value_from.hpp

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value_from.hpp

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1		//
2		// Copyright (c) 2019 Vinnie Falco (vinnie.falco@gmail.com)
3		// Copyright (c) 2020 Krystian Stasiowski (sdkrystian@gmail.com)
4		// Copyright (c) 2022 Dmitry Arkhipov (grisumbras@gmail.com)
5		//
6		// Distributed under the Boost Software License, Version 1.0. (See accompanying
7		// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
8		//
9		// Official repository: https://github.com/boostorg/json
10		//
11
12		#ifndef BOOST_JSON_VALUE_FROM_HPP
13		#define BOOST_JSON_VALUE_FROM_HPP
14
15		#include <boost/core/detail/static_assert.hpp>
16		#include <boost/json/detail/value_from.hpp>
17
18		namespace boost {
19		namespace json {
20
21		/** Convert an object of type `T` to @ref value.
22
23		This function attempts to convert an object of type `T` to @ref value using
24
25		- one of @ref value's constructors,
26		- a library-provided generic conversion, or
27		- a user-provided overload of `tag_invoke`.
28
29		In order to perform the conversion the function selects an appropriate
30		implementation based on the types `T` and `Context` (if provided).
31
32		1. If `Context` is available and is not `std::tuple<C...>`
33
34		a. check if `use_category<T, Context>::value` is not
35		@ref conversion_category::unknown; otherwise
36
37		b. check if a `tag_invoke` overload from the list below that takes a
38		`Context const&` exists.
39
40		2. Otherwise, if `Context` is available, and is `std::tuple<C...>` repeat
41		steps 1 and 2 recursively for every `C` until either
42		step 1.a or 1.b succeeds for some `C`.
43
44		3. Failing that,
45
46		a. check if `use_category<T>::value` is not
47		@ref conversion_category::unknown; otherwise
48
49		b. check if a `tag_invoke` overload from the list below that takes only
50		2 parameters exists; otherwise
51
52		c. check if `T` is one of @ref value, @ref array, @ref object,
53		or @ref string; otherwise
54
55		d. check if `T` matches one of the categories of types described in the
56		table "Conversion categories" in \<\<Value Conversion>> section.
57
58		These steps determine both the appropriate category of conversion for `T`,
59		and, if necessary, the effective context `C` that will be used for
60		conversion. If the category is selected on steps 1.a, 3.a, 3.c,
61		or 3.d, the library provides a suitable conversion implementation.
62		If the category is selected on steps 2.b or 3.b, then a
63		user-provided `tag_invoke` overload is used.
64
65		The overloads of `tag_invoke` that will be considered by this function
66		are in the following list. Overloads that appear higher in the list have
67		higher priority.
68
69		@code
70		template< class FullContext >
71		void tag_invoke( value_from_tag, value&, T, const Context&, const FullContext& );
72
73		void tag_invoke( value_from_tag, value&, T, const Context& );
74
75		void tag_invoke( value_from_tag, value&, T );
76		@endcode
77
78		The `ctx` argument can be used either as a tag type to provide conversions
79		for third-party types, or to pass extra data to the conversion function.
80
81		Overloads (2) and (4) construct their return value using the
82		@ref storage_ptr `sp`, which ensures that the memory resource is correctly
83		propagated.
84
85		@par Exception Safety
86		Strong guarantee.
87
88		@tparam T The type of the object to convert.
89
90		@tparam Context The type of context passed to the conversion function.
91
92		@param t The object to convert.
93
94		@param ctx Context passed to the conversion function.
95
96		@param jv @ref value out parameter.
97
98		@see @ref value_from_tag, @ref value_to,
99		<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1895r0.pdf">
100		tag_invoke: A general pattern for supporting customisable functions</a>
101		*/
102		/// @{
103		template< class T, class Context >
104		void
105	7056x	value_from(
106		T&& t,
107		Context const& ctx,
108		value& jv)
109		{
110		using bare_T = detail::remove_cvref<T>;
111		using cat = detail::value_from_category<Context, bare_T>;
112	7056x	detail::value_from_impl( cat(), jv, std::forward<T>(t), ctx );
113	7056x	}
114
115		/** Overload
116		@param t
117		@param ctx
118		@param sp A storage pointer referring to the memory resource to use for the
119		returned @ref value.
120
121		@return Overloads (2) and (4) return `t` converted to @ref value.
122		Overloads (1) and 3 return `void` instead and pass their result via
123		the out parameter `jv`.
124		*/
125		template< class T, class Context >
126		#ifndef BOOST_JSON_DOCS
127		typename std::enable_if<
128		!std::is_same< detail::remove_cvref<Context>, storage_ptr >::value &&
129		!std::is_same< detail::remove_cvref<Context>, value >::value,
130		value >::type
131		#else
132		value
133		#endif
134	7037x	value_from(
135		T&& t,
136		Context const& ctx,
137		storage_ptr sp = {})
138		{
139	7037x	value jv(std::move(sp));
140	7037x	value_from( static_cast<T&&>(t), ctx, jv );
141	7037x	return jv;
142	✗	}
143
144		/// Overload
145		template<class T>
146		void
147	19x	value_from(
148		T&& t,
149		value& jv)
150		{
151	19x	value_from( static_cast<T&&>(t), detail::no_context(), jv );
152	19x	}
153
154		/// Overload
155		template<class T>
156		value
157	297x	value_from(
158		T&& t,
159		storage_ptr sp = {})
160		{
161		return value_from(
162	297x	static_cast<T&&>(t), detail::no_context(), std::move(sp) );
163		}
164		/// @}
165
166		/** Determine if `T` can be converted to @ref value.
167
168		If `T` can be converted to @ref value via a call to @ref value_from, the
169		static data member `value` is defined as `true`. Otherwise, `value` is
170		defined as `false`.
171
172		@see @ref value_from.
173		*/
174		#ifdef BOOST_JSON_DOCS
175		template<class T>
176		using has_value_from = __see_below__;
177		#else
178		template<class T>
179		using has_value_from = detail::can_convert<
180		detail::remove_cvref<T>, detail::value_from_conversion>;
181		#endif
182
183		} // namespace json
184		} // namespace boost
185
186		#endif
187