Question

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Objective C message dispatch mechanism

My question IS NOT about the syntax. I’d like to learn how is calling a method in C++ different from sending a message to an object in Objective-C and how they are performed?

Answer 1

This is quite a complicated question, as there is no fix C++ calling convetion unlike C.

Objective-C is just a thin wrapper around C, so it uses the same convention.
Just one additional thing to now, when you send a message like:

[target selector];

It is the same as:

objc_msgSend(target, @selector(selector));

Then it’s just the traditional C calling convention, with a first table lookup for the function matching your message. The objc_msgSend is a bit more complicated as it keeps the arguments stack in place and pass it directly to the underlying function.

The C++ calling convention differs from one name-mangling to another, and even from one compiler to another.

From a performance point of view, C++ method call is faster as the link is resolved at compile-time (more exactly at link-time). The method either exists or not, which cause a linker error.

Objective-C method call include a method table lookup at runtime, thus your method may be added later in your code, which gives more flexibility but less performance.