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New casts for smart pointers.

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[SVN r12743]
This commit is contained in:
Darin Adler
2002-02-06 19:42:04 +00:00
parent 0a560dd543
commit 23e5f7cca3
1 changed files with 17 additions and 12 deletions
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cast.htm
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
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<body bgcolor="#FFFFFF" text="#000000">
<h1><img src="../../c++boost.gif" alt="c++boost.gif (8819 bytes)" align="center" width="277" height="86">Header
<h1><img src="../../c++boost.gif" alt="c++boost.gif (8819 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 <code>header <a href="../../boost/cast.hpp">boost/cast.hpp</a></code>
<p>The header <a href="../../boost/cast.hpp">boost/cast.hpp</a>
provides <a href="#Polymorphic_cast"><b>polymorphic_cast</b></a>, <a href="#Polymorphic_cast"><b>polymorphic_downcast</b></a>,
and <a href="#numeric_cast"><b>numeric_cast</b></a> function templates designed
to complement the C++ built-in casts.</p>
<p>The program&nbsp;<a href="cast_test.cpp">cast_test.cpp</a> can be used to
<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.&nbsp; Downcasting means
casting from a base class to a derived class.&nbsp; Crosscasting means casting
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
<b>Y</b> diagram hierarchy.</p>
<p>Such casts can be done with old-style casts, but this approach is never to be
recommended.&nbsp; Old-style casts are sorely lacking in type safety, suffer
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 <b>static_cast</b> 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 <b>polymorphic_downcast</b>
template retains the efficiency of <b>static_cast</b> for non-debug
compilations, but for debug compilations adds safety via an assert() that a <b>dynamic_cast</b>
succeeds.&nbsp;<b>&nbsp;</b></p>
succeeds.</p>
<p>The C++ built-in <b>dynamic_cast</b> 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.&nbsp; The <b>polymorphic_cast</b> template performs a <b>dynamic_cast</b>,
test. The <b>polymorphic_cast</b> template performs a <b>dynamic_cast</b>,
and throws an exception if the <b>dynamic_cast</b> returns 0.</p>
<p>A <b>polymorphic_downcast</b> is preferred when debug-mode tests will cover
100% of the object types possibly cast and when non-debug-mode efficiency is an
issue. If these two conditions are not present, <b>polymorphic_cast</b> is
preferred.&nbsp; It must also be used for crosscasts.&nbsp; It does an assert(
preferred. It must also be used for crosscasts. It does an assert(
dynamic_cast&lt;Derived&gt;(x) == x ) where x is the base pointer, ensuring that
not only is a non-zero pointer returned, but also that it correct in the
presence of multiple inheritance.<b> Warning:</b>: Because <b>polymorphic_downcast</b>
uses assert(), it violates the one definition rule (ODR) if NDEBUG is inconsistently
defined across translation units.&nbsp; [See ISO Std 3.2]</p>
defined across translation units. [See ISO Std 3.2]</p>
<p>The C++ built-in <b>dynamic_cast</b> must be used to cast references rather
than pointers.&nbsp; It is also the only cast that can be used to check whether
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>
@@ -132,7 +137,7 @@ Abrahams</a>.<b><br>
numeric_cast</b> was contributed by <a href="../../people/kevlin_henney.htm">Kevlin
Henney</a>.</p>
<hr>
<p>Revised&nbsp; <!--webbot bot="Timestamp" s-type="EDITED" s-format="%d %B, %Y" startspan
<p>Revised <!--webbot bot="Timestamp" s-type="EDITED" s-format="%d %B, %Y" startspan
-->06 January, 2001<!--webbot bot="Timestamp" endspan i-checksum="38320"
--></p>
<p><EFBFBD> Copyright boost.org 1999. Permission to copy, use, modify, sell and