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113 lines
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113 lines
5.2 KiB
HTML
4 years ago
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<title>Constant string objects - Using the GNU Compiler Collection (GCC)</title>
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<meta name="description" content="Using the GNU Compiler Collection (GCC)">
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<link rel="up" href="Objective_002dC.html#Objective_002dC" title="Objective-C">
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<link rel="prev" href="Garbage-Collection.html#Garbage-Collection" title="Garbage Collection">
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<link rel="next" href="compatibility_005falias.html#compatibility_005falias" title="compatibility_alias">
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Permission is granted to copy, distribute and/or modify this document
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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<div class="node">
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<a name="Constant-string-objects"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="compatibility_005falias.html#compatibility_005falias">compatibility_alias</a>,
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Previous: <a rel="previous" accesskey="p" href="Garbage-Collection.html#Garbage-Collection">Garbage Collection</a>,
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Up: <a rel="up" accesskey="u" href="Objective_002dC.html#Objective_002dC">Objective-C</a>
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<hr>
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</div>
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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><span class="samp">@</span></samp>’:
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<pre class="smallexample"> id myString = @"this is a constant string object";
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</pre>
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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><span class="file">objc/NXConstStr.h</span></samp> header file.
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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><span class="option">-fconstant-string-class=</span><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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<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>
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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>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><span class="option">-fconstant-string-class</span></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>As a result, when a file is compiled with the
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<samp><span class="option">-fconstant-string-class</span></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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<!-- ========================================================================= -->
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</body></html>
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