What's so special about message passing in smalltalk

Question

I was going through an introduction to Smalltalk. In C++, the functions declared inside a class can be called by objects of that class, and similarly in Smalltalk a keyword, termed as message, is written adjacent to the name of the object. (Don't know much but would also like to ask here whether in response to a message a unique method is there to be executed?)

Basically, to my naive mind, this seems to be only a difference in syntax style. But, I wonder if internally in terms of compilation or memory structure this difference in calling holds any significance.

Thanks in advance.

P.S : I bow down to all of you for your time and answers . Thanks a lot.

Answer 1

The fundamental difference is that in Smalltalk, the receiver of the message has complete control over how that message is handled. It's a true object, not a data structure with functions that operate on it.

That means that in Smalltalk you can send any message to any object. The compiler places no restrictions on that, it's all handled at runtime. In C++, you can only invoke functions that the compiler knows about.

Also, Smalltalk messages are simply symbols (unique character strings), not a function address in memory as in C++. That means it's easy to send messages interactively, or over a network connection. There is a perform: method that lets you send a message given its string name.

An object even receives messages it does not implement. The Virtual Machine detects that case and creates a Message object, and then sends the messageNotUnderstood: message. Again, it's the object's sole responsibility of how to handle that unknown message. Most objects simply inherit the default implementation which raises an error, but an object can also handle it itself. It could, for example, forward those messages to a remote object, or log them to a file, etc.

Answer 2

You call a function in C++ because during the compilation time you know which function will be called (or at least you have a finite set of functions defined in a class hierarchy.

Smalltalk is dynamically typed and late bound, so during the compilation time you have no idea which method is going to be evaluated (if one will be at all). Thus you send a message, and if the object has a method with that selector, it is evaluated. Otherwise, the "message not understood" exception is raised.