mirror of git://gcc.gnu.org/git/gcc.git
30 lines
3.6 KiB
HTML
30 lines
3.6 KiB
HTML
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<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="fundamental_types.html" title="Chapter 4. Types" /><link rel="prev" href="bk01pt02ch04s02.html" title="Numeric Properties" /><link rel="next" href="dynamic_memory.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="dynamic_memory.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>
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The only change that might affect people is the type of
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<code class="constant">NULL</code>: while it is required to be a macro,
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the definition of that macro is <span class="emphasis"><em>not</em></span> allowed
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to be <code class="constant">(void*)0</code>, which is often used in C.
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</p><p>
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For <span class="command"><strong>g++</strong></span>, <code class="constant">NULL</code> is
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</p><pre class="programlisting">#define</pre><p>'d to be
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<code class="constant">__null</code>, a magic keyword extension of
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<span class="command"><strong>g++</strong></span>.
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</p><p>
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The biggest problem of #defining <code class="constant">NULL</code> to be
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something like “<span class="quote">0L</span>” is that the compiler will view
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that as a long integer before it views it as a pointer, so
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overloading won't do what you expect. (This is why
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<span class="command"><strong>g++</strong></span> has a magic extension, so that
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<code class="constant">NULL</code> is always a pointer.)
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</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
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C++</em></span></a>, Scott Meyers points out that the best way
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to solve this problem is to not overload on pointer-vs-integer
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types to begin with. He also offers a way to make your own magic
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<code class="constant">NULL</code> that will match pointers before it
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matches integers.
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</p><p>See
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<a class="ulink" href="http://www.awprofessional.com/titles/0-201-31015-5/" target="_top">the
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Effective C++ CD example</a>
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</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="fundamental_types.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="dynamic_memory.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>
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