8.2 +load: Executing Code before main

This section is specific for the GNU Objective-C runtime. If you are using a different runtime, you can skip it.

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.