Objective-C. Forwarding Messages.

Forwarding messages. It's an absolutely new feature for me in Object-Oriented Programming. I found this thing working in Objective-C (thanks to the book - on the left).

#import <Foundation/Foundation.h>

@interface Cat : NSObject
- (void) meow;
@end

@interface Dog : NSObject
- (void) woof;
@end

@implementation Cat

- (void) meow
{
    NSLog( @"%@: Meow", [self className] );
}

@end

@implementation Dog

- (void) woof
{
    NSLog( @"%@: woof", [self className] );
}

- (NSMethodSignature*) methodSignatureForSelector:(SEL)selector
{
    NSString* name = NSStringFromSelector( selector );
    NSLog( @"-[%@: methodSignatureForSelector: %@]", [self className], name );
    
    if ([Cat instancesRespondToSelector: selector])
        return [Cat instanceMethodSignatureForSelector: selector];
    
    return [super methodSignatureForSelector: selector];
}

- (void) forwardInvocation:(NSInvocation *)anInvocation
{
    NSString* name = NSStringFromSelector( anInvocation.selector );
    NSLog( @"-[%@: forwardInvocation: %@]", [self className], name );
    
    id theCat = [[Cat alloc] init];
    NSLog( @"Forwarding '%@' to %@", name, theCat );
    [anInvocation invokeWithTarget: theCat];
    [theCat release];
}

@end

int main() 
{
    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
    
    id dog = [[Dog alloc] init];
    [dog woof];
    [dog meow];
    [dog release];

    [pool drain];
    return 0;
}

And here is the output:

Running…

Dog: woof

-[Dog: methodSignatureForSelector: meow]

-[Dog: forwardInvocation: meow]

Forwarding 'meow' to

Cat: Meow