What is the difference between Message Dispatch and Dynamic Dispatch mechanisms in message processing and method calls?

Message Dispatch and Dynamic Dispatch are mechanisms related to method invocation, but in different contexts.

Message Dispatch is often used in languages with dynamic message sending (e.g., Objective-C). Here, calling a method is considered as sending a message to an object, which at runtime decides which method to execute. This allows for dynamically changing object behavior and utilizing features like method swizzling.

Dynamic Dispatch is a mechanism where the method call is determined at runtime based on the actual type of the object (polymorphism). In languages like Java or Swift, this is implemented through virtual tables (vtable), allowing methods to be overridden in subclasses.

Main difference:

• Message Dispatch is more dynamic and flexible, allowing calls to be handled as messages that can be intercepted or redirected.
• Dynamic Dispatch is a more structured polymorphism mechanism, where the method choice is based on the object's type, but without the same flexibility in handling the call.

Example in Objective-C:

[obj performSelector:@selector(methodName)];

Here, performSelector: is a message sending, which at runtime determines which method to invoke.