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| <div class="section" title="In-Place Factories"> |
| <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
| <a name="boost_optional.in_place_factories"></a><a class="link" href="in_place_factories.html" title="In-Place Factories">In-Place Factories</a> |
| </h2></div></div></div> |
| <p> |
| One of the typical problems with wrappers and containers is that their interfaces |
| usually provide an operation to initialize or assign the contained object as |
| a copy of some other object. This not only requires the underlying type to |
| be <a href="../../../../utility/CopyConstructible.html" target="_top">Copy Constructible</a>, |
| but also requires the existence of a fully constructed object, often temporary, |
| just to follow the copy from: |
| </p> |
| <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">X</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="special">;</span> |
| <span class="special">}</span> <span class="special">;</span> |
| |
| <span class="keyword">class</span> <span class="identifier">W</span> |
| <span class="special">{</span> |
| <span class="identifier">X</span> <span class="identifier">wrapped_</span> <span class="special">;</span> |
| |
| <span class="keyword">public</span><span class="special">:</span> |
| |
| <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> |
| <span class="special">}</span> <span class="special">;</span> |
| |
| <span class="keyword">void</span> <span class="identifier">foo</span><span class="special">()</span> |
| <span class="special">{</span> |
| <span class="comment">// Temporary object created. |
| </span> <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> |
| <span class="special">}</span> |
| </pre> |
| <p> |
| A solution to this problem is to support direct construction of the contained |
| object right in the container's storage. In this scheme, the user only needs |
| to supply the arguments to the constructor to use in the wrapped object construction. |
| </p> |
| <pre class="programlisting"><span class="keyword">class</span> <span class="identifier">W</span> |
| <span class="special">{</span> |
| <span class="identifier">X</span> <span class="identifier">wrapped_</span> <span class="special">;</span> |
| |
| <span class="keyword">public</span><span class="special">:</span> |
| |
| <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> |
| <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> |
| <span class="special">}</span> <span class="special">;</span> |
| |
| <span class="keyword">void</span> <span class="identifier">foo</span><span class="special">()</span> |
| <span class="special">{</span> |
| <span class="comment">// Wrapped object constructed in-place |
| </span> <span class="comment">// No temporary created. |
| </span> <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> |
| <span class="special">}</span> |
| </pre> |
| <p> |
| A limitation of this method is that it doesn't scale well to wrapped objects |
| with multiple constructors nor to generic code were the constructor overloads |
| are unknown. |
| </p> |
| <p> |
| The solution presented in this library is the family of <span class="bold"><strong>InPlaceFactories</strong></span> |
| and <span class="bold"><strong>TypedInPlaceFactories</strong></span>. These factories |
| are a family of classes which encapsulate an increasing number of arbitrary |
| constructor parameters and supply a method to construct an object of a given |
| type using those parameters at an address specified by the user via placement |
| new. |
| </p> |
| <p> |
| For example, one member of this family looks like: |
| </p> |
| <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> |
| <span class="keyword">class</span> <span class="identifier">TypedInPlaceFactory2</span> |
| <span class="special">{</span> |
| <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> |
| |
| <span class="keyword">public</span><span class="special">:</span> |
| |
| <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> |
| |
| <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> |
| <span class="special">}</span> <span class="special">;</span> |
| </pre> |
| <p> |
| A wrapper class aware of this can use it as: |
| </p> |
| <pre class="programlisting"><span class="keyword">class</span> <span class="identifier">W</span> |
| <span class="special">{</span> |
| <span class="identifier">X</span> <span class="identifier">wrapped_</span> <span class="special">;</span> |
| |
| <span class="keyword">public</span><span class="special">:</span> |
| |
| <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> |
| <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> |
| <span class="special">}</span> <span class="special">;</span> |
| |
| <span class="keyword">void</span> <span class="identifier">foo</span><span class="special">()</span> |
| <span class="special">{</span> |
| <span class="comment">// Wrapped object constructed in-place via a TypedInPlaceFactory. |
| </span> <span class="comment">// No temporary created. |
| </span> <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> |
| <span class="special">}</span> |
| </pre> |
| <p> |
| The factories are divided in two groups: |
| </p> |
| <div class="itemizedlist"><ul class="itemizedlist" type="disc"> |
| <li class="listitem"> |
| <span class="underline">TypedInPlaceFactories</span>: those which |
| take the target type as a primary template parameter. |
| </li> |
| <li class="listitem"> |
| <span class="underline">InPlaceFactories</span>: those with a template |
| <code class="computeroutput"><span class="identifier">construct</span><span class="special">(</span><span class="keyword">void</span><span class="special">*)</span></code> member |
| function taking the target type. |
| </li> |
| </ul></div> |
| <p> |
| Within each group, all the family members differ only in the number of parameters |
| allowed. |
| </p> |
| <p> |
| This library provides an overloaded set of helper template functions to construct |
| these factories without requiring unnecessary template parameters: |
| </p> |
| <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> |
| <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> |
| |
| <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> |
| <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> |
| </pre> |
| <p> |
| In-place factories can be used generically by the wrapper and user as follows: |
| </p> |
| <pre class="programlisting"><span class="keyword">class</span> <span class="identifier">W</span> |
| <span class="special">{</span> |
| <span class="identifier">X</span> <span class="identifier">wrapped_</span> <span class="special">;</span> |
| |
| <span class="keyword">public</span><span class="special">:</span> |
| |
| <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> |
| |
| <span class="keyword">template</span><span class="special"><</span> <span class="keyword">class</span> <span class="identifier">InPlaceFactory</span> <span class="special">></span> |
| <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> |
| |
| <span class="special">}</span> <span class="special">;</span> |
| |
| <span class="keyword">void</span> <span class="identifier">foo</span><span class="special">()</span> |
| <span class="special">{</span> |
| <span class="comment">// Wrapped object constructed in-place via a InPlaceFactory. |
| </span> <span class="comment">// No temporary created. |
| </span> <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> |
| <span class="special">}</span> |
| </pre> |
| <p> |
| The factories are implemented in the headers: <a href="../../../../../boost/utility/in_place_factory.hpp" target="_top">in_place_factory.hpp</a> |
| and <a href="../../../../../boost/utility/typed_in_place_factory.hpp" target="_top">typed_in_place_factory.hpp</a> |
| </p> |
| </div> |
| <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> |
| <td align="left"></td> |
| <td align="right"><div class="copyright-footer">Copyright © 2003 -2007 Fernando Luis Cacciola Carballal<p> |
| Distributed under the Boost Software License, Version 1.0. (See accompanying |
| 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>) |
| </p> |
| </div></td> |
| </tr></table> |
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