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<title>Header boost/cast.hpp Documentation</title>
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<h1><img src="../../boost.png" alt="boost.png (6897 bytes)" align=
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"middle" width="277" height="86">Header <a href=
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"../../boost/cast.hpp">boost/cast.hpp</a></h1>
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<h2><a name="Cast Functions">Cast Functions</a></h2>
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<p>The header <a href="../../boost/cast.hpp">boost/cast.hpp</a> provides
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<a href="#Polymorphic_cast"><code>polymorphic_cast</code></a>, <a href=
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"#Polymorphic_cast"><code>polymorphic_downcast</code></a>, and <a href=
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"#numeric_cast"><code>numeric_cast</code></a> function templates designed to
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complement the C++ built-in casts.</p>
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<p>The program <a href="cast_test.cpp">cast_test.cpp</a> can be used to
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verify these function templates work as expected.</p>
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<h3><a name="Polymorphic_cast">Polymorphic casts</a></h3>
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<p>Pointers to polymorphic objects (objects of classes which define at
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least one virtual function) are sometimes downcast or crosscast.
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Downcasting means casting from a base class to a derived class.
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Crosscasting means casting across an inheritance hierarchy diagram, such
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as from one base to the other in a <code>Y</code> diagram hierarchy.</p>
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<p>Such casts can be done with old-style casts, but this approach is
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never to be recommended. Old-style casts are sorely lacking in type
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safety, suffer poor readability, and are difficult to locate with search
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tools.</p>
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<p>The C++ built-in <code>static_cast</code> can be used for efficiently
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downcasting pointers to polymorphic objects, but provides no error
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detection for the case where the pointer being cast actually points to
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the wrong derived class. The <code>polymorphic_downcast</code> template retains
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the efficiency of <code>static_cast</code> for non-debug compilations, but for
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debug compilations adds safety via an assert() that a <code>dynamic_cast</code>
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succeeds.</p>
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<p>The C++ built-in <code>dynamic_cast</code> can be used for downcasts and
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crosscasts of pointers to polymorphic objects, but error notification in
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the form of a returned value of 0 is inconvenient to test, or worse yet,
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easy to forget to test. The throwing form of <code>dynamic_cast</code>, which
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works on references, can be used on pointers through the ugly expression
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&<code>dynamic_cast<T&>(*p)</code>, which causes undefined
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behavior if <code>p</code> is <code>0</code>. The <code>polymorphic_cast</code>
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template performs a <code>dynamic_cast</code> on a pointer, and throws an
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exception if the <code>dynamic_cast</code> returns 0.</p>
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<p>A <code>polymorphic_downcast</code> should be used for
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downcasts that you are certain should succeed. Error checking is
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only performed in translation units where <code>NDEBUG</code> is
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not defined, via
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<pre>
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assert( dynamic_cast<Derived>(x) == x )
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</pre> where <code>x</code> is the source pointer. This approach
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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.
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Attempts to crosscast using <code>polymorphic_downcast</code> will
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fail to compile.
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<b>Warning:</b> Because <code>polymorphic_downcast</code> uses assert(), it
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violates the One Definition Rule (ODR) if NDEBUG is inconsistently
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defined across translation units. [See ISO Std 3.2]
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</p><p>
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For crosscasts, or when the success of a cast can only be known at
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runtime, or when efficiency is not important,
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<code>polymorphic_cast</code> is preferred. </p>
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<p>The C++ built-in <code>dynamic_cast</code> must be used to cast references
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rather than pointers. It is also the only cast that can be used to check
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whether a given interface is supported; in that case a return of 0 isn't
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an error condition.</p>
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<h3>polymorphic_cast and polymorphic_downcast synopsis</h3>
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<blockquote>
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<pre>
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namespace boost {
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template <class Derived, class Base>
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inline Derived polymorphic_cast(Base* x);
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// Throws: std::bad_cast if ( dynamic_cast<Derived>(x) == 0 )
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// Returns: dynamic_cast<Derived>(x)
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template <class Derived, class Base>
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inline Derived polymorphic_downcast(Base* x);
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// Effects: assert( dynamic_cast<Derived>(x) == x );
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// Returns: static_cast<Derived>(x)
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}
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</pre>
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</blockquote>
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<h3>polymorphic_downcast example</h3>
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<blockquote>
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<pre>
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#include <boost/cast.hpp>
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...
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class Fruit { public: virtual ~Fruit(){}; ... };
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class Banana : public Fruit { ... };
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...
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void f( Fruit * fruit ) {
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// ... logic which leads us to believe it is a Banana
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Banana * banana = boost::polymorphic_downcast<Banana*>(fruit);
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...
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</pre>
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</blockquote>
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<h3><a name="numeric_cast">numeric_cast</a></h3>
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<p>A <code>static_cast</code> or implicit conversion will not detect failure to
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preserve range for numeric casts. The <code>numeric_cast</code> function
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templates are similar to <code>static_cast</code> and certain (dubious)
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implicit conversions in this respect, except that they detect loss of
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numeric range. An exception is thrown when a runtime value-preservation
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check fails.</p>
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<p>The requirements on the argument and result types are:</p>
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<blockquote>
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<ul>
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<li>Both argument and result types are CopyConstructible [ISO Std
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20.1.3].</li>
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<li>Both argument and result types are Numeric, defined by
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<code>std::numeric_limits<>::is_specialized</code> being
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true.</li>
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<li>The argument can be converted to the result type using
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<code>static_cast</code>.</li>
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</ul>
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</blockquote>
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<h3>numeric_cast synopsis</h3>
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<blockquote>
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<pre>
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namespace boost {
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class bad_numeric_cast : public std::bad_cast {...};
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template<typename Target, typename Source>
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inline Target numeric_cast(Source arg);
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// Throws: bad_numeric_cast unless, in converting arg from Source to Target,
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// there is no loss of negative range, and no underflow, and no
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// overflow, as determined by std::numeric_limits
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// Returns: static_cast<Target>(arg)
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}
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</pre>
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</blockquote>
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<h3>numeric_cast example</h3>
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<blockquote>
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<pre>
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#include <boost/cast.hpp>
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using namespace boost::cast;
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void ariane(double vx)
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{
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...
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unsigned short dx = numeric_cast<unsigned short>(vx);
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...
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}
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</pre>
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</blockquote>
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<h3>numeric_cast rationale</h3>
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<p>The form of the throws condition is specified so that != is not a
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required operation.</p>
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<h3>History</h3>
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<p><code>polymorphic_cast</code> was suggested by Bjarne Stroustrup in "The C++
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Programming Language".<br>
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<code>polymorphic_downcast</code> was contributed by <a href=
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"../../people/dave_abrahams.htm">Dave Abrahams</a>.<code><br>
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numeric_cast</code> was contributed by <a href=
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"../../people/kevlin_henney.htm">Kevlin Henney</a>.</p>
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<hr>
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<p>Revised
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<!--webbot bot="Timestamp" s-type="EDITED" s-format="%d %B, %Y" startspan
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-->06 January, 2001
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<!--webbot bot="Timestamp" endspan i-checksum="38320"
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--></p>
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<p>© Copyright boost.org 1999. Permission to copy, use, modify, sell
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and distribute this document is granted provided this copyright notice
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appears in all copies. This document is provided "as is" without express
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or implied warranty, and with no claim as to its suitability for any
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purpose.</p>
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