| <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> |
| <html> |
| <!-- Copyright (C) 1988-2015 Free Software Foundation, Inc. |
| |
| Permission is granted to copy, distribute and/or modify this document |
| under the terms of the GNU Free Documentation License, Version 1.3 or |
| any later version published by the Free Software Foundation; with the |
| Invariant Sections being "Funding Free Software", the Front-Cover |
| Texts being (a) (see below), and with the Back-Cover Texts being (b) |
| (see below). A copy of the license is included in the section entitled |
| "GNU Free Documentation License". |
| |
| (a) The FSF's Front-Cover Text is: |
| |
| A GNU Manual |
| |
| (b) The FSF's Back-Cover Text is: |
| |
| 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. --> |
| <!-- Created by GNU Texinfo 5.2, http://www.gnu.org/software/texinfo/ --> |
| <head> |
| <title>Using the GNU Compiler Collection (GCC): Executing code before main</title> |
| |
| <meta name="description" content="Using the GNU Compiler Collection (GCC): Executing code before main"> |
| <meta name="keywords" content="Using the GNU Compiler Collection (GCC): Executing code before main"> |
| <meta name="resource-type" content="document"> |
| <meta name="distribution" content="global"> |
| <meta name="Generator" content="makeinfo"> |
| <meta http-equiv="Content-Type" content="text/html; charset=utf-8"> |
| <link href="index.html#Top" rel="start" title="Top"> |
| <link href="Option-Index.html#Option-Index" rel="index" title="Option Index"> |
| <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents"> |
| <link href="Objective_002dC.html#Objective_002dC" rel="up" title="Objective-C"> |
| <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"> |
| <link href="Traditional-GNU-Objective_002dC-runtime-API.html#Traditional-GNU-Objective_002dC-runtime-API" rel="prev" title="Traditional GNU Objective-C runtime API"> |
| <style type="text/css"> |
| <!-- |
| a.summary-letter {text-decoration: none} |
| blockquote.smallquotation {font-size: smaller} |
| div.display {margin-left: 3.2em} |
| div.example {margin-left: 3.2em} |
| div.indentedblock {margin-left: 3.2em} |
| div.lisp {margin-left: 3.2em} |
| div.smalldisplay {margin-left: 3.2em} |
| div.smallexample {margin-left: 3.2em} |
| div.smallindentedblock {margin-left: 3.2em; font-size: smaller} |
| div.smalllisp {margin-left: 3.2em} |
| kbd {font-style:oblique} |
| pre.display {font-family: inherit} |
| pre.format {font-family: inherit} |
| pre.menu-comment {font-family: serif} |
| pre.menu-preformatted {font-family: serif} |
| pre.smalldisplay {font-family: inherit; font-size: smaller} |
| pre.smallexample {font-size: smaller} |
| pre.smallformat {font-family: inherit; font-size: smaller} |
| pre.smalllisp {font-size: smaller} |
| span.nocodebreak {white-space:nowrap} |
| span.nolinebreak {white-space:nowrap} |
| span.roman {font-family:serif; font-weight:normal} |
| span.sansserif {font-family:sans-serif; font-weight:normal} |
| ul.no-bullet {list-style: none} |
| --> |
| </style> |
| |
| |
| </head> |
| |
| <body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000"> |
| <a name="Executing-code-before-main"></a> |
| <div class="header"> |
| <p> |
| 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> |
| </div> |
| <hr> |
| <a name="g_t_002bload_003a-Executing-code-before-main"></a> |
| <h3 class="section">8.2 <code>+load</code>: Executing code before main</h3> |
| |
| <p>This section is specific for the GNU Objective-C runtime. If you are |
| using a different runtime, you can skip it. |
| </p> |
| <p>The GNU Objective-C runtime provides a way that allows you to execute |
| code 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> |
| <p>This facility is very useful if you want to initialize global variables |
| which can be accessed by the program directly, without sending a message |
| to 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 a |
| class object, which in some cases could be too late. |
| </p> |
| <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, like |
| below: |
| </p> |
| <div class="smallexample"> |
| <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]; |
| } |
| |
| /* <span class="roman">Other methods here</span> */ |
| @end |
| |
| </pre></div> |
| |
| <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 can |
| send a message to one of these objects before the variables are actually |
| initialized, thus sending messages to the <code>nil</code> object. The |
| <code>+initialize</code> method which actually initializes the global |
| variables is not invoked until the first message is sent to the class |
| object. The solution would require these variables to be initialized |
| just before entering <code>main</code>. |
| </p> |
| <p>The correct solution of the above problem is to use the <code>+load</code> |
| method instead of <code>+initialize</code>: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample"> |
| @implementation FileStream |
| |
| + (void)load |
| { |
| Stdin = [[FileStream new] initWithFd:0]; |
| Stdout = [[FileStream new] initWithFd:1]; |
| Stderr = [[FileStream new] initWithFd:2]; |
| } |
| |
| /* <span class="roman">Other methods here</span> */ |
| @end |
| |
| </pre></div> |
| |
| <p>The <code>+load</code> is a method that is not overridden by categories. If a |
| class and a category of it both implement <code>+load</code>, both methods are |
| invoked. This allows some additional initializations to be performed in |
| a category. |
| </p> |
| <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 it |
| instead of <code>+initialize</code>. |
| </p> |
| <table class="menu" border="0" cellspacing="0"> |
| <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"> |
| </td></tr> |
| </table> |
| |
| |
| <hr> |
| <div class="header"> |
| <p> |
| 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> |
| </div> |
| |
| |
| |
| </body> |
| </html> |