What you can't do in C that you can do in Objective-C?

Question

What you can't do in C (C99 standard) that you can do in Objective-C? (with code example if you please)

Answer 1

There isn't such a thing. Generally, you can program anything in any programming language.

Answer 2

Absolutely nothing. Objective-C's OO features are implemented as a small runtime library written in C.

While some commenters might point out that Objective-C has blocks and C doesn't, that's actually a GCC/LLVM extension to C that Objective-C makes use of, it's not an ObjC feature.

Answer 3

As Graham said -- there is absolutely nothing that can be done in Objective-C that can't be done in C. Of course, there isn't anything you can do in C, either, that can't be done in assembly. It is just a matter of typing.

(I was hoping that the the assembly mention would be enough to indicate that, no, I'm not remotely suggesting that writing OO patterns in pure C is remotely a productive thing to do...)

There are a few additional points of interest:

• Objective-C started out as a preprocessor that turned Objective-C code into C code that could be compiled by a standard C compiler. That is, Objective-C was originally never directly compiled, but translated to straight C, then compiled.

• Objective-C objects can be thought of as C structures where the first entry in the structure -- the isa -- is always a pointer to the metadata -- the Class -- that describes the structure/instance.

• Blocks are an extension to C, now proposed as an addition to the C language in WG-14 (C standards committee). Blocks, as implemented by Apple, absolutely do not require an executable stack. It was a hard requirement in their design.

• LLVM has a rewriter that can actually rewrite Objective-C to C and has, apparently, been used to enable Objective-C coding in Xbox 360 games (see http://permalink.gmane.org/gmane.comp.compilers.llvm.devel/31996).

Answer 4

I'm going to be a little bit presumptuous and disagree with everyone else. While it's technically true that anything possible in one language is possible in another (where "possible" means "computable"), they differ in what you can express naturally and easily. The computer might be doing the same thing in response to the code you write in C, but you are writing radically different code to make it do those things.

As others have said, Objective-C provides a full runtime library written in C that will allow you to create Objective-C data structures and call C functions, but the code to do it will be very verbose, fairly roundabout and completely imperative. In Objective-C, the code is more declarative, more concise and far more readable.

In general, trying to write Objective-C things in C will only make your code worse than it would be using either language idiomatically. For example, here is a simple program written in Objective-C:

@interface NumberAdder : NSObject {
    int storedValue;
}

- (id)initWithStoredValue:(int)value;
- (int)resultOfAddingStoredValue:(int)numberToAdd;
@end

@implementation NumberAdder
- (int)resultOfAddingStoredValue:(int)numberToAdd {
    return numberToAdd + storedValue;
}

- (id)initWithStoredValue:(int)value {
    if (!(self = [super init])) return nil;
    storedValue = value;
    return self;
}
@end

int main() {
    id adder = [[NumberAdder alloc] initWithStoredValue:4];
    int result = [adder resultOfAddingStoredValue:3];
    printf("It is %d\n", result);
    return 0;
}

And here is the same thing written in C with the Objective-C runtime (not tested, but should be roughly correct):

int returnPlusStoredValueImp(id self, SEL _cmd, int arg) {
    int *storedValue = nil;
    object_getInstanceVariable(self, "storedValue", &storedValue)
    return arg + *storedValue;
}

id numberAdderInit(id self, SEL _cmd, int valueToStore) {
    objc_super superInfo = {self, objc_lookupClass("NSObject")};
    self = objc_msgSendSuper(super_info, sel_getName("init"));
    if (!self) return nil;
    object_setInstanceVariable(self, "storedValue", &valueToStore);
    return self;
}

void createNumberAdderClass() __attribute(constructor)__ {
    Class NumberAdder = objc_allocateClassPair(objc_lookupClass("NSObject"), "NumberAdder", 0);
    if (!NumberAdder) return;
    class_addIvar(NumberAdder, "storedValue", sizeof(int), 4, "i"); // I'm actually not sure if the fourth argument is correct, so it's probably wrong, but just take that as a sign of how much this way of coding sucks
    objc_registerClassPair(NumberAdder);
    SEL nameOfPlusStoredValue = sel_registerName("resultOfAddingStoredValue:");
    SEL nameOfInit = sel_registerName("initWithStoredValue:");
    class_addMethod(NumberAdder, nameOfPlusStoredValue, returnPlusStoredValueImp, "i@:i");
    class_addMethod(NumberAdder, nameOfInit, numberAdderInit, "@@:i");
}

int main() {
    id adder = objc_msgSend(objc_lookupClass("NumberAdder"), sel_getName"alloc");
    adder = objc_msgSend(adder, sel_getName("initWithStoredValue:"), 4);
    int result = (int)objc_msgSend(adder, sel_getName("resultOfAddingStoredValue:"), 3);
    printf("It is %d\n", result);
    return 0;
}