C function vs. Objective-C method?

Question

What is the difference between the two? If I'm writing a program, when would I need a this:

void aFunction() {     //do something } 

and when would I need this:

-(void)aMethod {     //do something else } 

Answer 1

Actually, an Objective-C method is just a C function with two arguments always present at the beginning.

This:

-(void)aMethod; 

Is exactly equivalent to this:

void function(id self, SEL _cmd); 

Objective-C's messaging is such that this:

[someObject aMethod]; 

Is exactly equivalent to this (almost -- there is a variadic argument ABI issue beyond the scope of this answer):

objc_msgSend(someObject, @selector(aMethod)); 

objc_msgSend() finds the appropriate implementation of the method (by looking it up on someObject) and then, through the magic of a tail call optimization, jumps to the implementation of the method which, for all intents and purposes, works exactly like a C function call that looks like this:

function(someObject, @selector(aMethod)); 

Quite literally, Objective-C was originally implemented as nothing but a C preprocessor. Anything you can do in Objective-C could be rewritten as straight C.

Doing so, however, would be a complete pain in the ass and not worth your time beyond the incredibly educational experience of doing so.

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In general, you use Objective-C methods when talking to objects and function when working with straight C goop. Given that pretty much all of Mac OS X and iOS provide Objective-C APIs -- certainly entirely so for the UI level programming entry points -- then you use Obj-C most of the time.

Even when writing your own model level code that is relatively standalone, you'll typically use Objective-C simply because it provides a very natural glue between state/data & functionality, a fundamental tenant of object oriented programming.