<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<meta name="generator" content=
"Microsoft FrontPage 5.0">
<meta http-equiv="Content-Type" content=
"text/html; charset=windows-1252">
<meta name="GENERATOR" content="Microsoft FrontPage 4.0">
<meta name="ProgId" content="FrontPage.Editor.Document">
<title>Header boost/cast.hpp Documentation</title>
</head>
<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>
