what was the second parameter in "id (*IMP)(id, SEL, ...) " used for?

Question

my question as the title says.obviously, the first parameter was used for this pointer , in some taste of c++.what about the second one? thak you.

Answer 1

The signature of objc_msgSend() is:

id objc_msgSend(id self, SEL op, ...);

Every method call is compiled down to a call to this function. I.e., if you call:

[anArray objectAtIndex:42];

That will be compiled as if it were:

objc_msgSend(anArray, @selector(objectAtIndex:), 42);

Now, to your question, why do methods get compiled down to a function that has the SEL as the second argument. Or, more specifically, why is this method:

- (id)objectAtIndex:(NSUInteger)index;

Exactly equivalent to this C function:

id object_at_index(id object, SEL _cmd, NSUInteger index);

The answer is speed speed speed.

Speed

Specifically, by doing this, then objc_msgSend() never has to rewrite the stack frame* and it can also use a tail call optimization to jump directly to the method invocation. This is the same reason why you never see objc_msgSend() in backtraces in the debugger (save for when you actually crash/break in the messenger).

objc_msgSend() uses the object and the _cmd to look up the implementation of the method and then, quite literally, jumps to that implementation.

Very fast. Stack frame untouched.

And, as others have stated, having _cmd around in the method implementation can be handy for a variety of reasons. As well, it also means that the messenger can do neat tricks like proxy support via NSInvocation and the like.

*rewriting the stack frame can be insanely complex and expensive. Some of the arguments might be in registers some of the time, etc... All architecture dependent ABI nastiness. One of the biggest challenges to writing things like imp_implementationWithBlock() was figuring out how to do so without touching the stack because doing so would have been too slow and too bloated to be viable.

Answer 2

The purpose of having the second parameter contain the selector is to enable a common dispatch mechanism. As such, the method dispatch code always expects the second parameter to be the selector, and dispatches based on that, or follows the inheritance chain up, or even creates an NSInvocation and calls forwardInvocation:.

Generally, only system-level routines use the selector argument, although it's rather nice to have it when you hit an exception or are in the debugger trying to figure out what routine is giving you difficulties if you are using forwardInvocation

Answer 3

From the documentation:

Discussion

This data type is a pointer to the start of the function that implements the method. This function uses standard C calling conventions as implemented for the current CPU architecture. The first argument is a pointer to self (that is, the memory for the particular instance of this class, or, for a class method, a pointer to the metaclass). The second argument is the method selector. The method arguments follow.

In Objective-C when you call a method you need to know the target, the selector and the eventual arguments. Let's suppose that you are trying to do this manually: how can you know which method to call if you don't know the selector? Do you call some random method? No, you call the right method because you know the method name.