Objective-C. Execute code before main

Interesting for me Objective-C run-time feature - execute code before the main:
Objective-C class may have two special class methods:
+ (void)load;
+ (void)initialize;

Here is a simple program demonstrating this language feature:

#import <Foundation/Foundation.h>

@interface First : NSObject
{
    int x;
}

- (void)printX;
- (void)setX:(int)value;

@end

@implementation First

+ (void)load
{
    NSLog(@"First +load");
}

+ (void)initialize
{
    NSLog(@"First. +initialize");
}

- (void)printX
{
    NSLog(@"First -printX");
    NSLog(@"x=%i", x);
}

- (void)setX:(int)value
{
    NSLog(@"First -setX");
    x = value;
}

@end

@interface Second : NSObject
{

}

@end

@implementation Second

+ (void)load
{
    NSLog(@"Second +load");
}

+ (void)initialize
{
    NSLog(@"Second initialize");
}

@end



int main (int argc, const char * argv[]) {
    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];

    NSLog(@"Program starts");
    First *first = [First new];
    [first printX];
    [first setX: 123];
    [first printX];
    [first release];
    [pool drain];
    return 0;
}

Here is the program output:

You see text "First +load" before "Program starts". It means that the class method load was called before I instantiated first object. Then, you see text "First +initialize" - the class method initialize was called when the class receives a first message. You also see text "Second +load". I do not create any object of Second class, but the method load was called anyway.

Reference:
Max OS X Reference Library. 7.1 +load: Executing code before main