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Second-Chance Dispatch

In C++, you don't send messages, you call member functions. This is an important distinction, because it means that calling methods is semantically similar to a function call. Objective-C adds another layer of abstraction. If you send a message to an Objective-C object that it doesn't understand, an exception will be thrown. This isn't done by the language, however.

The runtime library has a fallback mechanism when no method is found for a selector. It calls a method that introspects the receiver for some type information, wraps up the call in an NSInvocation object, and then passes this to the object's -forwardInvocation: method.

The NSInvocation object encapsulates the receiver, the selector, and the arguments. You can use this idea for things like higher-order messaging. Consider the following example usage:

[[anArray map] toUppercase];

The -map method on the array returns a proxy object with a forwardInvocation: method implemented like this:

- (void) forwardInvocation:(NSInvocation*)anInvocation
{
 SEL selector = [anInvocation selector];
 NSMutableArray * mappedArray = [NSMutableArray array];
 FOREACHI(array, object)
 {
  if([object respondsToSelector:selector])
  {
   [anInvocation invokeWithTarget:object];
   id mapped;
   [anInvocation getReturnValue:&mapped];
   [mappedArray addObject:mapped];
  }
 }
 [anInvocation setReturnValue:mappedArray];
}

The FOREACHI macro is from Étoilé, and simply performs some IMP caching on an NSEnumerator. When you send the -toUppercase message to the map proxy, it iterates over every object in the array; checks whether it responds to the selector; and, if it does, calls the method with the arguments. The return value is then added to a new array.

This is pretty much impossible to do in C++. You can use the command pattern to do something similar, but only by implementing your own dispatch mechanism on top of C++.

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