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  76. <a name="Constant-String-Objects"></a>

  77. <h3 class="section">8.5 Constant String Objects</h3>

  78. 

  79. <p>GNU Objective-C provides constant string objects that are generated

  80. directly by the compiler.  You declare a constant string object by

  81. prefixing a C constant string with the character &lsquo;<samp>@</samp>&rsquo;:

  82. </p>

  83. <div class="smallexample">

  84. <pre class="smallexample">  id myString = @&quot;this is a constant string object&quot;;

  85. </pre></div>

  86. 

  87. <p>The constant string objects are by default instances of the

  88. <code>NXConstantString</code> class which is provided by the GNU Objective-C

  89. runtime.  To get the definition of this class you must include the

  90. <samp>objc/NXConstStr.h</samp> header file.

  91. </p>

  92. <p>User defined libraries may want to implement their own constant string

  93. class.  To be able to support them, the GNU Objective-C compiler provides

  94. a new command line options <samp>-fconstant-string-class=<var>class-name</var></samp>.

  95. The provided class should adhere to a strict structure, the same

  96. as <code>NXConstantString</code>&rsquo;s structure:

  97. </p>

  98. <div class="smallexample">

  99. <pre class="smallexample">

  100. 

  101. @interface MyConstantStringClass

  102. {

  103.   Class isa;

  104.   char *c_string;

  105.   unsigned int len;

  106. }

  107. @end

  108. 

  109. </pre></div>

  110. 

  111. <p><code>NXConstantString</code> inherits from <code>Object</code>; user class

  112. libraries may choose to inherit the customized constant string class

  113. from a different class than <code>Object</code>.  There is no requirement in

  114. the methods the constant string class has to implement, but the final

  115. ivar layout of the class must be the compatible with the given

  116. structure.

  117. </p>

  118. <p>When the compiler creates the statically allocated constant string

  119. object, the <code>c_string</code> field will be filled by the compiler with

  120. the string; the <code>length</code> field will be filled by the compiler with

  121. the string length; the <code>isa</code> pointer will be filled with

  122. <code>NULL</code> by the compiler, and it will later be fixed up automatically

  123. at runtime by the GNU Objective-C runtime library to point to the class

  124. which was set by the <samp>-fconstant-string-class</samp> option when the

  125. object file is loaded (if you wonder how it works behind the scenes, the

  126. name of the class to use, and the list of static objects to fixup, are

  127. stored by the compiler in the object file in a place where the GNU

  128. runtime library will find them at runtime).

  129. </p>

  130. <p>As a result, when a file is compiled with the

  131. <samp>-fconstant-string-class</samp> option, all the constant string objects

  132. will be instances of the class specified as argument to this option.  It

  133. is possible to have multiple compilation units referring to different

  134. constant string classes, neither the compiler nor the linker impose any

  135. restrictions in doing this.

  136. </p>

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