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<html lang="en"><head><title>Using the GNU Compiler Collection (GCC)</title><meta http-equiv="Content-Type" content="text/html"><meta name="description" content="Using the GNU Compiler Collection (GCC)"><meta name="generator" content="makeinfo 4.6"><!--Copyright © 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998,1999, 2000, 2001, 2002, 2003, 2004 Free Software Foundation, Inc. <p>Permission is granted to copy, distribute and/or modify this documentunder the terms of the GNU Free Documentation License, Version 1.2 orany later version published by the Free Software Foundation; with theInvariant Sections being "GNU General Public License" and "FundingFree Software", the Front-Cover texts being (a) (see below), and withthe Back-Cover Texts being (b) (see below). A copy of the license isincluded in the section entitled "GNU Free Documentation License". <p>(a) The FSF's Front-Cover Text is: <p>A GNU Manual <p>(b) The FSF's Back-Cover Text is: <p>You have freedom to copy and modify this GNU Manual, like GNU software. Copies published by the Free Software Foundation raise funds for GNU development.--><meta http-equiv="Content-Style-Type" content="text/css"><style type="text/css"><!-- pre.display { font-family:inherit } pre.format { font-family:inherit } pre.smalldisplay { font-family:inherit; font-size:smaller } pre.smallformat { font-family:inherit; font-size:smaller } pre.smallexample { font-size:smaller } pre.smalllisp { font-size:smaller }--></style></head><body><div class="node"><p>Node: <a name="Executing%20code%20before%20main">Executing code before main</a>,Next: <a rel="next" accesskey="n" href="Type-encoding.html#Type%20encoding">Type encoding</a>,Previous: <a rel="previous" accesskey="p" href="Objective-C.html#Objective-C">Objective-C</a>,Up: <a rel="up" accesskey="u" href="Objective-C.html#Objective-C">Objective-C</a><hr><br></div><h3 class="section"><code>+load</code>: Executing code before main</h3><p>The GNU Objective-C runtime provides a way that allows you to executecode before the execution of the program enters the <code>main</code>function. The code is executed on a per-class and a per-category basis,through a special class method <code>+load</code>. <p>This facility is very useful if you want to initialize global variableswhich can be accessed by the program directly, without sending a messageto the class first. The usual way to initialize global variables, in the<code>+initialize</code> method, might not be useful because<code>+initialize</code> is only called when the first message is sent to aclass object, which in some cases could be too late. <p>Suppose for example you have a <code>FileStream</code> class that declares<code>Stdin</code>, <code>Stdout</code> and <code>Stderr</code> as global variables, likebelow:<pre class="smallexample"> FileStream *Stdin = nil; FileStream *Stdout = nil; FileStream *Stderr = nil; @implementation FileStream + (void)initialize { Stdin = [[FileStream new] initWithFd:0]; Stdout = [[FileStream new] initWithFd:1]; Stderr = [[FileStream new] initWithFd:2]; } /* Other methods here */ @end </pre> <p>In this example, the initialization of <code>Stdin</code>, <code>Stdout</code> and<code>Stderr</code> in <code>+initialize</code> occurs too late. The programmer cansend a message to one of these objects before the variables are actuallyinitialized, thus sending messages to the <code>nil</code> object. The<code>+initialize</code> method which actually initializes the globalvariables is not invoked until the first message is sent to the classobject. The solution would require these variables to be initializedjust before entering <code>main</code>. <p>The correct solution of the above problem is to use the <code>+load</code>method instead of <code>+initialize</code>:<pre class="smallexample"> @implementation FileStream + (void)load { Stdin = [[FileStream new] initWithFd:0]; Stdout = [[FileStream new] initWithFd:1]; Stderr = [[FileStream new] initWithFd:2]; } /* Other methods here */ @end </pre> <p>The <code>+load</code> is a method that is not overridden by categories. If aclass and a category of it both implement <code>+load</code>, both methods areinvoked. This allows some additional initializations to be performed ina category. <p>This mechanism is not intended to be a replacement for <code>+initialize</code>. You should be aware of its limitations when you decide to use itinstead of <code>+initialize</code>.<ul class="menu"><li><a accesskey="1" href="What-you-can-and-what-you-cannot-do-in--load.html#What%20you%20can%20and%20what%20you%20cannot%20do%20in%20+load">What you can and what you cannot do in +load</a>: </ul> </body></html>⌨️ 快捷键说明
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