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     <h1><img src="../../boost.png" alt="boost.png (6897 bytes)" align=
     "middle" width="277" height="86">Header <a href=
     "../../boost/cast.hpp">boost/cast.hpp</a></h1>

     <h2><a name="Cast Functions">Cast Functions</a></h2>

     <p>The header <a href="../../boost/cast.hpp">boost/cast.hpp</a> provides <code>
     <a href="#Polymorphic_cast">polymorphic_cast</a> and</code> <a href=
     "#Polymorphic_cast"><code>polymorphic_downcast</code></a> function templates designed to
     complement the C++ built-in casts.</p>

     <p>The program <a href="cast_test.cpp">cast_test.cpp</a> can be used to
     verify these function templates work as expected.</p>

     <h3><a name="Polymorphic_cast">Polymorphic casts</a></h3>

     <p>Pointers to polymorphic objects (objects of classes which define at
     least one virtual function) are sometimes downcast or crosscast.
     Downcasting means casting from a base class to a derived class.
     Crosscasting means casting across an inheritance hierarchy diagram, such
     as from one base to the other in a <code>Y</code> diagram hierarchy.</p>

     <p>Such casts can be done with old-style casts, but this approach is
     never to be recommended. Old-style casts are sorely lacking in type
     safety, suffer poor readability, and are difficult to locate with search
     tools.</p>

     <p>The C++ built-in <code>static_cast</code> can be used for efficiently
     downcasting pointers to polymorphic objects, but provides no error
     detection for the case where the pointer being cast actually points to
     the wrong derived class. The <code>polymorphic_downcast</code> template retains
     the efficiency of <code>static_cast</code> for non-debug compilations, but for
     debug compilations adds safety via an assert() that a <code>dynamic_cast</code>
     succeeds.</p>

     <p>The C++ built-in <code>dynamic_cast</code> can be used for downcasts and
     crosscasts of pointers to polymorphic objects, but error notification in
     the form of a returned value of 0 is inconvenient to test, or worse yet,
     easy to forget to test. The throwing form of <code>dynamic_cast</code>, which
     works on references, can be used on pointers through the ugly expression
     &amp;<code>dynamic_cast&lt;T&amp;&gt;(*p)</code>, which causes undefined
     behavior if <code>p</code> is <code>0</code>. The <code>polymorphic_cast</code>
     template performs a <code>dynamic_cast</code> on a pointer, and throws an
     exception if the <code>dynamic_cast</code> returns 0.</p>

     <p>A <code>polymorphic_downcast</code> should be used for
     downcasts that you are certain should succeed.  Error checking is
     only performed in translation units where <code>NDEBUG</code> is
     not defined, via
 <pre>  assert( dynamic_cast&lt;Derived&gt;(x) == x )
 </pre> where <code>x</code> is the source pointer.  This approach
     ensures that not only is a non-zero pointer returned, but also
     that it is correct in the presence of multiple inheritance.
     Attempts to crosscast using <code>polymorphic_downcast</code> will
     fail to compile.
     <b>Warning:</b> Because <code>polymorphic_downcast</code> uses assert(), it
     violates the One Definition Rule (ODR) if NDEBUG is inconsistently
     defined across translation units. [See ISO Std 3.2]
 </p><p>
     For crosscasts, or when the success of a cast can only be known at
     runtime, or when efficiency is not important,
     <code>polymorphic_cast</code> is preferred. </p>

     <p>The C++ built-in <code>dynamic_cast</code> must be used to cast references
     rather than pointers. It is also the only cast that can be used to check
     whether a given interface is supported; in that case a return of 0 isn't
     an error condition.</p>

     <h3>polymorphic_cast and polymorphic_downcast synopsis</h3>

     <blockquote>
 <pre>namespace boost {

 template &lt;class Derived, class Base&gt;
 inline Derived polymorphic_cast(Base* x);
 // Throws: std::bad_cast if ( dynamic_cast&lt;Derived&gt;(x) == 0 )
 // Returns: dynamic_cast&lt;Derived&gt;(x)

 template &lt;class Derived, class Base&gt;
 inline Derived polymorphic_downcast(Base* x);
 // Effects: assert( dynamic_cast&lt;Derived&gt;(x) == x );
 // Returns: static_cast&lt;Derived&gt;(x)

 }
 </pre>
     </blockquote>

     <h3>polymorphic_downcast example</h3>

     <blockquote>
 <pre>#include &lt;boost/cast.hpp&gt;
 ...
 class Fruit { public: virtual ~Fruit(){}; ... };
 class Banana : public Fruit { ... };
 ...
 void f( Fruit * fruit ) {
 // ... logic which leads us to believe it is a Banana
   Banana * banana = boost::polymorphic_downcast&lt;Banana*&gt;(fruit);
   ...
 </pre>
     </blockquote>

     <h3>History</h3>

     <p><code>polymorphic_cast</code> was suggested by Bjarne Stroustrup in "The C++
     Programming Language".<br>
      <code>polymorphic_downcast</code> was contributed by <a href=
     "http://www.boost.org/people/dave_abrahams.htm">Dave Abrahams</a>.<code><br>
      An old
      numeric_cast</code> that was contributed by <a href=
     "http://www.boost.org/people/kevlin_henney.htm">Kevlin Henney</a> is now superseeded by the <a href="../numeric/conversion/doc/html/index.html">Boost Numeric Conversion Library</a></p>
     <hr>

     <p>Revised
     <!--webbot bot="Timestamp" s-type="EDITED" s-format="%d %B, %Y" startspan
                     -->June 23, 2005<!--webbot bot="Timestamp" endspan i-checksum="30348"
                     --></p>

     <p>&copy; Copyright boost.org 1999. Permission to copy, use, modify, sell
     and distribute this document is granted provided this copyright notice
     appears in all copies. This document is provided "as is" without express
     or implied warranty, and with no claim as to its suitability for any
     purpose.</p>
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	<title>Header boost/cast.hpp Documentation</title>
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	<body bgcolor="#FFFFFF" text="#000000">
	<h1><img src="../../boost.png" alt="boost.png (6897 bytes)" align=
	"middle" width="277" height="86">Header <a href=
	"../../boost/cast.hpp">boost/cast.hpp</a></h1>

	<h2><a name="Cast Functions">Cast Functions</a></h2>

	<p>The header <a href="../../boost/cast.hpp">boost/cast.hpp</a> provides <code>
	<a href="#Polymorphic_cast">polymorphic_cast</a> and</code> <a href=
	"#Polymorphic_cast"><code>polymorphic_downcast</code></a> function templates designed to
	complement the C++ built-in casts.</p>

	<p>The program <a href="cast_test.cpp">cast_test.cpp</a> can be used to
	verify these function templates work as expected.</p>

	<h3><a name="Polymorphic_cast">Polymorphic casts</a></h3>

	<p>Pointers to polymorphic objects (objects of classes which define at
	least one virtual function) are sometimes downcast or crosscast.
	Downcasting means casting from a base class to a derived class.
	Crosscasting means casting across an inheritance hierarchy diagram, such
	as from one base to the other in a <code>Y</code> diagram hierarchy.</p>

	<p>Such casts can be done with old-style casts, but this approach is
	never to be recommended. Old-style casts are sorely lacking in type
	safety, suffer poor readability, and are difficult to locate with search
	tools.</p>

	<p>The C++ built-in <code>static_cast</code> can be used for efficiently
	downcasting pointers to polymorphic objects, but provides no error
	detection for the case where the pointer being cast actually points to
	the wrong derived class. The <code>polymorphic_downcast</code> template retains
	the efficiency of <code>static_cast</code> for non-debug compilations, but for
	debug compilations adds safety via an assert() that a <code>dynamic_cast</code>
	succeeds.</p>

	<p>The C++ built-in <code>dynamic_cast</code> can be used for downcasts and
	crosscasts of pointers to polymorphic objects, but error notification in
	the form of a returned value of 0 is inconvenient to test, or worse yet,
	easy to forget to test. The throwing form of <code>dynamic_cast</code>, which
	works on references, can be used on pointers through the ugly expression
	&<code>dynamic_cast<T&>(*p)</code>, which causes undefined
	behavior if <code>p</code> is <code>0</code>. The <code>polymorphic_cast</code>
	template performs a <code>dynamic_cast</code> on a pointer, and throws an
	exception if the <code>dynamic_cast</code> returns 0.</p>

	<p>A <code>polymorphic_downcast</code> should be used for
	downcasts that you are certain should succeed. Error checking is
	only performed in translation units where <code>NDEBUG</code> is
	not defined, via
	<pre> assert( dynamic_cast<Derived>(x) == x )
	</pre> where <code>x</code> is the source pointer. This approach
	ensures that not only is a non-zero pointer returned, but also
	that it is correct in the presence of multiple inheritance.
	Attempts to crosscast using <code>polymorphic_downcast</code> will
	fail to compile.
	<b>Warning:</b> Because <code>polymorphic_downcast</code> uses assert(), it
	violates the One Definition Rule (ODR) if NDEBUG is inconsistently
	defined across translation units. [See ISO Std 3.2]
	</p><p>
	For crosscasts, or when the success of a cast can only be known at
	runtime, or when efficiency is not important,
	<code>polymorphic_cast</code> is preferred. </p>

	<p>The C++ built-in <code>dynamic_cast</code> must be used to cast references
	rather than pointers. It is also the only cast that can be used to check
	whether a given interface is supported; in that case a return of 0 isn't
	an error condition.</p>

	<h3>polymorphic_cast and polymorphic_downcast synopsis</h3>

	<blockquote>
	<pre>namespace boost {

	template <class Derived, class Base>
	inline Derived polymorphic_cast(Base* x);
	// Throws: std::bad_cast if ( dynamic_cast<Derived>(x) == 0 )
	// Returns: dynamic_cast<Derived>(x)

	template <class Derived, class Base>
	inline Derived polymorphic_downcast(Base* x);
	// Effects: assert( dynamic_cast<Derived>(x) == x );
	// Returns: static_cast<Derived>(x)

	}
	</pre>
	</blockquote>

	<h3>polymorphic_downcast example</h3>

	<blockquote>
	<pre>#include <boost/cast.hpp>
	...
	class Fruit { public: virtual ~Fruit(){}; ... };
	class Banana : public Fruit { ... };
	...
	void f( Fruit * fruit ) {
	// ... logic which leads us to believe it is a Banana
	Banana * banana = boost::polymorphic_downcast<Banana*>(fruit);
	...
	</pre>
	</blockquote>

	<h3>History</h3>

	<p><code>polymorphic_cast</code> was suggested by Bjarne Stroustrup in "The C++
	Programming Language".<br>
	<code>polymorphic_downcast</code> was contributed by <a href=
	"http://www.boost.org/people/dave_abrahams.htm">Dave Abrahams</a>.<code><br>
	An old
	numeric_cast</code> that was contributed by <a href=
	"http://www.boost.org/people/kevlin_henney.htm">Kevlin Henney</a> is now superseeded by the <a href="../numeric/conversion/doc/html/index.html">Boost Numeric Conversion Library</a></p>
	<hr>

	<p>Revised
	<!--webbot bot="Timestamp" s-type="EDITED" s-format="%d %B, %Y" startspan
	-->June 23, 2005<!--webbot bot="Timestamp" endspan i-checksum="30348"
	--></p>

	<p>© Copyright boost.org 1999. Permission to copy, use, modify, sell
	and distribute this document is granted provided this copyright notice
	appears in all copies. This document is provided "as is" without express
	or implied warranty, and with no claim as to its suitability for any
	purpose.</p>
	</body>
	</html>