@implementation MyClass

+ (void)load {
    [[self sharedInstance] run];
}

+ (instancetype)sharedInstance {
    static MyClass *sharedClass;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        sharedClass = [self new];
    });
    return sharedClass;
}

- (void)run {
    // Do stuff
}

@end

Upon MyClass getting loaded into memory, +load is called by the runtime, which allocates, instantiates, and retains an instance of MyClass and then calls -run to do some arbitrary work. MyClass is not #imported from anywhere else, and it is also not retained by anyone else either. It’s there but you can’t see it.

In practice this is probably a bad idea because of the sneaky nature of things. So have a good reason before deciding to use something like this.