Is objc_msgSend() the significant piece that makes Objective-C object oriented?

Question

While reading the documentation, I wonder if objc_msgSend() is actually the "core technology" in delivering the functionality for making Objective-C "object oriented". Maybe someone can explain in more detail which other pieces come into place to enable the object oriented paradigm of Objective-C?

Answer 1

Not entirely.

Objective-C is object oriented solely because it encapsulates data and functionality into a single container; a class.

That is pretty much all there is to "object oriented programming".

Now, there are many different kinds of object oriented programming and one critical aspect is whether or not a language uses dynamic or static dispatch.

In a statically dispatched language -- C++ is the best example (yes, I know it has virtual methods that give a form of dynamic dispatch) -- a method call is wired up at compile time and cannot change at runtime. That is, the implementation of the method that will be used to fulfill the method call is fixed during compilation and cannot change at runtime.

With a dynamically dispatched language like Objective-C, the implementation of the method that will be used to fulfill a method call is determined each time the method call happens. Thus, through the use of categories or the runtime's API, it is possible to change a method's implementation while an application is running (this is actually how Key Value Observation works, for example).

objc_msgSend() is the hook that does the dynamic dispatch. It takes a reference to an object or a class & a method name -- a selector or SEL, as it is called -- and looks up the implementation on the object or class that goes by that method name. Once the implementation is found, it is called.

If no implementation is found, objc_msgSend() will then take a series of steps to see if the class or instance wants to handle the unrecognized method call somehow, allowing one object to stand in for another (proxying) or similar functionality.

There is a lot more to it than that. I would suggest you read Apple's documentation for more information.