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1 | <?xml version="1.0" encoding="utf-8" ?> | |
2 | <!DOCTYPE header PUBLIC "-//Boost//DTD BoostBook XML V1.0//EN" | |
3 | "http://www.boost.org/tools/boostbook/dtd/boostbook.dtd"> | |
4 | <!-- | |
5 | Copyright 2003, Eric Friedman, Itay Maman. | |
6 | ||
7 | Distributed under the Boost Software License, Version 1.0. (See accompanying | |
8 | file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) | |
9 | --> | |
10 | <section id="variant.concepts"> | |
11 | <title>Concepts</title> | |
12 | ||
13 | <using-namespace name="boost"/> | |
14 | ||
15 | <section id="variant.concepts.bounded-type"> | |
16 | <title><emphasis>BoundedType</emphasis></title> | |
17 | ||
18 | <para>The requirements on a <emphasis role="bold">bounded type</emphasis> | |
19 | are as follows:</para> | |
20 | ||
21 | <itemizedlist> | |
22 | <listitem><conceptname>CopyConstructible</conceptname> or <conceptname>MoveConstructible</conceptname>.</listitem> | |
23 | <listitem>Destructor upholds the no-throw exception-safety | |
24 | guarantee.</listitem> | |
25 | <listitem>Complete at the point of <code>variant</code> template | |
26 | instantiation. (See | |
27 | <code><classname>boost::recursive_wrapper</classname><T></code> | |
28 | for a type wrapper that accepts incomplete types to enable recursive | |
29 | <code>variant</code> types.)</listitem> | |
30 | </itemizedlist> | |
31 | ||
32 | <para>Every type specified as a template argument to | |
33 | <code><classname>variant</classname></code> must at minimum fulfill the | |
34 | above requirements. In addition, certain features of <code>variant</code> | |
35 | are available only if its bounded types meet the requirements of these | |
36 | following additional concepts:</para> | |
37 | ||
38 | <itemizedlist> | |
39 | <listitem><conceptname>Assignable</conceptname>: | |
40 | <code>variant</code> is itself <emphasis>Assignable</emphasis> if and | |
41 | only if every one of its bounded types meets the requirements of the | |
42 | concept. (Note that top-level <code>const</code>-qualified types and | |
43 | reference types do <emphasis>not</emphasis> meet these | |
44 | requirements.)</listitem> | |
45 | <listitem><conceptname>MoveAssignable</conceptname>: | |
46 | <code>variant</code> is itself <emphasis>MoveAssignable</emphasis> if and | |
47 | only if every one of its bounded types meets the requirements of the | |
48 | concept. (Note that top-level <code>const</code>-qualified types and | |
49 | reference types do <emphasis>not</emphasis> meet these | |
50 | requirements.)</listitem> | |
51 | <listitem><conceptname>DefaultConstructible</conceptname> [20.1.4]: | |
52 | <code>variant</code> is itself | |
53 | <conceptname>DefaultConstructible</conceptname> if and only if its first | |
54 | bounded type (i.e., <code>T1</code>) meets the requirements of the | |
55 | concept.</listitem> | |
56 | <listitem><conceptname>EqualityComparable</conceptname>: | |
57 | <code>variant</code> is itself <conceptname>EqualityComparable</conceptname> | |
58 | if and only if every one of its bounded types meets the requirements | |
59 | of the concept.</listitem> | |
60 | <listitem><conceptname>LessThanComparable</conceptname>: | |
61 | <code>variant</code> is itself <conceptname>LessThanComparable</conceptname> | |
62 | if and only if every one of its bounded types meets the requirements | |
63 | of the concept.</listitem> | |
64 | <listitem><link linkend="variant.concepts.output-streamable"><emphasis>OutputStreamable</emphasis></link>: | |
65 | <code>variant</code> is itself <emphasis>OutputStreamable</emphasis> | |
66 | if and only if every one of its bounded types meets the requirements | |
67 | of the concept.</listitem> | |
68 | <listitem><link linkend="variant.concepts.hashable"><emphasis>Hashable</emphasis></link>: | |
69 | <code>variant</code> is itself <emphasis>Hashable</emphasis> | |
70 | if and only if every one of its bounded types meets the requirements | |
71 | of the concept.</listitem> | |
72 | </itemizedlist> | |
73 | </section> | |
74 | ||
75 | <section id="variant.concepts.static-visitor"> | |
76 | <title><emphasis>StaticVisitor</emphasis></title> | |
77 | ||
78 | <para>The requirements on a <emphasis role="bold">static | |
79 | visitor</emphasis> of a type <code>T</code> are as follows:</para> | |
80 | ||
81 | <itemizedlist> | |
82 | <listitem>Must allow invocation as a function by overloading | |
83 | <code>operator()</code>, unambiguously accepting any value of type | |
84 | <code>T</code>.</listitem> | |
85 | <listitem>Must expose inner type <code>result_type</code>. C++14 compatible compilers | |
86 | could detect <code>result_type</code> automatically, but will stick to | |
87 | <code>result_type</code> if it is defined. (See | |
88 | <code><functionname>boost::visitor_ptr</functionname></code> for a | |
89 | solution to using functions as visitors.)</listitem> | |
90 | <listitem>If <code>result_type</code> is not <code>void</code>, then | |
91 | each operation of the function object must return a value implicitly | |
92 | convertible to <code>result_type</code>.</listitem> | |
93 | </itemizedlist> | |
94 | ||
95 | <section id="variant.concepts.static-visitor.examples"> | |
96 | <title>Examples</title> | |
97 | ||
98 | <para>The following class satisfies the requirements of a static visitor | |
99 | of several types (i.e., explicitly: <code>int</code> and | |
100 | <code>std::string</code>; or, e.g., implicitly: <code>short</code> and | |
101 | <code>const char *</code>; etc.):</para> | |
102 | ||
103 | <programlisting>class my_visitor | |
104 | : public <classname>boost::static_visitor</classname><int> | |
105 | { | |
106 | public: | |
107 | ||
108 | int operator()(int i) | |
109 | { | |
110 | return i * 2; | |
111 | } | |
112 | ||
113 | int operator()(const std::string& s) | |
114 | { | |
115 | return s.length(); | |
116 | } | |
117 | ||
118 | };</programlisting> | |
119 | ||
120 | <para>Another example is the following class, whose function-call | |
121 | operator is a member template, allowing it to operate on values of many | |
122 | types. Thus, the following class is a visitor of any type that supports | |
123 | streaming output (e.g., <code>int</code>, <code>double</code>, | |
124 | <code>std::string</code>, etc.):</para> | |
125 | ||
126 | <programlisting>class printer | |
127 | : public <classname>boost::static_visitor</classname><> | |
128 | { | |
129 | template <typename T> | |
130 | void operator()(const T& t) | |
131 | { | |
132 | std::cout << t << std::endl; | |
133 | } | |
134 | };</programlisting> | |
135 | ||
136 | <para>C++14 compatible compilers detect <code>result_type</code> automatically:</para> | |
137 | ||
138 | <programlisting> | |
139 | <classname>boost::variant</classname><int, float> v; | |
140 | // ... | |
141 | ||
142 | <functionname>boost::apply_visitor</functionname>( | |
143 | [](auto val) { return std::to_string(val); }, | |
144 | v | |
145 | ); | |
146 | </programlisting> | |
147 | ||
148 | </section> | |
149 | </section> | |
150 | ||
151 | <section id="variant.concepts.output-streamable"> | |
152 | <title><emphasis>OutputStreamable</emphasis></title> | |
153 | ||
154 | <para>The requirements on an <emphasis role="bold">output | |
155 | streamable</emphasis> type <code>T</code> are as follows:</para> | |
156 | ||
157 | <itemizedlist> | |
158 | <listitem>For any object <code>t</code> of type <code>T</code>, | |
159 | <code>std::cout << t</code> must be a valid | |
160 | expression.</listitem> | |
161 | </itemizedlist> | |
162 | </section> | |
163 | ||
164 | <section id="variant.concepts.hashable"> | |
165 | <title><emphasis>Hashable</emphasis></title> | |
166 | ||
167 | <para>The requirements on an <emphasis role="bold">hashable</emphasis> type <code>T</code> are as follows:</para> | |
168 | ||
169 | <itemizedlist> | |
170 | <listitem>For any object <code>t</code> of type <code>T</code>, | |
171 | <code>boost::hash<T>()(t)</code> must be a valid | |
172 | expression.</listitem> | |
173 | </itemizedlist> | |
174 | </section> | |
175 | ||
176 | </section> |