Current File : //usr/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
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