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<html lang="en"><head><title>Using the GNU Compiler Collection (GCC)</title><meta http-equiv="Content-Type" content="text/html"><meta name="description" content="Using the GNU Compiler Collection (GCC)"><meta name="generator" content="makeinfo 4.6"><!--Copyright &copy; 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998,1999, 2000, 2001, 2002, 2003, 2004 Free Software Foundation, Inc.   <p>Permission is granted to copy, distribute and/or modify this documentunder the terms of the GNU Free Documentation License, Version 1.2 orany later version published by the Free Software Foundation; with theInvariant Sections being "GNU General Public License" and "FundingFree Software", the Front-Cover texts being (a) (see below), and withthe Back-Cover Texts being (b) (see below).  A copy of the license isincluded in the section entitled "GNU Free Documentation License".   <p>(a) The FSF's Front-Cover Text is:   <p>A GNU Manual   <p>(b) The FSF's Back-Cover Text is:   <p>You have freedom to copy and modify this GNU Manual, like GNU     software.  Copies published by the Free Software Foundation raise     funds for GNU development.--><meta http-equiv="Content-Style-Type" content="text/css"><style type="text/css"><!--  pre.display { font-family:inherit }  pre.format  { font-family:inherit }  pre.smalldisplay { font-family:inherit; font-size:smaller }  pre.smallformat  { font-family:inherit; font-size:smaller }  pre.smallexample { font-size:smaller }  pre.smalllisp    { font-size:smaller }--></style></head><body><div class="node"><p>Node:&nbsp;<a name="Constant%20string%20objects">Constant string objects</a>,Next:&nbsp;<a rel="next" accesskey="n" href="compatibility_alias.html#compatibility_alias">compatibility_alias</a>,Previous:&nbsp;<a rel="previous" accesskey="p" href="Garbage-Collection.html#Garbage%20Collection">Garbage Collection</a>,Up:&nbsp;<a rel="up" accesskey="u" href="Objective-C.html#Objective-C">Objective-C</a><hr><br></div><h3 class="section">Constant string objects</h3><p>GNU Objective-C provides constant string objects that are generateddirectly by the compiler.  You declare a constant string object byprefixing a C constant string with the character <code>@</code>:<pre class="smallexample">       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-Cruntime.  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 stringclass.  To be able to support them, the GNU Objective-C compiler providesa 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 sameas <code>NXConstantString</code>'s structure:<pre class="smallexample">          @interface MyConstantStringClass     {       Class isa;       char *c_string;       unsigned int len;     }     @end          </pre>   <p><code>NXConstantString</code> inherits from <code>Object</code>; user classlibraries may choose to inherit the customized constant string classfrom a different class than <code>Object</code>.  There is no requirement inthe methods the constant string class has to implement, but the finalivar layout of the class must be the compatible with the givenstructure.   <p>When the compiler creates the statically allocated constant stringobject, the <code>c_string</code> field will be filled by the compiler withthe string; the <code>length</code> field will be filled by the compiler withthe string length; the <code>isa</code> pointer will be filled with<code>NULL</code> by the compiler, and it will later be fixed up automaticallyat runtime by the GNU Objective-C runtime library to point to the classwhich was set by the <code>-fconstant-string-class</code> option when theobject file is loaded (if you wonder how it works behind the scenes, thename of the class to use, and the list of static objects to fixup, arestored by the compiler in the object file in a place where the GNUruntime 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 objectswill be instances of the class specified as argument to this option.  Itis possible to have multiple compilation units referring to differentconstant string classes, neither the compiler nor the linker impose anyrestrictions in doing this.   </body></html>
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