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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
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<a name="conversion.polymorphic_casts"></a><a class="link" href="polymorphic_casts.html" title="Polymorphic casts">Polymorphic casts</a>
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</h2></div></div></div>
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<div class="toc"><dl class="toc">
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<dt><span class="section"><a href="polymorphic_casts.html#conversion.polymorphic_casts.polymorphic_downcast">polymorphic_downcast</a></span></dt>
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<dt><span class="section"><a href="polymorphic_casts.html#conversion.polymorphic_casts.polymorphic_cast">polymorphic_cast</a></span></dt>
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<dt><span class="section"><a href="polymorphic_casts.html#conversion.polymorphic_casts.polymorphic_pointer_cast">polymorphic_pointer_cast</a></span></dt>
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</dl></div>
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<p>
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Pointers to polymorphic objects (objects of classes which define at least one
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virtual function) are sometimes downcast or crosscast. Downcasting means casting
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from a base class to a derived class. Crosscasting means casting across an
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inheritance hierarchy diagram, such as from one base to the other in a <code class="literal">Y</code>
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diagram hierarchy.
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</p>
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<p>
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Such casts can be done with old-style casts, but this approach is never to
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be recommended. Old-style casts are sorely lacking in type safety, suffer poor
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readability, and are difficult to locate with search tools.
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</p>
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<div class="section">
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<div class="titlepage"><div><div><h3 class="title">
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<a name="conversion.polymorphic_casts.polymorphic_downcast"></a><a name="polymorphic_downcast"></a><a class="link" href="polymorphic_casts.html#conversion.polymorphic_casts.polymorphic_downcast" title="polymorphic_downcast">polymorphic_downcast</a>
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</h3></div></div></div>
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<p>
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The C++ built-in <code class="computeroutput"><span class="keyword">static_cast</span></code>
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can be used for efficiently downcasting pointers to polymorphic objects,
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but provides no error detection for the case where the pointer being cast
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actually points to the wrong derived class. The <code class="computeroutput"><span class="identifier">polymorphic_downcast</span></code>
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template retains the efficiency of <code class="computeroutput"><span class="keyword">static_cast</span></code>
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for non-debug compilations, but for debug compilations adds safety via an
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<code class="computeroutput"><span class="identifier">assert</span><span class="special">()</span></code>
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that a <code class="computeroutput"><span class="keyword">dynamic_cast</span></code> succeeds.
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</p>
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<p>
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A <code class="computeroutput"><span class="identifier">polymorphic_downcast</span></code> should
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be used for downcasts that you are certain should succeed. Error checking
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is only performed in translation units where <code class="computeroutput"><span class="identifier">NDEBUG</span></code>
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is not defined, via
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</p>
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<pre class="programlisting"><span class="identifier">assert</span><span class="special">(</span> <span class="keyword">dynamic_cast</span><span class="special"><</span><span class="identifier">Derived</span><span class="special">>(</span><span class="identifier">x</span><span class="special">)</span> <span class="special">==</span> <span class="identifier">x</span> <span class="special">)</span>
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</pre>
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<p>
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where <code class="computeroutput"><span class="identifier">x</span></code> is the source pointer.
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This approach ensures that not only is a non-zero pointer returned, but also
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that it is correct in the presence of multiple inheritance. Attempts to crosscast
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using <code class="computeroutput"><span class="identifier">polymorphic_downcast</span></code>
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will fail to compile.
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</p>
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<div class="warning"><table border="0" summary="Warning">
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<tr>
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<td rowspan="2" align="center" valign="top" width="25"><img alt="[Warning]" src="../../../doc/src/images/warning.png"></td>
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<th align="left">Warning</th>
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</tr>
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<tr><td align="left" valign="top"><p>
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Because <code class="computeroutput"><span class="identifier">polymorphic_downcast</span></code>
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uses <code class="computeroutput"><span class="identifier">assert</span><span class="special">()</span></code>,
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it violates the One Definition Rule (ODR) if <code class="computeroutput"><span class="identifier">NDEBUG</span></code>
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is inconsistently defined across translation units. See ISO Std 3.2
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</p></td></tr>
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</table></div>
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<h5>
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<a name="conversion.polymorphic_casts.polymorphic_downcast.h0"></a>
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<span class="phrase"><a name="conversion.polymorphic_casts.polymorphic_downcast.example_"></a></span><a class="link" href="polymorphic_casts.html#conversion.polymorphic_casts.polymorphic_downcast.example_">Example:</a>
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</h5>
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<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">polymorphic_cast</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
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<span class="special">...</span>
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<span class="keyword">class</span> <span class="identifier">Fruit</span> <span class="special">{</span> <span class="keyword">public</span><span class="special">:</span> <span class="keyword">virtual</span> <span class="special">~</span><span class="identifier">Fruit</span><span class="special">(){};</span> <span class="special">...</span> <span class="special">};</span>
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<span class="keyword">class</span> <span class="identifier">Banana</span> <span class="special">:</span> <span class="keyword">public</span> <span class="identifier">Fruit</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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<span class="keyword">void</span> <span class="identifier">f</span><span class="special">(</span> <span class="identifier">Fruit</span> <span class="special">*</span> <span class="identifier">fruit</span> <span class="special">)</span> <span class="special">{</span>
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<span class="comment">// ... logic which leads us to believe it is a Banana</span>
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<span class="identifier">Banana</span> <span class="special">*</span> <span class="identifier">banana</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">polymorphic_downcast</span><span class="special"><</span><span class="identifier">Banana</span><span class="special">*>(</span><span class="identifier">fruit</span><span class="special">);</span>
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<span class="special">...</span>
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<span class="special">}</span>
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</pre>
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</div>
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<div class="section">
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<div class="titlepage"><div><div><h3 class="title">
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<a name="conversion.polymorphic_casts.polymorphic_cast"></a><a name="polymorphic_cast"></a><a class="link" href="polymorphic_casts.html#conversion.polymorphic_casts.polymorphic_cast" title="polymorphic_cast">polymorphic_cast</a>
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</h3></div></div></div>
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<p>
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The C++ built-in <code class="computeroutput"><span class="keyword">dynamic_cast</span></code>
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can be used for downcasts and crosscasts of pointers to polymorphic objects,
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but error notification in the form of a returned value of 0 is inconvenient
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to test, or worse yet, easy to forget to test. The throwing form of <code class="computeroutput"><span class="keyword">dynamic_cast</span></code>, which works on references, can
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be used on pointers through the ugly expression <code class="computeroutput"><span class="special">&</span><span class="keyword">dynamic_cast</span><span class="special"><</span><span class="identifier">T</span><span class="special">&>(*</span><span class="identifier">p</span><span class="special">)</span></code>, which
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causes undefined behavior if <code class="computeroutput"><span class="identifier">p</span></code>
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is <code class="computeroutput"><span class="number">0</span></code>. The <code class="computeroutput"><span class="identifier">polymorphic_cast</span></code>
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template performs a <code class="computeroutput"><span class="keyword">dynamic_cast</span></code>
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on a pointer, and throws an exception if the <code class="computeroutput"><span class="keyword">dynamic_cast</span></code>
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returns 0.
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</p>
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<p>
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For crosscasts, or when the success of a cast can only be known at runtime,
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or when efficiency is not important, <code class="computeroutput"><span class="identifier">polymorphic_cast</span></code>
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is preferred.
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</p>
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<p>
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The C++ built-in <code class="computeroutput"><span class="keyword">dynamic_cast</span></code>
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must be used to cast references rather than pointers. It is also the only
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cast that can be used to check whether a given interface is supported; in
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that case a return of 0 isn't an error condition.
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</p>
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</div>
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<div class="section">
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<div class="titlepage"><div><div><h3 class="title">
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<a name="conversion.polymorphic_casts.polymorphic_pointer_cast"></a><a name="polymorphic_pointer_cast"></a><a class="link" href="polymorphic_casts.html#conversion.polymorphic_casts.polymorphic_pointer_cast" title="polymorphic_pointer_cast">polymorphic_pointer_cast</a>
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</h3></div></div></div>
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<p>
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While <code class="computeroutput"><span class="identifier">polymorphic_downcast</span></code>
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and <code class="computeroutput"><span class="identifier">polymorphic_cast</span></code> work
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with built-in pointer types only, <code class="computeroutput"><span class="identifier">polymorphic_pointer_downcast</span></code>
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and <code class="computeroutput"><span class="identifier">polymorphic_pointer_cast</span></code>
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are more generic versions with support for any pointer type for which the
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following expressions would be valid:
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</p>
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<p>
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For <code class="computeroutput"><span class="identifier">polymorphic_pointer_downcast</span></code>:
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</p>
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<pre class="programlisting"><span class="identifier">static_pointer_cast</span><span class="special"><</span><span class="identifier">Derived</span><span class="special">>(</span><span class="identifier">p</span><span class="special">);</span>
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<span class="identifier">dynamic_pointer_cast</span><span class="special"><</span><span class="identifier">Derived</span><span class="special">>(</span><span class="identifier">p</span><span class="special">);</span>
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</pre>
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<p>
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For <code class="computeroutput"><span class="identifier">polymorphic_pointer_cast</span></code>:
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</p>
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<pre class="programlisting"><span class="identifier">dynamic_pointer_cast</span><span class="special"><</span><span class="identifier">Derived</span><span class="special">>(</span><span class="identifier">p</span><span class="special">);</span>
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<span class="special">!</span><span class="identifier">p</span><span class="special">;</span> <span class="comment">// conversion to bool with negation</span>
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</pre>
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<p>
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This includes C++ built-in pointers, <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">shared_ptr</span></code>,
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<code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">shared_ptr</span></code>, <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">intrusive_ptr</span></code>,
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etc.
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</p>
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<h5>
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<a name="conversion.polymorphic_casts.polymorphic_pointer_cast.h0"></a>
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<span class="phrase"><a name="conversion.polymorphic_casts.polymorphic_pointer_cast.example_"></a></span><a class="link" href="polymorphic_casts.html#conversion.polymorphic_casts.polymorphic_pointer_cast.example_">Example:</a>
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</h5>
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<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">polymorphic_pointer_cast</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
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<span class="keyword">class</span> <span class="identifier">Fruit</span> <span class="special">{</span> <span class="keyword">public</span><span class="special">:</span> <span class="keyword">virtual</span> <span class="special">~</span><span class="identifier">Fruit</span><span class="special">(){}</span> <span class="special">};</span>
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<span class="keyword">class</span> <span class="identifier">Banana</span> <span class="special">:</span> <span class="keyword">public</span> <span class="identifier">Fruit</span> <span class="special">{};</span>
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<span class="comment">// Use one of these:</span>
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<span class="keyword">typedef</span> <span class="identifier">Fruit</span><span class="special">*</span> <span class="identifier">FruitPtr</span><span class="special">;</span>
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<span class="keyword">typedef</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">shared_ptr</span><span class="special"><</span><span class="identifier">Fruit</span><span class="special">></span> <span class="identifier">FruitPtr</span><span class="special">;</span>
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<span class="keyword">typedef</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">shared_ptr</span><span class="special"><</span><span class="identifier">Fruit</span><span class="special">></span> <span class="identifier">FruitPtr</span><span class="special">;</span>
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<span class="keyword">typedef</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">intrusive_ptr</span><span class="special"><</span><span class="identifier">Fruit</span><span class="special">></span> <span class="identifier">FruitPtr</span><span class="special">;</span>
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<span class="keyword">void</span> <span class="identifier">f</span><span class="special">(</span><span class="identifier">FruitPtr</span> <span class="identifier">fruit</span><span class="special">)</span> <span class="special">{</span>
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<span class="comment">// ... logic which leads us to believe it is a banana</span>
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<span class="keyword">auto</span> <span class="identifier">banana</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">polymorphic_pointer_downcast</span><span class="special"><</span><span class="identifier">Banana</span><span class="special">>(</span><span class="identifier">fruit</span><span class="special">);</span>
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<span class="special">...</span>
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<span class="special">}</span>
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</pre>
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</div>
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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"><div class="copyright-footer">Copyright © 2001 Beman Dawes<br>Copyright © 2014-2021 Antony Polukhin<p>
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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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>)
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