Question

In Objective-C, you can invoke class methods with:

[MyClass aClassMethod];

And you can query an instance’s kind with:

[someInstance isKindOfClass:[MyClass class]];

But, why do we need to do [MyClass class], and not simply provide MyClass like this:

[someInstance isKindOfClass:MyClass];

Is there a reason that the compiler is fine with encountering MyClass as a receiver (a pointer type) but not as an argument? Is it a limitation of parsing the language? Or perhaps a limitation of the compiler?

Answer 1

Ooooh… fun question. The answer is a c-ism.

Consider:

@interface MyClass : NSObject
@end
@implementation MyClass
@end

Now, say you have:

...
MyClass *m = nil;
...

In that context, the compiler sees MyClass as a type definition. The * says that the variable m is a pointer to a hunk o' memory that contains one (or many -- don't forget your C pointer-fu) MyClass instances.

In other words, MyClass is a type.

But, in the context of something like:

[someInstance isKindOfClass: x ];

x must be an rvalue or, in human terms, the value of an expression. A type, however, cannot be used as an rvalue.

That [MyClass class] works is actually a bit of a hack, both in the language and the compiler in that the grammar specifically allows a type name to be the message receiver (to be the target of a method call).

And, as a matter of fact, you can do:

typedef MyClass Foo;
....
[MyClass class];
[Foo Class];

It’ll all work. However, you can’t do the following but the error message is illuminating:

[NSUInteger class];

error: ‘NSUInteger’ is not an Objective-C class name or alias

---

Now, why not special case it everywhere as a bare name?

That colludes type names and rvalues and you quickly end up having to swallow something like [foo isKindOfClass: (MyClass)]; while barfing on [foo isKindOfClass: (MyClass *)]; which then encroaches upon typecasting territory in a rather uncomfortable fashion.