0075846f90
2009-03-17 Benjamin Kosnik <bkoz@redhat.com> * doc/xml/manual/appendix_contributing.xml: Add docbook style sheet version information. Table-ize docbook element examples. * doc/xml/manual/using.xml: Human-readable header markup. Alphabetized. Add new headers. * doc/html: Regenerate. From-SVN: r144923
29 lines
3.5 KiB
HTML
29 lines
3.5 KiB
HTML
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
|
||
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
|
||
<html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>NULL</title><meta name="generator" content="DocBook XSL Stylesheets V1.74.0" /><meta name="keywords" content=" ISO C++ , library " /><link rel="home" href="../spine.html" title="The GNU C++ Library Documentation" /><link rel="up" href="bk01pt02ch04.html" title="Chapter 4. Types" /><link rel="prev" href="bk01pt02ch04s02.html" title="Numeric Properties" /><link rel="next" href="bk01pt02ch05.html" title="Chapter 5. Dynamic Memory" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">NULL</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="bk01pt02ch04s02.html">Prev</a> </td><th width="60%" align="center">Chapter 4. Types</th><td width="20%" align="right"> <a accesskey="n" href="bk01pt02ch05.html">Next</a></td></tr></table><hr /></div><div class="sect1" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="manual.support.types.null"></a>NULL</h2></div></div></div><p>
|
||
The only change that might affect people is the type of
|
||
<code class="constant">NULL</code>: while it is required to be a macro,
|
||
the definition of that macro is <span class="emphasis"><em>not</em></span> allowed
|
||
to be <code class="constant">(void*)0</code>, which is often used in C.
|
||
</p><p>
|
||
For <span class="command"><strong>g++</strong></span>, <code class="constant">NULL</code> is
|
||
</p><pre class="programlisting">#define</pre><p>'d to be
|
||
<code class="constant">__null</code>, a magic keyword extension of
|
||
<span class="command"><strong>g++</strong></span>.
|
||
</p><p>
|
||
The biggest problem of #defining <code class="constant">NULL</code> to be
|
||
something like “<span class="quote">0L</span>” is that the compiler will view
|
||
that as a long integer before it views it as a pointer, so
|
||
overloading won't do what you expect. (This is why
|
||
<span class="command"><strong>g++</strong></span> has a magic extension, so that
|
||
<code class="constant">NULL</code> is always a pointer.)
|
||
</p><p>In his book <a class="ulink" href="http://www.awprofessional.com/titles/0-201-92488-9/" target="_top"><span class="emphasis"><em>Effective
|
||
C++</em></span></a>, Scott Meyers points out that the best way
|
||
to solve this problem is to not overload on pointer-vs-integer
|
||
types to begin with. He also offers a way to make your own magic
|
||
<code class="constant">NULL</code> that will match pointers before it
|
||
matches integers.
|
||
</p><p>See
|
||
<a class="ulink" href="http://www.awprofessional.com/titles/0-201-31015-5/" target="_top">the
|
||
Effective C++ CD example</a>
|
||
</p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="bk01pt02ch04s02.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="bk01pt02ch04.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="bk01pt02ch05.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Numeric Properties </td><td width="20%" align="center"><a accesskey="h" href="../spine.html">Home</a></td><td width="40%" align="right" valign="top"> Chapter 5. Dynamic Memory</td></tr></table></div></body></html>
|