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147 lines
6.7 KiB
HTML
147 lines
6.7 KiB
HTML
4 years ago
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<!-- Copyright (C) 1988-2018 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation; with the
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Invariant Sections being "Funding Free Software", the Front-Cover
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Texts being (a) (see below), and with the Back-Cover Texts being (b)
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(see below). A copy of the license is included in the section entitled
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"GNU Free Documentation License".
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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(b) The FSF's Back-Cover Text is:
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You have freedom to copy and modify this GNU Manual, like GNU
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software. Copies published by the Free Software Foundation raise
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funds for GNU development. -->
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<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>Constant string objects (Using the GNU Compiler Collection (GCC))</title>
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<meta name="description" content="Constant string objects (Using the GNU Compiler Collection (GCC))">
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<meta name="keywords" content="Constant string objects (Using the GNU Compiler Collection (GCC))">
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<meta name="resource-type" content="document">
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<meta name="distribution" content="global">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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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="compatibility_005falias.html#compatibility_005falias" rel="next" title="compatibility_alias">
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<link href="Garbage-Collection.html#Garbage-Collection" rel="prev" title="Garbage Collection">
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</head>
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<body lang="en">
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<a name="Constant-string-objects"></a>
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<div class="header">
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<p>
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Next: <a href="compatibility_005falias.html#compatibility_005falias" accesskey="n" rel="next">compatibility_alias</a>, Previous: <a href="Garbage-Collection.html#Garbage-Collection" accesskey="p" rel="prev">Garbage Collection</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="Constant-String-Objects"></a>
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<h3 class="section">8.5 Constant String Objects</h3>
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<p>GNU Objective-C provides constant string objects that are generated
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directly by the compiler. You declare a constant string object by
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prefixing a C constant string with the character ‘<samp>@</samp>’:
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</p>
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<div class="smallexample">
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<pre class="smallexample"> id myString = @"this is a constant string object";
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</pre></div>
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<p>The constant string objects are by default instances of the
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<code>NXConstantString</code> class which is provided by the GNU Objective-C
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runtime. To get the definition of this class you must include the
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<samp>objc/NXConstStr.h</samp> header file.
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</p>
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<p>User defined libraries may want to implement their own constant string
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class. To be able to support them, the GNU Objective-C compiler provides
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a new command line options <samp>-fconstant-string-class=<var>class-name</var></samp>.
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The provided class should adhere to a strict structure, the same
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as <code>NXConstantString</code>’s structure:
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</p>
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<div class="smallexample">
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<pre class="smallexample">
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@interface MyConstantStringClass
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{
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Class isa;
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char *c_string;
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unsigned int len;
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}
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@end
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</pre></div>
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<p><code>NXConstantString</code> inherits from <code>Object</code>; user class
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libraries may choose to inherit the customized constant string class
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from a different class than <code>Object</code>. There is no requirement in
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the methods the constant string class has to implement, but the final
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ivar layout of the class must be the compatible with the given
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structure.
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</p>
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<p>When the compiler creates the statically allocated constant string
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object, the <code>c_string</code> field will be filled by the compiler with
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the string; the <code>length</code> field will be filled by the compiler with
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the string length; the <code>isa</code> pointer will be filled with
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<code>NULL</code> by the compiler, and it will later be fixed up automatically
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at runtime by the GNU Objective-C runtime library to point to the class
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which was set by the <samp>-fconstant-string-class</samp> option when the
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object file is loaded (if you wonder how it works behind the scenes, the
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name of the class to use, and the list of static objects to fixup, are
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stored by the compiler in the object file in a place where the GNU
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runtime library will find them at runtime).
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</p>
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<p>As a result, when a file is compiled with the
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<samp>-fconstant-string-class</samp> option, all the constant string objects
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will be instances of the class specified as argument to this option. It
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is possible to have multiple compilation units referring to different
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constant string classes, neither the compiler nor the linker impose any
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restrictions in doing this.
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</p>
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<hr>
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<div class="header">
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<p>
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Next: <a href="compatibility_005falias.html#compatibility_005falias" accesskey="n" rel="next">compatibility_alias</a>, Previous: <a href="Garbage-Collection.html#Garbage-Collection" accesskey="p" rel="prev">Garbage Collection</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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