]>
Commit | Line | Data |
---|---|---|
7c673cae FG |
1 | <html> |
2 | <head> | |
3 | <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> | |
4 | <title>In-Place Factories</title> | |
5 | <link rel="stylesheet" href="../../../../../doc/src/boostbook.css" type="text/css"> | |
6 | <meta name="generator" content="DocBook XSL Stylesheets V1.78.1"> | |
7 | <link rel="home" href="../index.html" title="Chapter 1. Boost.Optional"> | |
8 | <link rel="up" href="../index.html" title="Chapter 1. Boost.Optional"> | |
9 | <link rel="prev" href="detailed_semantics.html" title="Detailed Semantics"> | |
10 | <link rel="next" href="a_note_about_optional_bool_.html" title="A note about optional<bool>"> | |
11 | </head> | |
12 | <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> | |
13 | <table cellpadding="2" width="100%"><tr> | |
14 | <td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../boost.png"></td> | |
15 | <td align="center"><a href="../../../../../index.html">Home</a></td> | |
16 | <td align="center"><a href="../../../../../libs/libraries.htm">Libraries</a></td> | |
17 | <td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> | |
18 | <td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> | |
19 | <td align="center"><a href="../../../../../more/index.htm">More</a></td> | |
20 | </tr></table> | |
21 | <hr> | |
22 | <div class="spirit-nav"> | |
23 | <a accesskey="p" href="detailed_semantics.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="a_note_about_optional_bool_.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a> | |
24 | </div> | |
25 | <div class="section"> | |
26 | <div class="titlepage"><div><div><h2 class="title" style="clear: both"> | |
27 | <a name="boost_optional.in_place_factories"></a><a class="link" href="in_place_factories.html" title="In-Place Factories">In-Place Factories</a> | |
28 | </h2></div></div></div> | |
29 | <p> | |
30 | One of the typical problems with wrappers and containers is that their interfaces | |
31 | usually provide an operation to initialize or assign the contained object as | |
32 | a copy of some other object. This not only requires the underlying type to | |
33 | be <a href="../../../../utility/CopyConstructible.html" target="_top">Copy Constructible</a>, | |
34 | but also requires the existence of a fully constructed object, often temporary, | |
35 | just to follow the copy from: | |
36 | </p> | |
37 | <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">X</span> | |
38 | <span class="special">{</span> | |
39 | <span class="identifier">X</span> <span class="special">(</span> <span class="keyword">int</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="special">)</span> <span class="special">;</span> | |
40 | <span class="special">}</span> <span class="special">;</span> | |
41 | ||
42 | <span class="keyword">class</span> <span class="identifier">W</span> | |
43 | <span class="special">{</span> | |
44 | <span class="identifier">X</span> <span class="identifier">wrapped_</span> <span class="special">;</span> | |
45 | ||
46 | <span class="keyword">public</span><span class="special">:</span> | |
47 | ||
48 | <span class="identifier">W</span> <span class="special">(</span> <span class="identifier">X</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span> <span class="special">)</span> <span class="special">:</span> <span class="identifier">wrapped_</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <span class="special">{}</span> | |
49 | <span class="special">}</span> <span class="special">;</span> | |
50 | ||
51 | <span class="keyword">void</span> <span class="identifier">foo</span><span class="special">()</span> | |
52 | <span class="special">{</span> | |
53 | <span class="comment">// Temporary object created.</span> | |
54 | <span class="identifier">W</span> <span class="special">(</span> <span class="identifier">X</span><span class="special">(</span><span class="number">123</span><span class="special">,</span><span class="string">"hello"</span><span class="special">)</span> <span class="special">)</span> <span class="special">;</span> | |
55 | <span class="special">}</span> | |
56 | </pre> | |
57 | <p> | |
58 | A solution to this problem is to support direct construction of the contained | |
59 | object right in the container's storage. In this scheme, the user only needs | |
60 | to supply the arguments to the constructor to use in the wrapped object construction. | |
61 | </p> | |
62 | <pre class="programlisting"><span class="keyword">class</span> <span class="identifier">W</span> | |
63 | <span class="special">{</span> | |
64 | <span class="identifier">X</span> <span class="identifier">wrapped_</span> <span class="special">;</span> | |
65 | ||
66 | <span class="keyword">public</span><span class="special">:</span> | |
67 | ||
68 | <span class="identifier">W</span> <span class="special">(</span> <span class="identifier">X</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span> <span class="special">)</span> <span class="special">:</span> <span class="identifier">wrapped_</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <span class="special">{}</span> | |
69 | <span class="identifier">W</span> <span class="special">(</span> <span class="keyword">int</span> <span class="identifier">a0</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">a1</span><span class="special">)</span> <span class="special">:</span> <span class="identifier">wrapped_</span><span class="special">(</span><span class="identifier">a0</span><span class="special">,</span><span class="identifier">a1</span><span class="special">)</span> <span class="special">{}</span> | |
70 | <span class="special">}</span> <span class="special">;</span> | |
71 | ||
72 | <span class="keyword">void</span> <span class="identifier">foo</span><span class="special">()</span> | |
73 | <span class="special">{</span> | |
74 | <span class="comment">// Wrapped object constructed in-place</span> | |
75 | <span class="comment">// No temporary created.</span> | |
76 | <span class="identifier">W</span> <span class="special">(</span><span class="number">123</span><span class="special">,</span><span class="string">"hello"</span><span class="special">)</span> <span class="special">;</span> | |
77 | <span class="special">}</span> | |
78 | </pre> | |
79 | <p> | |
80 | A limitation of this method is that it doesn't scale well to wrapped objects | |
81 | with multiple constructors nor to generic code were the constructor overloads | |
82 | are unknown. | |
83 | </p> | |
84 | <p> | |
85 | The solution presented in this library is the family of <span class="bold"><strong>InPlaceFactories</strong></span> | |
86 | and <span class="bold"><strong>TypedInPlaceFactories</strong></span>. These factories | |
87 | are a family of classes which encapsulate an increasing number of arbitrary | |
88 | constructor parameters and supply a method to construct an object of a given | |
89 | type using those parameters at an address specified by the user via placement | |
90 | new. | |
91 | </p> | |
92 | <p> | |
93 | For example, one member of this family looks like: | |
94 | </p> | |
95 | <pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">,</span><span class="keyword">class</span> <span class="identifier">A0</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">A1</span><span class="special">></span> | |
96 | <span class="keyword">class</span> <span class="identifier">TypedInPlaceFactory2</span> | |
97 | <span class="special">{</span> | |
98 | <span class="identifier">A0</span> <span class="identifier">m_a0</span> <span class="special">;</span> <span class="identifier">A1</span> <span class="identifier">m_a1</span> <span class="special">;</span> | |
99 | ||
100 | <span class="keyword">public</span><span class="special">:</span> | |
101 | ||
102 | <span class="identifier">TypedInPlaceFactory2</span><span class="special">(</span> <span class="identifier">A0</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">a0</span><span class="special">,</span> <span class="identifier">A1</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">a1</span> <span class="special">)</span> <span class="special">:</span> <span class="identifier">m_a0</span><span class="special">(</span><span class="identifier">a0</span><span class="special">),</span> <span class="identifier">m_a1</span><span class="special">(</span><span class="identifier">a1</span><span class="special">)</span> <span class="special">{}</span> | |
103 | ||
104 | <span class="keyword">void</span> <span class="identifier">construct</span> <span class="special">(</span> <span class="keyword">void</span><span class="special">*</span> <span class="identifier">p</span> <span class="special">)</span> <span class="special">{</span> <span class="keyword">new</span> <span class="special">(</span><span class="identifier">p</span><span class="special">)</span> <span class="identifier">T</span><span class="special">(</span><span class="identifier">m_a0</span><span class="special">,</span><span class="identifier">m_a1</span><span class="special">)</span> <span class="special">;</span> <span class="special">}</span> | |
105 | <span class="special">}</span> <span class="special">;</span> | |
106 | </pre> | |
107 | <p> | |
108 | A wrapper class aware of this can use it as: | |
109 | </p> | |
110 | <pre class="programlisting"><span class="keyword">class</span> <span class="identifier">W</span> | |
111 | <span class="special">{</span> | |
112 | <span class="identifier">X</span> <span class="identifier">wrapped_</span> <span class="special">;</span> | |
113 | ||
114 | <span class="keyword">public</span><span class="special">:</span> | |
115 | ||
116 | <span class="identifier">W</span> <span class="special">(</span> <span class="identifier">X</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span> <span class="special">)</span> <span class="special">:</span> <span class="identifier">wrapped_</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <span class="special">{}</span> | |
117 | <span class="identifier">W</span> <span class="special">(</span> <span class="identifier">TypedInPlaceFactory2</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">fac</span> <span class="special">)</span> <span class="special">{</span> <span class="identifier">fac</span><span class="special">.</span><span class="identifier">construct</span><span class="special">(&</span><span class="identifier">wrapped_</span><span class="special">)</span> <span class="special">;</span> <span class="special">}</span> | |
118 | <span class="special">}</span> <span class="special">;</span> | |
119 | ||
120 | <span class="keyword">void</span> <span class="identifier">foo</span><span class="special">()</span> | |
121 | <span class="special">{</span> | |
122 | <span class="comment">// Wrapped object constructed in-place via a TypedInPlaceFactory.</span> | |
123 | <span class="comment">// No temporary created.</span> | |
124 | <span class="identifier">W</span> <span class="special">(</span> <span class="identifier">TypedInPlaceFactory2</span><span class="special"><</span><span class="identifier">X</span><span class="special">,</span><span class="keyword">int</span><span class="special">,</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">>(</span><span class="number">123</span><span class="special">,</span><span class="string">"hello"</span><span class="special">))</span> <span class="special">;</span> | |
125 | <span class="special">}</span> | |
126 | </pre> | |
127 | <p> | |
128 | The factories are divided in two groups: | |
129 | </p> | |
130 | <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> | |
131 | <li class="listitem"> | |
132 | <span class="underline">TypedInPlaceFactories</span>: those which | |
133 | take the target type as a primary template parameter. | |
134 | </li> | |
135 | <li class="listitem"> | |
136 | <span class="underline">InPlaceFactories</span>: those with a template | |
137 | <code class="computeroutput"><span class="identifier">construct</span><span class="special">(</span><span class="keyword">void</span><span class="special">*)</span></code> member | |
138 | function taking the target type. | |
139 | </li> | |
140 | </ul></div> | |
141 | <p> | |
142 | Within each group, all the family members differ only in the number of parameters | |
143 | allowed. | |
144 | </p> | |
145 | <p> | |
146 | This library provides an overloaded set of helper template functions to construct | |
147 | these factories without requiring unnecessary template parameters: | |
148 | </p> | |
149 | <pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">A0</span><span class="special">,...,</span><span class="keyword">class</span> <span class="identifier">AN</span><span class="special">></span> | |
150 | <span class="identifier">InPlaceFactoryN</span> <span class="special"><</span><span class="identifier">A0</span><span class="special">,...,</span><span class="identifier">AN</span><span class="special">></span> <span class="identifier">in_place</span> <span class="special">(</span> <span class="identifier">A0</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">a0</span><span class="special">,</span> <span class="special">...,</span> <span class="identifier">AN</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">aN</span><span class="special">)</span> <span class="special">;</span> | |
151 | ||
152 | <span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">,</span><span class="keyword">class</span> <span class="identifier">A0</span><span class="special">,...,</span><span class="keyword">class</span> <span class="identifier">AN</span><span class="special">></span> | |
153 | <span class="identifier">TypedInPlaceFactoryN</span> <span class="special"><</span><span class="identifier">T</span><span class="special">,</span><span class="identifier">A0</span><span class="special">,...,</span><span class="identifier">AN</span><span class="special">></span> <span class="identifier">in_place</span> <span class="special">(</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">a0</span><span class="special">,</span> <span class="identifier">A0</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">a0</span><span class="special">,</span> <span class="special">...,</span> <span class="identifier">AN</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">aN</span><span class="special">)</span> <span class="special">;</span> | |
154 | </pre> | |
155 | <p> | |
156 | In-place factories can be used generically by the wrapper and user as follows: | |
157 | </p> | |
158 | <pre class="programlisting"><span class="keyword">class</span> <span class="identifier">W</span> | |
159 | <span class="special">{</span> | |
160 | <span class="identifier">X</span> <span class="identifier">wrapped_</span> <span class="special">;</span> | |
161 | ||
162 | <span class="keyword">public</span><span class="special">:</span> | |
163 | ||
164 | <span class="identifier">W</span> <span class="special">(</span> <span class="identifier">X</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span> <span class="special">)</span> <span class="special">:</span> <span class="identifier">wrapped_</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <span class="special">{}</span> | |
165 | ||
166 | <span class="keyword">template</span><span class="special"><</span> <span class="keyword">class</span> <span class="identifier">InPlaceFactory</span> <span class="special">></span> | |
167 | <span class="identifier">W</span> <span class="special">(</span> <span class="identifier">InPlaceFactory</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">fac</span> <span class="special">)</span> <span class="special">{</span> <span class="identifier">fac</span><span class="special">.</span><span class="keyword">template</span> <span class="special"><</span><span class="identifier">X</span><span class="special">></span><span class="identifier">construct</span><span class="special">(&</span><span class="identifier">wrapped_</span><span class="special">)</span> <span class="special">;</span> <span class="special">}</span> | |
168 | ||
169 | <span class="special">}</span> <span class="special">;</span> | |
170 | ||
171 | <span class="keyword">void</span> <span class="identifier">foo</span><span class="special">()</span> | |
172 | <span class="special">{</span> | |
173 | <span class="comment">// Wrapped object constructed in-place via a InPlaceFactory.</span> | |
174 | <span class="comment">// No temporary created.</span> | |
175 | <span class="identifier">W</span> <span class="special">(</span> <span class="identifier">in_place</span><span class="special">(</span><span class="number">123</span><span class="special">,</span><span class="string">"hello"</span><span class="special">)</span> <span class="special">)</span> <span class="special">;</span> | |
176 | <span class="special">}</span> | |
177 | </pre> | |
178 | <p> | |
179 | The factories are implemented in the headers: <a href="../../../../../boost/utility/in_place_factory.hpp" target="_top">in_place_factory.hpp</a> | |
180 | and <a href="../../../../../boost/utility/typed_in_place_factory.hpp" target="_top">typed_in_place_factory.hpp</a> | |
181 | </p> | |
182 | </div> | |
183 | <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> | |
184 | <td align="left"></td> | |
185 | <td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014 Andrzej Krzemieński<p> | |
186 | Distributed under the Boost Software License, Version 1.0. (See accompanying | |
187 | file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>) | |
188 | </p> | |
189 | </div></td> | |
190 | </tr></table> | |
191 | <hr> | |
192 | <div class="spirit-nav"> | |
193 | <a accesskey="p" href="detailed_semantics.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="a_note_about_optional_bool_.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a> | |
194 | </div> | |
195 | </body> | |
196 | </html> |