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<div class="section" lang="en">
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
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<a name="boost_optional.in_place_factories"></a><a href="in_place_factories.html" title="In-Place Factories">In-Place Factories</a>
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</h2></div></div></div>
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<p>
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One of the typical problems with wrappers and containers is that their interfaces
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usually provide an operation to initialize or assign the contained object as
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a copy of some other object. This not only requires the underlying type to
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be <a href="../../../../utility/CopyConstructible.html" target="_top">Copy Constructible</a>,
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but also requires the existence of a fully constructed object, often temporary,
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just to follow the copy from:
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</p>
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<pre class="programlisting">
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<span class="keyword">struct</span> <span class="identifier">X</span>
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<span class="special">{</span>
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<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>
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<span class="special">}</span> <span class="special">;</span>
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<span class="keyword">class</span> <span class="identifier">W</span>
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<span class="special">{</span>
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<span class="identifier">X</span> <span class="identifier">wrapped_</span> <span class="special">;</span>
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<span class="keyword">public</span><span class="special">:</span>
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<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>
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<span class="special">}</span> <span class="special">;</span>
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<span class="keyword">void</span> <span class="identifier">foo</span><span class="special">()</span>
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<span class="special">{</span>
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<span class="comment">// Temporary object created.
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</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>
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<span class="special">}</span>
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</pre>
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<p>
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A solution to this problem is to support direct construction of the contained
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object right in the container's storage. In this scheme, the user only needs
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to supply the arguments to the constructor to use in the wrapped object construction.
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</p>
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<pre class="programlisting">
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<span class="keyword">class</span> <span class="identifier">W</span>
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<span class="special">{</span>
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<span class="identifier">X</span> <span class="identifier">wrapped_</span> <span class="special">;</span>
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<span class="keyword">public</span><span class="special">:</span>
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<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>
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<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>
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<span class="special">}</span> <span class="special">;</span>
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<span class="keyword">void</span> <span class="identifier">foo</span><span class="special">()</span>
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<span class="special">{</span>
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<span class="comment">// Wrapped object constructed in-place
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</span> <span class="comment">// No temporary created.
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</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>
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<span class="special">}</span>
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</pre>
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<p>
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A limitation of this method is that it doesn't scale well to wrapped objects
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with multiple constructors nor to generic code were the constructor overloads
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are unknown.
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</p>
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<p>
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The solution presented in this library is the family of <span class="bold"><strong>InPlaceFactories</strong></span>
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and <span class="bold"><strong>TypedInPlaceFactories</strong></span>. These factories
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are a family of classes which encapsulate an increasing number of arbitrary
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constructor parameters and supply a method to construct an object of a given
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type using those parameters at an address specified by the user via placement
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new.
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</p>
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<p>
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For example, one member of this family looks like:
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</p>
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<pre class="programlisting">
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<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>
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<span class="keyword">class</span> <span class="identifier">TypedInPlaceFactory2</span>
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<span class="special">{</span>
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<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>
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<span class="keyword">public</span><span class="special">:</span>
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<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>
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<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>
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<span class="special">}</span> <span class="special">;</span>
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</pre>
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<p>
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A wrapper class aware of this can use it as:
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</p>
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<pre class="programlisting">
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<span class="keyword">class</span> <span class="identifier">W</span>
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<span class="special">{</span>
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<span class="identifier">X</span> <span class="identifier">wrapped_</span> <span class="special">;</span>
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<span class="keyword">public</span><span class="special">:</span>
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<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>
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<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>
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<span class="special">}</span> <span class="special">;</span>
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<span class="keyword">void</span> <span class="identifier">foo</span><span class="special">()</span>
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<span class="special">{</span>
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<span class="comment">// Wrapped object constructed in-place via a TypedInPlaceFactory.
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</span> <span class="comment">// No temporary created.
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</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="identifier">rt</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>
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<span class="special">}</span>
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</pre>
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<p>
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The factories are divided in two groups:
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</p>
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<div class="itemizedlist"><ul type="disc">
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<li>
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<span class="underline">TypedInPlaceFactories</span>: those which
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take the target type as a primary template parameter.
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</li>
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<li>
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<span class="underline">InPlaceFactories</span>: those with a template
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<code class="computeroutput"><span class="identifier">construct</span><span class="special">(</span><span class="keyword">void</span><span class="special">*)</span></code> member
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function taking the target type.
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</li>
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</ul></div>
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<p>
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Within each group, all the family members differ only in the number of parameters
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allowed.
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</p>
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<p>
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This library provides an overloaded set of helper template functions to construct
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these factories without requiring unnecessary template parameters:
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</p>
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<pre class="programlisting">
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<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>
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<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>
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<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>
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<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>
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</pre>
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<p>
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In-place factories can be used generically by the wrapper and user as follows:
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</p>
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<pre class="programlisting">
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<span class="keyword">class</span> <span class="identifier">W</span>
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<span class="special">{</span>
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<span class="identifier">X</span> <span class="identifier">wrapped_</span> <span class="special">;</span>
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<span class="keyword">public</span><span class="special">:</span>
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<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>
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<span class="keyword">template</span><span class="special"><</span> <span class="keyword">class</span> <span class="identifier">InPlaceFactory</span> <span class="special">></span>
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<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>
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<span class="special">}</span> <span class="special">;</span>
|
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|
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<span class="keyword">void</span> <span class="identifier">foo</span><span class="special">()</span>
|
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<span class="special">{</span>
|
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<span class="comment">// Wrapped object constructed in-place via a InPlaceFactory.
|
||||
</span> <span class="comment">// No temporary created.
|
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</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>
|
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<span class="special">}</span>
|
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</pre>
|
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<p>
|
||||
The factories are implemented in the headers: <a href="../../../../../boost/utility/in_place_factory.hpp" target="_top">in_place_factory.hpp</a>
|
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and <a href="../../../../../boost/utility/typed_in_place_factory.hpp" target="_top">typed_in_place_factory.hpp</a>
|
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</p>
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</div>
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<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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<td align="left"></td>
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||||
<td align="right"><small>Copyright <20> 2003 -2007 Fernando Luis Cacciola Carballal</small></td>
|
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</tr></table>
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