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| <title>Using the GNU Compiler Collection (GCC): Constant string objects</title> |
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| <a name="Constant-string-objects"></a> |
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| <a name="Constant-string-objects-1"></a> |
| <h3 class="section">8.5 Constant string objects</h3> |
| |
| <p>GNU Objective-C provides constant string objects that are generated |
| directly by the compiler. You declare a constant string object by |
| prefixing a C constant string with the character ‘<samp>@</samp>’: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample"> id myString = @"this is a constant string object"; |
| </pre></div> |
| |
| <p>The constant string objects are by default instances of the |
| <code>NXConstantString</code> class which is provided by the GNU Objective-C |
| runtime. To get the definition of this class you must include the |
| <samp>objc/NXConstStr.h</samp> header file. |
| </p> |
| <p>User defined libraries may want to implement their own constant string |
| class. To be able to support them, the GNU Objective-C compiler provides |
| a new command line options <samp>-fconstant-string-class=<var>class-name</var></samp>. |
| The provided class should adhere to a strict structure, the same |
| as <code>NXConstantString</code>’s structure: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample"> |
| @interface MyConstantStringClass |
| { |
| Class isa; |
| char *c_string; |
| unsigned int len; |
| } |
| @end |
| |
| </pre></div> |
| |
| <p><code>NXConstantString</code> inherits from <code>Object</code>; user class |
| libraries may choose to inherit the customized constant string class |
| from a different class than <code>Object</code>. There is no requirement in |
| the methods the constant string class has to implement, but the final |
| ivar layout of the class must be the compatible with the given |
| structure. |
| </p> |
| <p>When the compiler creates the statically allocated constant string |
| object, the <code>c_string</code> field will be filled by the compiler with |
| the string; the <code>length</code> field will be filled by the compiler with |
| the string length; the <code>isa</code> pointer will be filled with |
| <code>NULL</code> by the compiler, and it will later be fixed up automatically |
| at runtime by the GNU Objective-C runtime library to point to the class |
| which was set by the <samp>-fconstant-string-class</samp> option when the |
| object file is loaded (if you wonder how it works behind the scenes, the |
| name of the class to use, and the list of static objects to fixup, are |
| stored by the compiler in the object file in a place where the GNU |
| runtime library will find them at runtime). |
| </p> |
| <p>As a result, when a file is compiled with the |
| <samp>-fconstant-string-class</samp> option, all the constant string objects |
| will be instances of the class specified as argument to this option. It |
| is possible to have multiple compilation units referring to different |
| constant string classes, neither the compiler nor the linker impose any |
| restrictions in doing this. |
| </p> |
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