#endif
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