@c Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998,
-@c 1999, 2000, 2001 Free Software Foundation, Inc.
+@c 1999, 2000, 2001, 2002, 2003, 2004, 2005 Free Software Foundation, Inc.
@c This is part of the GCC manual.
@c For copying conditions, see the file gcc.texi.
@node Executing code before main, Type encoding, Objective-C, Objective-C
@section @code{+load}: Executing code before main
-
The GNU Objective-C runtime provides a way that allows you to execute
code before the execution of the program enters the @code{main}
function. The code is executed on a per-class and a per-category basis,
@code{Stdin}, @code{Stdout} and @code{Stderr} as global variables, like
below:
-@example
+@smallexample
FileStream *Stdin = nil;
FileStream *Stdout = nil;
Stderr = [[FileStream new] initWithFd:2];
@}
-/* Other methods here */
+/* @r{Other methods here} */
@@end
-@end example
+@end smallexample
In this example, the initialization of @code{Stdin}, @code{Stdout} and
@code{Stderr} in @code{+initialize} occurs too late. The programmer can
The correct solution of the above problem is to use the @code{+load}
method instead of @code{+initialize}:
-@example
+@smallexample
@@implementation FileStream
Stderr = [[FileStream new] initWithFd:2];
@}
-/* Other methods here */
+/* @r{Other methods here} */
@@end
-@end example
+@end smallexample
The @code{+load} is a method that is not overridden by categories. If a
class and a category of it both implement @code{+load}, both methods are
@c @sp 1
@multitable @columnfractions .25 .75
+@item @code{_Bool}
+@tab @code{B}
@item @code{char}
@tab @code{c}
@item @code{unsigned char}
@tab @code{*}
@item unknown type
@tab @code{?}
+@item Complex types
+@tab @code{j} followed by the inner type. For example @code{_Complex double} is encoded as "jd".
@item bit-fields
@tab @code{b} followed by the starting position of the bit-field, the type of the bit-field and the size of the bit-field (the bit-fields encoding was changed from the NeXT's compiler encoding, see below)
@end multitable
@item Objective-C type
@tab Compiler encoding
@item
-@example
+@smallexample
int a[10];
-@end example
+@end smallexample
@tab @code{[10i]}
@item
-@example
+@smallexample
struct @{
int i;
float f[3];
int b:2;
char c;
@}
-@end example
+@end smallexample
@tab @code{@{?=i[3f]b128i3b131i2c@}}
@end multitable
implement a class whose instances hold a weak pointer reference; the
following class does this:
-@example
+@smallexample
@@interface WeakPointer : Object
@{
@@end
-@end example
+@end smallexample
Weak pointers are supported through a new type character specifier
represented by the @samp{!} character. The
directly by the compiler. You declare a constant string object by
prefixing a C constant string with the character @samp{@@}:
-@example
+@smallexample
id myString = @@"this is a constant string object";
-@end example
+@end smallexample
-The constant string objects are usually instances of the
+The constant string objects are by default instances of the
@code{NXConstantString} class which is provided by the GNU Objective-C
runtime. To get the definition of this class you must include the
@file{objc/NXConstStr.h} header file.
The provided class should adhere to a strict structure, the same
as @code{NXConstantString}'s structure:
-@example
+@smallexample
-@@interface NXConstantString : Object
+@@interface MyConstantStringClass
@{
+ Class isa;
char *c_string;
unsigned int len;
@}
@@end
-@end example
-
-User class libraries may choose to inherit the customized constant
-string class from a different class than @code{Object}. There is no
-requirement in the methods the constant string class has to implement.
-
-When a file is compiled with the @option{-fconstant-string-class} 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.
+@end smallexample
+
+@code{NXConstantString} inherits from @code{Object}; user class
+libraries may choose to inherit the customized constant string class
+from a different class than @code{Object}. 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.
+
+When the compiler creates the statically allocated constant string
+object, the @code{c_string} field will be filled by the compiler with
+the string; the @code{length} field will be filled by the compiler with
+the string length; the @code{isa} pointer will be filled with
+@code{NULL} 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 @option{-fconstant-string-class} 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).
+
+As a result, when a file is compiled with the
+@option{-fconstant-string-class} 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.
@c =========================================================================
@node compatibility_alias
The keyword @code{@@compatibility_alias} allows you to define a class name
as equivalent to another class name. For example:
-@example
+@smallexample
@@compatibility_alias WOApplication GSWApplication;
-@end example
+@end smallexample
tells the compiler that each time it encounters @code{WOApplication} as
a class name, it should replace it with @code{GSWApplication} (that is,
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