Vehicle-cpp/ZJ_PRO/thirdparty/Boost/include/boost/json/value_from.hpp

//
// Copyright (c) 2019 Vinnie Falco (vinnie.falco@gmail.com)
// Copyright (c) 2020 Krystian Stasiowski (sdkrystian@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_VALUE_FROM_HPP
#define BOOST_JSON_VALUE_FROM_HPP

#include <boost/json/detail/config.hpp>
#include <boost/json/value.hpp>
#include <boost/json/detail/value_from.hpp>

BOOST_JSON_NS_BEGIN

/** Customization point tag.
    
    This tag type is used by the function
    @ref value_from to select overloads
    of `tag_invoke`.

    @note This type is empty; it has no members.

    @see @ref value_from, @ref value_to, @ref value_to_tag,
    <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1895r0.pdf">
        tag_invoke: A general pattern for supporting customisable functions</a>
*/
#ifndef BOOST_JSON_DOCS
struct value_from_tag;
#else
// VFALCO Doc toolchain doesn't like
// forward declared ordinary classes.
struct value_from_tag {};
#endif

/** Convert an object of type `T` to @ref value.

    This function attempts to convert an object
    of type `T` to @ref value using

    @li one of @ref value's constructors,

    @li a library-provided generic conversion, or

    @li a user-provided overload of `tag_invoke`.
    
    In all cases, the conversion is done by calling
    an overload of `tag_invoke` found by argument-dependent
    lookup. Its signature should be similar to:
 
    @code
    void tag_invoke( value_from_tag, value&, T );
    @endcode

    A @ref value constructed
    with the @ref storage_ptr passed to @ref value_from is
    passed as the second argument to ensure that the memory
    resource is correctly propagated.

    @par Exception Safety
    Strong guarantee.

    @tparam T The type of the object to convert.
    
    @returns `t` converted to @ref value.

    @param t The object to convert.

    @param sp A storage pointer referring to the memory resource
    to use for the returned @ref value. The default argument for this
    parameter is `{}`.

    @see @ref value_from_tag, @ref value_to,
    <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1895r0.pdf">
        tag_invoke: A general pattern for supporting customisable functions</a>
*/
template<class T>
value
value_from(
    T&& t,
    storage_ptr sp = {})
{
    return detail::value_from_impl(
        std::forward<T>(t), std::move(sp));
}

/** Determine if `T` can be converted to @ref value.

    If `T` can be converted to @ref value via a 
    call to @ref value_from, the static data member `value`
    is defined as `true`. Otherwise, `value` is
    defined as `false`.

    @see @ref value_from
*/
#ifdef BOOST_JSON_DOCS
template<class T>
using has_value_from = __see_below__;
#else
template<class T, class>
struct has_value_from : std::false_type { };

template<class T>
struct has_value_from<T, detail::void_t<
    decltype(detail::value_from_impl(std::declval<T&&>(),
        std::declval<storage_ptr>()))>>
    : std::true_type { };
#endif

BOOST_JSON_NS_END

#endif