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<h3 class="section">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 <code>@</code>:



<pre class="example">       id myString = @"this is a constant string object";

     </pre>



   <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

<code>objc/NXConstStr.h</code> header file.



   <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 <code>-fconstant-string-class=</code><var>class-name</var><code></code>. 

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

as <code>NXConstantString</code>'s structure:



<pre class="example">     

     @interface MyConstantStringClass

     {

       Class isa;

       char *c_string;

       unsigned int len;

     }

     @end

     

     </pre>



   <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>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 <code>-fconstant-string-class</code> 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>As a result, when a file is compiled with the

<code>-fconstant-string-class</code> 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.



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