feat():initial version

install/boost_1_75_0/include/boost/json/pilfer.hpp

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+//
+// Copyright (c) 2019 Vinnie Falco (vinnie.falco@gmail.com)
+//
+// Distributed under the Boost Software License, Version 1.0. (See accompanying
+// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+//
+// Official repository: https://github.com/boostorg/json
+//
+
+#ifndef BOOST_JSON_PILFER_HPP
+#define BOOST_JSON_PILFER_HPP
+
+#include <boost/json/detail/config.hpp>
+#include <type_traits>
+#include <utility>
+
+/*
+    Implements "pilfering" from P0308R0
+
+    @see
+        http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0308r0.html
+*/
+
+namespace boost {
+namespace json {
+
+/** Tag wrapper to specify pilfer-construction.
+
+    This wrapper is used to specify a pilfer constructor
+    overload.
+
+    @par Example
+
+    A pilfer constructor accepts a single argument
+    of type @ref pilfered and throws nothing:
+
+    @code
+    struct T
+    {
+        T( pilfered<T> ) noexcept;
+    };
+    @endcode
+
+    @note
+
+    The constructor should not be marked explicit.
+
+    @see @ref pilfer, @ref is_pilfer_constructible,
+    <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0308r0.html">
+        Valueless Variants Considered Harmful</a>
+*/
+template<class T>
+class pilfered
+{
+    T& t_;
+
+public:
+    /** Constructor
+
+        Construct the wrapper from `t`.
+
+        @param t The pilferable object. Ownership
+        is not transferred.
+    */
+    explicit
+    constexpr
+    pilfered(T&& t) noexcept
+        : t_(t)
+    {
+    }
+
+    /** Return a reference to the pilferable object.
+
+        This returns a reference to the wrapped object.
+    */
+    constexpr T&
+    get() const noexcept
+    {
+        return t_;
+    }
+
+    /** Return a pointer to the pilferable object.
+
+        This returns a pointer to the wrapped object.
+    */
+    constexpr T*
+    operator->() const noexcept
+    {
+        //return std::addressof(t_);
+        return reinterpret_cast<T*>(
+            const_cast<char *>(
+                &reinterpret_cast<
+                    const volatile char &>(t_)));
+    }
+};
+
+#ifndef BOOST_JSON_DOCS
+// VFALCO Renamed this to work around an msvc bug 
+namespace detail_pilfer {
+template<class>
+struct not_pilfered
+{
+};
+} // detail_pilfer
+#endif
+
+/** Metafunction returning `true` if `T` is <em>PilferConstructible</em>
+
+    If `T` can be pilfer constructed, this metafunction is
+    equal to `std::true_type`. Otherwise it is equal to
+    `std::false_type`.
+
+    @see @ref pilfer, @ref pilfered,
+    <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0308r0.html">
+        Valueless Variants Considered Harmful</a>
+*/
+template<class T>
+struct is_pilfer_constructible
+#ifndef BOOST_JSON_DOCS
+    : std::integral_constant<bool,
+        std::is_nothrow_move_constructible<T>::value ||
+        (
+            std::is_nothrow_constructible<
+                T, pilfered<T> >::value &&
+            ! std::is_nothrow_constructible<
+                T, detail_pilfer::not_pilfered<T> >::value
+        )>
+#endif
+{
+};
+
+/** Indicate that an object `t` may be pilfered from.
+
+    A <em>pilfer</em> operation is the construction
+    of a new object of type `T` from an existing
+    object `t`. After the construction, the only
+    valid operation on the pilfered-from object is
+    destruction. This permits optimizations beyond
+    those available for a move-construction, as the
+    pilfered-from object is not required to be in
+    a "usable" state.
+\n
+    This is used similarly to `std::move`.
+
+    @par Example
+
+    A pilfer constructor accepts a single argument
+    of type @ref pilfered and throws nothing:
+
+    @code
+    struct T
+    {
+        T( pilfered<T> ) noexcept;
+    };
+    @endcode
+
+    Pilfer construction is performed using @ref pilfer :
+
+    @code
+    {
+        T t1;                       // default construction
+        T t2( pilfer( t1 ) );       // pilfer-construct from t1
+
+        // At this point, t1 may only be destroyed
+    }
+    @endcode
+
+    @see @ref pilfered, @ref is_pilfer_constructible,
+    <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0308r0.html">
+        Valueless Variants Considered Harmful</a>
+*/
+template<class T>
+auto
+pilfer(T&& t) noexcept ->
+    typename std::conditional<
+        std::is_nothrow_constructible<
+            typename std::remove_reference<T>::type,
+            pilfered<typename
+                std::remove_reference<T>::type> >::value &&
+        ! std::is_nothrow_constructible<
+            typename std::remove_reference<T>::type,
+            detail_pilfer::not_pilfered<typename
+                std::remove_reference<T>::type> >::value,
+        pilfered<typename std::remove_reference<T>::type>,
+        typename std::remove_reference<T>::type&&
+            >::type
+{
+    using U =
+        typename std::remove_reference<T>::type;
+    static_assert(
+        is_pilfer_constructible<U>::value, "");
+    return typename std::conditional<
+        std::is_nothrow_constructible<
+            U, pilfered<U> >::value &&
+        ! std::is_nothrow_constructible<
+            U, detail_pilfer::not_pilfered<U> >::value,
+        pilfered<U>, U&&
+            >::type(std::move(t));
+}
+
+/*
+template<class T>
+void
+relocate(T* dest, T& src) noexcept
+{
+    static_assert(
+        is_pilfer_constructible<T>::value, "");
+    ::new(dest) T(pilfer(src));
+    src.~T();
+}
+*/
+
+} // json
+} // boost
+
+
+#endif