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(gcc-300.info)Executing code before main

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`+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 `main' function.
The code is executed on a per-class and a per-category basis, through a
special class method `+load'.

   This facility is very useful if you want to initialize global
variables which can be accessed by the program directly, without
sending a message to the class first.  The usual way to initialize
global variables, in the `+initialize' method, might not be useful
because `+initialize' is only called when the first message is sent to a
class object, which in some cases could be too late.

   Suppose for example you have a `FileStream' class that declares
`Stdin', `Stdout' and `Stderr' as global variables, like below:


     FileStream *Stdin = nil;
     FileStream *Stdout = nil;
     FileStream *Stderr = nil;
     
     @implementation FileStream
     
     + (void)initialize
     {
         Stdin = [[FileStream new] initWithFd:0];
         Stdout = [[FileStream new] initWithFd:1];
         Stderr = [[FileStream new] initWithFd:2];
     }
     
     /* Other methods here */
     @end

   In this example, the initialization of `Stdin', `Stdout' and
`Stderr' in `+initialize' occurs too late.  The programmer can send a
message to one of these objects before the variables are actually
initialized, thus sending messages to the `nil' object.  The
`+initialize' method which actually initializes the global variables is
not invoked until the first message is sent to the class object.  The
solution would require these variables to be initialized just before
entering `main'.

   The correct solution of the above problem is to use the `+load'
method instead of `+initialize':


     @implementation FileStream
     
     + (void)load
     {
         Stdin = [[FileStream new] initWithFd:0];
         Stdout = [[FileStream new] initWithFd:1];
         Stderr = [[FileStream new] initWithFd:2];
     }
     
     /* Other methods here */
     @end

   The `+load' is a method that is not overridden by categories.  If a
class and a category of it both implement `+load', both methods are
invoked.  This allows some additional initializations to be performed in
a category.

   This mechanism is not intended to be a replacement for `+initialize'.
You should be aware of its limitations when you decide to use it
instead of `+initialize'.

What you can and what you cannot do in +load