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Improved justification for polymorphic_cast.

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[SVN r20068]
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Dave Abrahams
2003-09-15 16:30:22 +00:00
parent 13796f3f98
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<h1><img src="../../c++boost.gif" alt="c++boost.gif (8819 bytes)" align=
<h1><img src="../../c++boost.gif" alt="c++boost.gif (8819 bytes)" align="middle" width="277" height="86">Header "middle" width="277" height="86">Header <a href=
<a href="../../boost/cast.hpp">boost/cast.hpp</a></h1> "../../boost/cast.hpp">boost/cast.hpp</a></h1>
<h2><a name="Cast Functions">Cast Functions</a></h2> <h2><a name="Cast Functions">Cast Functions</a></h2>
<p>The header <a href="../../boost/cast.hpp">boost/cast.hpp</a> <p>The header <a href="../../boost/cast.hpp">boost/cast.hpp</a> provides
provides <a href="#Polymorphic_cast"><b>polymorphic_cast</b></a>, <a href="#Polymorphic_cast"><b>polymorphic_downcast</b></a>, <a href="#Polymorphic_cast"><b>polymorphic_cast</b></a>, <a href=
and <a href="#numeric_cast"><b>numeric_cast</b></a> function templates designed "#Polymorphic_cast"><b>polymorphic_downcast</b></a>, and <a href=
to complement the C++ built-in casts.</p> "#numeric_cast"><b>numeric_cast</b></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 <p>The program <a href="cast_test.cpp">cast_test.cpp</a> can be used to
verify these function templates work as expected.</p> verify these function templates work as expected.</p>
<h3><a name="Polymorphic_cast">Polymorphic casts</a></h3> <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 <p>Pointers to polymorphic objects (objects of classes which define at
casting from a base class to a derived class. Crosscasting means casting least one virtual function) are sometimes downcast or crosscast.
across an inheritance hierarchy diagram, such as from one base to the other in a Downcasting means casting from a base class to a derived class.
<b>Y</b> diagram hierarchy.</p> Crosscasting means casting across an inheritance hierarchy diagram, such
<p>Such casts can be done with old-style casts, but this approach is never to be as from one base to the other in a <b>Y</b> diagram hierarchy.</p>
recommended. Old-style casts are sorely lacking in type safety, suffer
poor readability, and are difficult to locate with search tools.</p> <p>Such casts can be done with old-style casts, but this approach is
<p>The C++ built-in <b>static_cast</b> can be used for efficiently downcasting never to be recommended. Old-style casts are sorely lacking in type
pointers to polymorphic objects, but provides no error detection for the case safety, suffer poor readability, and are difficult to locate with search
where the pointer being cast actually points to the wrong derived class. The <b>polymorphic_downcast</b> tools.</p>
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> <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.</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 <p>The C++ built-in <b>dynamic_cast</b> can be used for downcasts and
returned value of 0 is inconvenient to test, or worse yet, easy to forget to crosscasts of pointers to polymorphic objects, but error notification in
test. The <b>polymorphic_cast</b> template performs a <b>dynamic_cast</b>, the form of a returned value of 0 is inconvenient to test, or worse yet,
and throws an exception if the <b>dynamic_cast</b> returns 0.</p> easy to forget to test. The throwing form of <b>dynamic_cast</b>, which
<p>A <b>polymorphic_downcast</b> is preferred when debug-mode tests will cover works on references, can be used on pointers through the ugly expression
100% of the object types possibly cast and when non-debug-mode efficiency is an &amp;<code>dynamic_cast&lt;T&amp;&gt;(*p)</code>, which causes undefined
issue. If these two conditions are not present, <b>polymorphic_cast</b> is behavior if <code>p</code> is <code>0</code>. The <b>polymorphic_cast</b>
preferred. It must also be used for crosscasts. It does an assert( template performs a <b>dynamic_cast</b> on a pointer, and throws an
dynamic_cast&lt;Derived&gt;(x) == x ) where x is the base pointer, ensuring that exception if the <b>dynamic_cast</b> returns 0.</p>
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> <p>A <b>polymorphic_downcast</b> is preferred when debug-mode tests will
uses assert(), it violates the one definition rule (ODR) if NDEBUG is inconsistently cover 100% of the object types possibly cast and when non-debug-mode
defined across translation units. [See ISO Std 3.2]</p> efficiency is an issue. If these two conditions are not present,
<p>The C++ built-in <b>dynamic_cast</b> must be used to cast references rather <b>polymorphic_cast</b> is preferred. It must also be used for
than pointers. It is also the only cast that can be used to check whether crosscasts. It does an assert( dynamic_cast&lt;Derived&gt;(x) == x )
a given interface is supported; in that case a return of 0 isn't an error where x is the base pointer, ensuring that not only is a non-zero pointer
condition.</p> 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. [See ISO Std 3.2]</p>
<p>The C++ built-in <b>dynamic_cast</b> 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> <h3>polymorphic_cast and polymorphic_downcast synopsis</h3>
<blockquote> <blockquote>
<pre>namespace boost { <pre>
namespace boost {
template &lt;class Derived, class Base&gt; template &lt;class Derived, class Base&gt;
inline Derived polymorphic_cast(Base* x); inline Derived polymorphic_cast(Base* x);
@@ -70,11 +91,15 @@ inline Derived polymorphic_downcast(Base* x);
// Effects: assert( dynamic_cast&lt;Derived&gt;(x) == x ); // Effects: assert( dynamic_cast&lt;Derived&gt;(x) == x );
// Returns: static_cast&lt;Derived&gt;(x) // Returns: static_cast&lt;Derived&gt;(x)
}</pre> }
</pre>
</blockquote> </blockquote>
<h3>polymorphic_downcast example</h3> <h3>polymorphic_downcast example</h3>
<blockquote> <blockquote>
<pre>#include &lt;boost/cast.hpp&gt; <pre>
#include &lt;boost/cast.hpp&gt;
... ...
class Fruit { public: virtual ~Fruit(){}; ... }; class Fruit { public: virtual ~Fruit(){}; ... };
class Banana : public Fruit { ... }; class Banana : public Fruit { ... };
@@ -82,26 +107,40 @@ class Banana : public Fruit { ... };
void f( Fruit * fruit ) { void f( Fruit * fruit ) {
// ... logic which leads us to believe it is a Banana // ... logic which leads us to believe it is a Banana
Banana * banana = boost::polymorphic_downcast&lt;Banana*&gt;(fruit); Banana * banana = boost::polymorphic_downcast&lt;Banana*&gt;(fruit);
...</pre> ...
</pre>
</blockquote> </blockquote>
<h3><a name="numeric_cast">numeric_cast</a></h3> <h3><a name="numeric_cast">numeric_cast</a></h3>
<p>A <b>static_cast</b> or implicit conversion will not
detect failure to preserve range for numeric casts. The <b>numeric_cast</b> function <p>A <b>static_cast</b> or implicit conversion will not detect failure to
preserve range for numeric casts. The <b>numeric_cast</b> function
templates are similar to <b>static_cast</b> and certain (dubious) templates are similar to <b>static_cast</b> and certain (dubious)
implicit conversions in this respect, except that they detect loss of numeric implicit conversions in this respect, except that they detect loss of
range. An exception is thrown when a runtime value-preservation check fails.</p> numeric range. An exception is thrown when a runtime value-preservation
check fails.</p>
<p>The requirements on the argument and result types are:</p> <p>The requirements on the argument and result types are:</p>
<blockquote> <blockquote>
<ul> <ul>
<li>Both argument and result types are CopyConstructible [ISO Std 20.1.3].</li> <li>Both argument and result types are CopyConstructible [ISO Std
<li>Both argument and result types are Numeric, defined by <code>std::numeric_limits&lt;&gt;::is_specialized</code> 20.1.3].</li>
being true.</li>
<li>The argument can be converted to the result type using <b>static_cast</b>.</li> <li>Both argument and result types are Numeric, defined by
<code>std::numeric_limits&lt;&gt;::is_specialized</code> being
true.</li>
<li>The argument can be converted to the result type using
<b>static_cast</b>.</li>
</ul> </ul>
</blockquote> </blockquote>
<h3>numeric_cast synopsis</h3> <h3>numeric_cast synopsis</h3>
<blockquote> <blockquote>
<pre>namespace boost { <pre>
namespace boost {
class bad_numeric_cast : public std::bad_cast {...}; class bad_numeric_cast : public std::bad_cast {...};
@@ -112,11 +151,15 @@ template&lt;typename Target, typename Source&gt;
// overflow, as determined by std::numeric_limits // overflow, as determined by std::numeric_limits
// Returns: static_cast&lt;Target&gt;(arg) // Returns: static_cast&lt;Target&gt;(arg)
}</pre> }
</pre>
</blockquote> </blockquote>
<h3>numeric_cast example</h3> <h3>numeric_cast example</h3>
<blockquote> <blockquote>
<pre>#include &lt;boost/cast.hpp&gt; <pre>
#include &lt;boost/cast.hpp&gt;
using namespace boost::cast; using namespace boost::cast;
void ariane(double vx) void ariane(double vx)
@@ -124,27 +167,36 @@ void ariane(double vx)
... ...
unsigned short dx = numeric_cast&lt;unsigned short&gt;(vx); unsigned short dx = numeric_cast&lt;unsigned short&gt;(vx);
... ...
}</pre> }
</pre>
</blockquote> </blockquote>
<h3>numeric_cast rationale</h3> <h3>numeric_cast rationale</h3>
<p>The form of the throws condition is specified so that != is not a required
operation.</p> <p>The form of the throws condition is specified so that != is not a
required operation.</p>
<h3>History</h3> <h3>History</h3>
<p><b>polymorphic_cast</b> was suggested by Bjarne Stroustrup in &quot;The C++
Programming Language&quot;.<br> <p><b>polymorphic_cast</b> was suggested by Bjarne Stroustrup in "The C++
<b>polymorphic_downcast</b> was contributed by <a href="../../people/dave_abrahams.htm">Dave Programming Language".<br>
Abrahams</a>.<b><br> <b>polymorphic_downcast</b> was contributed by <a href=
numeric_cast</b> was contributed by <a href="../../people/kevlin_henney.htm">Kevlin "../../people/dave_abrahams.htm">Dave Abrahams</a>.<b><br>
Henney</a>.</p> numeric_cast</b> was contributed by <a href=
"../../people/kevlin_henney.htm">Kevlin Henney</a>.</p>
<hr> <hr>
<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>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>
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<p>&copy; Copyright boost.org 1999. Permission to copy, use, modify, sell
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