How to tell if a Class inherits from NSObject (Objective-C)

Question

I'm working in Objective-C on the iPhone and need to know whether a 'Class' inherits from 'NSObject'.

I tried checking if it responds to an NSObject selector:

bool success = [myClass respondsToSelector:@selector(class)];

but you can guess what happened... it didn't even respond to "respondsToSelector:" so it throws a "does not implement doesNotRecognizeSelector:" exception.

I tried to catch that exception, but it looks like it can't be caught with a @try-@catch.

Any ideas?

Answer 1

Go direct to the Objective-C runtime:

#import <objc/runtime.h>

/* originally posted version — works because eventually class_getSuperclass(class)
returns nil, and class_getSuperclass(nil) does so also. */
BOOL classDescendsFromClass(Class classA, Class classB)
{
    while(1)
    {
        if(classA == classB) return YES;
        id superClass = class_getSuperclass(classA);
        if(classA == superClass) return (superClass == classB);
        classA = superClass;
    }
}

/* shorter version; exits straight after classA turns into nil */
BOOL classDescendsFromClassShorter(Class classA, Class classB)
{
    while(classA)
    {
        if(classA == classB) return YES;
        classA = class_getSuperclass(classA);
    }

    return NO;
}
...

if(classDescendsFromClass(classToTest->isa, [NSObject class]) ...

class_getSuperclass does what it says, and it's safe to compare metaclasses by pointer in the Objective-C runtime because there is only exactly one instance of the metaclass for each class. The isa pointer is the only thing that's definitely in struct objc_object.

EDIT: additionally, there are known bugs in the iPhone simulator that cause some exceptions not to be caught by try/catch blocks. I've reported them as a bug to Apple and been told that mine was a duplicate, so they are definitely aware. Did you try your code on a real device or just in the simulator?

EDIT2: from the wider context given elsewhere in this conversation, something like this might be smarter:

#import <objc/runtime.h>

BOOL classRespondsToSelector(Class classA, SEL selector)
{
    return class_getInstanceMethod(classA, selector) ? YES : NO;
}

....
if(classRespondsToSelector(instance->isa, @selector(respondsToSelector:))
{
     // great, we've got something that responds to respondsToSelector:; do the
     // rest of our querying through there
}

Answer 2

You can use the methods isKindOfClass: and isMemberOfClass: to determine whether a class is a subclass of another class or if it is a particular class.

Answer 3

respondsToSelector: is itself an NSObject-defined selector, so you can't use it. I don't believe there's a way to do this without getting very deep into the internals of Objective-C.

May I ask why you have objects that aren't descendants of NSObject? Apple very strongly recommends you don't attempt to create them, and with good reason.

Answer 4

The class 'Class' does not inherit from NSObject. That means methods defined by NSObject (such as isKindOfClass or respondsToSelector) cannot be used on it.

What are you trying to do with it in the first place?