172 lines
7.5 KiB
HTML
172 lines
7.5 KiB
HTML
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<!-- Created by GNU Texinfo 5.2, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>Using the GNU Compiler Collection (GCC): Executing code before main</title>
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<meta name="description" content="Using the GNU Compiler Collection (GCC): Executing code before main">
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<meta name="keywords" content="Using the GNU Compiler Collection (GCC): Executing code before main">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="Option-Index.html#Option-Index" rel="index" title="Option Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Objective_002dC.html#Objective_002dC" rel="up" title="Objective-C">
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<link href="What-you-can-and-what-you-cannot-do-in-_002bload.html#What-you-can-and-what-you-cannot-do-in-_002bload" rel="next" title="What you can and what you cannot do in +load">
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<link href="Traditional-GNU-Objective_002dC-runtime-API.html#Traditional-GNU-Objective_002dC-runtime-API" rel="prev" title="Traditional GNU Objective-C runtime API">
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<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
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<a name="Executing-code-before-main"></a>
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<div class="header">
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<p>
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Next: <a href="Type-encoding.html#Type-encoding" accesskey="n" rel="next">Type encoding</a>, Previous: <a href="GNU-Objective_002dC-runtime-API.html#GNU-Objective_002dC-runtime-API" accesskey="p" rel="prev">GNU Objective-C runtime API</a>, Up: <a href="Objective_002dC.html#Objective_002dC" accesskey="u" rel="up">Objective-C</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="g_t_002bload_003a-Executing-Code-before-main"></a>
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<h3 class="section">8.2 <code>+load</code>: Executing Code before <code>main</code></h3>
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<p>This section is specific for the GNU Objective-C runtime. If you are
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using a different runtime, you can skip it.
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</p>
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<p>The GNU Objective-C runtime provides a way that allows you to execute
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code before the execution of the program enters the <code>main</code>
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function. The code is executed on a per-class and a per-category basis,
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through a special class method <code>+load</code>.
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</p>
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<p>This facility is very useful if you want to initialize global variables
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which can be accessed by the program directly, without sending a message
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to the class first. The usual way to initialize global variables, in the
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<code>+initialize</code> method, might not be useful because
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<code>+initialize</code> is only called when the first message is sent to a
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class object, which in some cases could be too late.
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</p>
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<p>Suppose for example you have a <code>FileStream</code> class that declares
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<code>Stdin</code>, <code>Stdout</code> and <code>Stderr</code> as global variables, like
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below:
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</p>
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<div class="smallexample">
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<pre class="smallexample">
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FileStream *Stdin = nil;
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FileStream *Stdout = nil;
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FileStream *Stderr = nil;
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@implementation FileStream
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+ (void)initialize
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{
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Stdin = [[FileStream new] initWithFd:0];
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Stdout = [[FileStream new] initWithFd:1];
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Stderr = [[FileStream new] initWithFd:2];
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}
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/* <span class="roman">Other methods here</span> */
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@end
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</pre></div>
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<p>In this example, the initialization of <code>Stdin</code>, <code>Stdout</code> and
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<code>Stderr</code> in <code>+initialize</code> occurs too late. The programmer can
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send a message to one of these objects before the variables are actually
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initialized, thus sending messages to the <code>nil</code> object. The
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<code>+initialize</code> method which actually initializes the global
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variables is not invoked until the first message is sent to the class
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object. The solution would require these variables to be initialized
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just before entering <code>main</code>.
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</p>
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<p>The correct solution of the above problem is to use the <code>+load</code>
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method instead of <code>+initialize</code>:
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</p>
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<div class="smallexample">
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<pre class="smallexample">
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@implementation FileStream
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+ (void)load
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{
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Stdin = [[FileStream new] initWithFd:0];
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Stdout = [[FileStream new] initWithFd:1];
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Stderr = [[FileStream new] initWithFd:2];
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}
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/* <span class="roman">Other methods here</span> */
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@end
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</pre></div>
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<p>The <code>+load</code> is a method that is not overridden by categories. If a
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class and a category of it both implement <code>+load</code>, both methods are
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invoked. This allows some additional initializations to be performed in
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a category.
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</p>
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<p>This mechanism is not intended to be a replacement for <code>+initialize</code>.
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You should be aware of its limitations when you decide to use it
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instead of <code>+initialize</code>.
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</p>
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<table class="menu" border="0" cellspacing="0">
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<tr><td align="left" valign="top">• <a href="What-you-can-and-what-you-cannot-do-in-_002bload.html#What-you-can-and-what-you-cannot-do-in-_002bload" accesskey="1">What you can and what you cannot do in +load</a>:</td><td> </td><td align="left" valign="top">
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</td></tr>
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</table>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Type-encoding.html#Type-encoding" accesskey="n" rel="next">Type encoding</a>, Previous: <a href="GNU-Objective_002dC-runtime-API.html#GNU-Objective_002dC-runtime-API" accesskey="p" rel="prev">GNU Objective-C runtime API</a>, Up: <a href="Objective_002dC.html#Objective_002dC" accesskey="u" rel="up">Objective-C</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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</body>
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</html>
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