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3 Commits
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 nobody
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c35f326490 | ||
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nobody
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175c00ea3e | ||
 Darin Adler
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23e5f7cca3 |
@@ -1,3 +1,5 @@
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<html>
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<head>
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@@ -9,46 +11,49 @@
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<body bgcolor="#FFFFFF" text="#000000">
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<h1><img src="../../c++boost.gif" alt="c++boost.gif (8819 bytes)" align="center" width="277" height="86">Header
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<h1><img src="../../c++boost.gif" alt="c++boost.gif (8819 bytes)" align="middle" width="277" height="86">Header
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<a href="../../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 <code>header <a href="../../boost/cast.hpp">boost/cast.hpp</a></code>
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<p>The header <a href="../../boost/cast.hpp">boost/cast.hpp</a>
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provides <a href="#Polymorphic_cast"><b>polymorphic_cast</b></a>, <a href="#Polymorphic_cast"><b>polymorphic_downcast</b></a>,
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and <a href="#numeric_cast"><b>numeric_cast</b></a> function templates designed
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to 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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<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 least one
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virtual function) are sometimes downcast or crosscast. Downcasting means
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casting from a base class to a derived class. Crosscasting means casting
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virtual function) are sometimes downcast or crosscast. Downcasting means
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casting from a base class to a derived class. Crosscasting means casting
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across an inheritance hierarchy diagram, such as from one base to the other in a
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<b>Y</b> diagram hierarchy.</p>
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<p>Such casts can be done with old-style casts, but this approach is never to be
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recommended. Old-style casts are sorely lacking in type safety, suffer
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recommended. Old-style casts are sorely lacking in type safety, suffer
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poor readability, and are difficult to locate with search tools.</p>
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<p>The C++ built-in <b>static_cast</b> can be used for efficiently downcasting
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pointers to polymorphic objects, but provides no error detection for the case
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where the pointer being cast actually points to the wrong derived class. The <b>polymorphic_downcast</b>
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template retains the efficiency of <b>static_cast</b> for non-debug
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compilations, but for debug compilations adds safety via an assert() that a <b>dynamic_cast</b>
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succeeds. <b> </b></p>
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succeeds.</p>
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<p>The C++ built-in <b>dynamic_cast</b> can be used for downcasts and crosscasts
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of pointers to polymorphic objects, but error notification in the form of a
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returned value of 0 is inconvenient to test, or worse yet, easy to forget to
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test. The <b>polymorphic_cast</b> template performs a <b>dynamic_cast</b>,
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test. The <b>polymorphic_cast</b> template performs a <b>dynamic_cast</b>,
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and throws an exception if the <b>dynamic_cast</b> returns 0.</p>
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<p>A <b>polymorphic_downcast</b> is preferred when debug-mode tests will cover
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100% of the object types possibly cast and when non-debug-mode efficiency is an
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issue. If these two conditions are not present, <b>polymorphic_cast</b> is
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preferred. It must also be used for crosscasts. It does an assert(
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preferred. It must also be used for crosscasts. It does an assert(
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dynamic_cast<Derived>(x) == x ) where x is the base pointer, ensuring that
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not only is a non-zero pointer returned, but also that it correct in the
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presence of multiple inheritance.<b> Warning:</b>: Because <b>polymorphic_downcast</b>
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uses assert(), it violates the one definition rule (ODR) if NDEBUG is inconsistently
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defined across translation units. [See ISO Std 3.2]</p>
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defined across translation units. [See ISO Std 3.2]</p>
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<p>The C++ built-in <b>dynamic_cast</b> must be used to cast references rather
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than pointers. It is also the only cast that can be used to check whether
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than pointers. It is also the only cast that can be used to check whether
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a given interface is supported; in that case a return of 0 isn't an error
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condition.</p>
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<h3>polymorphic_cast and polymorphic_downcast synopsis</h3>
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@@ -132,7 +137,7 @@ Abrahams</a>.<b><br>
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numeric_cast</b> was contributed by <a href="../../people/kevlin_henney.htm">Kevlin
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Henney</a>.</p>
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<hr>
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<p>Revised <!--webbot bot="Timestamp" s-type="EDITED" s-format="%d %B, %Y" startspan
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<p>Revised <!--webbot bot="Timestamp" s-type="EDITED" s-format="%d %B, %Y" startspan
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-->06 January, 2001<!--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 and
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