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24 | </div> | |
25 | <div class="section"> | |
26 | <div class="titlepage"><div><div><h3 class="title"> | |
27 | <a name="boost_optional.tutorial.in_place_factories"></a><a class="link" href="in_place_factories.html" title="In-Place Factories">In-Place | |
28 | Factories</a> | |
29 | </h3></div></div></div> | |
30 | <p> | |
31 | One of the typical problems with wrappers and containers is that their interfaces | |
32 | usually provide an operation to initialize or assign the contained object | |
33 | as a copy of some other object. This not only requires the underlying type | |
34 | to be <a href="../../../../../utility/CopyConstructible.html" target="_top"><code class="computeroutput"><span class="identifier">CopyConstructible</span></code></a>, but also requires | |
35 | the existence of a fully constructed object, often temporary, just to follow | |
36 | the copy from: | |
37 | </p> | |
38 | <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">X</span> | |
39 | <span class="special">{</span> | |
40 | <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> | |
41 | <span class="special">}</span> <span class="special">;</span> | |
42 | ||
43 | <span class="keyword">class</span> <span class="identifier">W</span> | |
44 | <span class="special">{</span> | |
45 | <span class="identifier">X</span> <span class="identifier">wrapped_</span> <span class="special">;</span> | |
46 | ||
47 | <span class="keyword">public</span><span class="special">:</span> | |
48 | ||
49 | <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> | |
50 | <span class="special">}</span> <span class="special">;</span> | |
51 | ||
52 | <span class="keyword">void</span> <span class="identifier">foo</span><span class="special">()</span> | |
53 | <span class="special">{</span> | |
54 | <span class="comment">// Temporary object created.</span> | |
55 | <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> | |
56 | <span class="special">}</span> | |
57 | </pre> | |
58 | <p> | |
59 | A solution to this problem is to support direct construction of the contained | |
60 | object right in the container's storage. In this scheme, the user only needs | |
61 | to supply the arguments to the constructor to use in the wrapped object construction. | |
62 | </p> | |
63 | <pre class="programlisting"><span class="keyword">class</span> <span class="identifier">W</span> | |
64 | <span class="special">{</span> | |
65 | <span class="identifier">X</span> <span class="identifier">wrapped_</span> <span class="special">;</span> | |
66 | ||
67 | <span class="keyword">public</span><span class="special">:</span> | |
68 | ||
69 | <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> | |
70 | <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> | |
71 | <span class="special">}</span> <span class="special">;</span> | |
72 | ||
73 | <span class="keyword">void</span> <span class="identifier">foo</span><span class="special">()</span> | |
74 | <span class="special">{</span> | |
75 | <span class="comment">// Wrapped object constructed in-place</span> | |
76 | <span class="comment">// No temporary created.</span> | |
77 | <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> | |
78 | <span class="special">}</span> | |
79 | </pre> | |
80 | <p> | |
81 | A limitation of this method is that it doesn't scale well to wrapped objects | |
82 | with multiple constructors nor to generic code were the constructor overloads | |
83 | are unknown. | |
84 | </p> | |
85 | <p> | |
86 | The solution presented in this library is the family of <span class="bold"><strong>InPlaceFactories</strong></span> | |
87 | and <span class="bold"><strong>TypedInPlaceFactories</strong></span>. These factories | |
88 | are a family of classes which encapsulate an increasing number of arbitrary | |
89 | constructor parameters and supply a method to construct an object of a given | |
90 | type using those parameters at an address specified by the user via placement | |
91 | new. | |
92 | </p> | |
93 | <p> | |
94 | For example, one member of this family looks like: | |
95 | </p> | |
96 | <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> | |
97 | <span class="keyword">class</span> <span class="identifier">TypedInPlaceFactory2</span> | |
98 | <span class="special">{</span> | |
99 | <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> | |
100 | ||
101 | <span class="keyword">public</span><span class="special">:</span> | |
102 | ||
103 | <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> | |
104 | ||
105 | <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> | |
106 | <span class="special">}</span> <span class="special">;</span> | |
107 | </pre> | |
108 | <p> | |
109 | A wrapper class aware of this can use it as: | |
110 | </p> | |
111 | <pre class="programlisting"><span class="keyword">class</span> <span class="identifier">W</span> | |
112 | <span class="special">{</span> | |
113 | <span class="identifier">X</span> <span class="identifier">wrapped_</span> <span class="special">;</span> | |
114 | ||
115 | <span class="keyword">public</span><span class="special">:</span> | |
116 | ||
117 | <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> | |
118 | <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> | |
119 | <span class="special">}</span> <span class="special">;</span> | |
120 | ||
121 | <span class="keyword">void</span> <span class="identifier">foo</span><span class="special">()</span> | |
122 | <span class="special">{</span> | |
123 | <span class="comment">// Wrapped object constructed in-place via a TypedInPlaceFactory.</span> | |
124 | <span class="comment">// No temporary created.</span> | |
125 | <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> | |
126 | <span class="special">}</span> | |
127 | </pre> | |
128 | <p> | |
129 | The factories are divided in two groups: | |
130 | </p> | |
131 | <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> | |
132 | <li class="listitem"> | |
133 | <span class="underline">TypedInPlaceFactories</span>: those which | |
134 | take the target type as a primary template parameter. | |
135 | </li> | |
136 | <li class="listitem"> | |
137 | <span class="underline">InPlaceFactories</span>: those with a | |
138 | template <code class="computeroutput"><span class="identifier">construct</span><span class="special">(</span><span class="keyword">void</span><span class="special">*)</span></code> | |
139 | member function taking the target type. | |
140 | </li> | |
141 | </ul></div> | |
142 | <p> | |
143 | Within each group, all the family members differ only in the number of parameters | |
144 | allowed. | |
145 | </p> | |
146 | <p> | |
147 | This library provides an overloaded set of helper template functions to construct | |
148 | these factories without requiring unnecessary template parameters: | |
149 | </p> | |
150 | <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> | |
151 | <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> | |
152 | ||
153 | <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> | |
154 | <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> | |
155 | </pre> | |
156 | <p> | |
157 | In-place factories can be used generically by the wrapper and user as follows: | |
158 | </p> | |
159 | <pre class="programlisting"><span class="keyword">class</span> <span class="identifier">W</span> | |
160 | <span class="special">{</span> | |
161 | <span class="identifier">X</span> <span class="identifier">wrapped_</span> <span class="special">;</span> | |
162 | ||
163 | <span class="keyword">public</span><span class="special">:</span> | |
164 | ||
165 | <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> | |
166 | ||
167 | <span class="keyword">template</span><span class="special"><</span> <span class="keyword">class</span> <span class="identifier">InPlaceFactory</span> <span class="special">></span> | |
168 | <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> | |
169 | ||
170 | <span class="special">}</span> <span class="special">;</span> | |
171 | ||
172 | <span class="keyword">void</span> <span class="identifier">foo</span><span class="special">()</span> | |
173 | <span class="special">{</span> | |
174 | <span class="comment">// Wrapped object constructed in-place via a InPlaceFactory.</span> | |
175 | <span class="comment">// No temporary created.</span> | |
176 | <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> | |
177 | <span class="special">}</span> | |
178 | </pre> | |
179 | <p> | |
180 | The factories are implemented in the headers: <a href="../../../../../../boost/utility/in_place_factory.hpp" target="_top">in_place_factory.hpp</a> | |
181 | and <a href="../../../../../../boost/utility/typed_in_place_factory.hpp" target="_top">typed_in_place_factory.hpp</a> | |
182 | </p> | |
183 | </div> | |
184 | <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> | |
185 | <td align="left"></td> | |
186 | <td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2016 Andrzej Krzemieński<p> | |
187 | Distributed under the Boost Software License, Version 1.0. (See accompanying | |
188 | 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>) | |
189 | </p> | |
190 | </div></td> | |
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