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
}
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3 Answers
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.
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.
1 Comment
AlanObject
Very concise and helpful. I recall that original C++ was just a preprocessor as well.
In Objective-C each function operates on an object, like
[myObject myFunction]
A C method has the form:
return-type function-name(argument1, argument2, etc) {}
An Objective-C instance method has the form:
-(return-type)function-name:argument1 {}
or for a multi-argument function
-(return-type)function-name:argument1 function-name:argument2 {}
I always use Objective-C-style methods in Obj-C programming, even though you can still use C-type functions as well.
I suppose the equivalent in C to [myObject myMethod:arg] might be myObject.myMethod(arg)
8 Comments
Matteo Italia
Actually in C there are no methods, just freestanding functions; the last "equivalent C" syntax you wrote is the C++ syntax for instance methods.
aqua
To me, methods and functions are the same things. i.e. interchangeable in language. Some programming languages do make distinctions, but across the board you'll have some people using function where method is appropriate and vice versa.
Matteo Italia
@aqua: AFAIK, you talk about methods when you're talking about objects (en.wikipedia.org/wiki/Method_%28computer_programming%29), otherwise you have a function or a subroutine. Still, my fundamental point was that the last "equivalent C" syntax you wrote doesn't make sense, because in C there are no objects (
structs don't have instance methods). Instead, that is the syntax used in C++ for instance methods.
mu is too short
@Matteo Italia: But
myObject could be an instance of a struct with a member called myMethod that is a pointer to a function, then myObject.myMethod(arg) would be perfectly valid C. Such constructs are quite common when implementing objects in C.Matteo Italia
@mu: I may still say that yours is a usage point of view while mine is a language POV. But I'll just say "lesson learned".
:) |
The first is a freestanding function. The second is an instance method for an Objective-C class. So I guess you would need the second version if you're actually writing a class.
answered Jan 31, 2011 at 1:23
Etienne de Martel
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