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Finish up the isa/cast/dyn_cast section

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@3645 91177308-0d34-0410-b5e6-96231b3b80d8
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Chris Lattner 2002-09-10 15:20:46 +00:00
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@ -97,8 +97,8 @@
<li>Important iterator invalidation semantics to be aware of
</ul>
<p><b>Written by <a href="mailto:dhurjati@cs.uiuc.edu">Dinakar Dhurjati</a>
<a href="mailto:sabre@nondot.org">Chris Lattner</a>, and
<p><b>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a>,
<a href="mailto:dhurjati@cs.uiuc.edu">Dinakar Dhurjati</a>, and
<a href="mailto:jstanley@cs.uiuc.edu">Joel Stanley</a></b><p>
</ol>
@ -235,7 +235,20 @@ checks to see if the operand is of the specified type, and if so, returns a
pointer to it (this operator does not work with references). If the operand is
not of the correct type, a null pointer is returned. Thus, this works very much
like the <tt>dynamic_cast</tt> operator in C++, and should be used in the same
circumstances. An example is:<p>
circumstances. Typically, the <tt>dyn_cast&lt;&gt;</tt> operator is used in an
<tt>if</tt> statement or some other flow control statement like this:<p>
<pre>
if (<a href="#AllocationInst">AllocationInst</a> *AI = dyn_cast&lt;<a href="#AllocationInst">AllocationInst</a>&gt;(Val)) {
...
}
</pre><p>
This form of the <tt>if</tt> statement effectively combines together a call to
<tt>isa&lt;&gt;</tt> and a call to <tt>cast&lt;&gt;</tt> into one statement,
which is very convenient.<p>
Another common example is:<p>
<pre>
<i>// Loop over all of the phi nodes in a basic block</i>
@ -244,12 +257,36 @@ circumstances. An example is:<p>
cerr &lt;&lt; *PN;
</pre><p>
Note that you should not use the <tt>dyn_cast&lt;&gt;</tt> operator in a series
of chained if statements, use an visitor instead... FIXME: continue.<p>
Note that the <tt>dyn_cast&lt;&gt;</tt> operator, like C++'s
<tt>dynamic_cast</tt> or Java's <tt>instanceof</tt> operator, can be abused. In
particular you should not use big chained <tt>if/then/else</tt> blocks to check
for lots of different variants of classes. If you find yourself wanting to do
this, it is much cleaner and more efficient to use the InstVisitor class to
dispatch over the instruction type directly.<p>
<dt><tt>cast_or_null&lt;&gt;</tt>:
<dd>The <tt>cast_or_null&lt;&gt;</tt> operator works just like the
<tt>cast&lt;&gt;</tt> operator, except that it allows for a null pointer as an
argument (which it then propogates). This can sometimes be useful, allowing you
to combine several null checks into one.<p>
<dt><tt>dyn_cast_or_null&lt;&gt;</tt>:
<dd>The <tt>dyn_cast_or_null&lt;&gt;</tt> operator works just like the
<tt>dyn_cast&lt;&gt;</tt> operator, except that it allows for a null pointer as
an argument (which it then propogates). This can sometimes be useful, allowing
you to combine several null checks into one.<p>
</dl>
These five templates can be used with any classes, whether they have a v-table
or not. To add support for these templates, you simply need to add
<tt>classof</tt> static methods to the class you are interested casting to.
Describing this is currently outside the scope of this document, but there are
lots of examples in the LLVM sourcebase.<p>
@ -1352,6 +1389,6 @@ pointer to the parent Function.
<a href="mailto:sabre@nondot.org">Chris Lattner</a></address>
<!-- Created: Tue Aug 6 15:00:33 CDT 2002 -->
<!-- hhmts start -->
Last modified: Mon Sep 9 19:38:23 CDT 2002
Last modified: Tue Sep 10 10:19:56 CDT 2002
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</font></body></html>