Question

I use the following code to iterate through all the properties of an object. I successfully retrieve the property name as char but I have no idea how to get the property value which is of id type. Any ideas on how can I achieve this?

objc_property_t *allProperties = class_copyPropertyList([currentObject class], &allPropertyCount);

for (unsigned int i = 0; i < allPropertyCount; i++) {

      objc_property_t property = allProperties[i];
      const char * propertyName = property_getName(property);

}

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EDIT: Thank you all for the great comments and answers. Some of you asked why do I need this. Well, here is the reason:

I have several objects of the same class. Let’s say the class is Person and its instances are Mary, John and David. The properties of each object are set as follows:

mary.age = [NSNumber numberWithInt:20];
john.age = [NSNumber numberWithInt:45];
david.age = [NSNumber numberWithInt:20];

mary.gender = @"female";
john.gender = @"male";
david.gender = @"male";

My purpose is to find a generic way to group the objects based on a given property name Eg. this will create 2 groups [david and mary] and [john]:

[self groupBaseDataObjects:self.persons withPropertyName:"age"];

and this:

[self groupBaseDataObjects:self.persons withPropertyName:"gender"]; 

will also create 2 groups [john and david] and [mary]

Answer 1

OK — there appears to be no need to do any funky runtime gymnastics to solve your problem (don’t get me wrong — funky runtime gymnastics can be fun…. but it is probably just a waste of time here).

To restate the question:

You have a single class (MyClass) with many instances of that class. The class
provides several attributes with instances of the class having many
different values for that attribute. You have a collection of
instances and you want to easily grab a collection of multiple subsets
where the objects in each subset have the same value for a particular
attribute.

To that end, I’d just loop over the original question and do something like (this code has never seen a compiler; it’ll probably not compile):

// implement this on MyClass
+ (NSDictionary*)subdivideArrayOfMyClass:(NSArray*) aCollection byAttribute:(NSString*) anAttribute
{
    NSMutableDictionary *collector = [NSMutableDictionary dictionary];
    for (MyClass *anObject in aCollection) {
        id value = [anObject valueForKey: anAttribute];
        // you'd have to special case for nil here if you wanted.
        NSMutableArray *sameValueCollector = [collector objectForKey: value];
        if (!sameValueCollector) {
             sameValueCollector = [NSMutableArray array];
             [collector setObject: sameValueCollector forKey:value];
        }

        [sameValueCollector addObject:anObject];
    }
    return collector; // no need to turn it into an immutable dict
}

This does assume that all the values are safe for use as keys in a dictionary. That would be true for the examples you gave.

If you need all objects with a single value, you could do valueForKey: on the array. If you wanted, you could do some funky set arithmetic to sub-divide the objects. However, that’ll likely incur multiple passes over the collection whereas the above does the work with one pass (though a potentially large quantity of hash lookups in the collector). Go with the straightforward until performance analysis indicates that it is too slow.