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I have an array, arry1 that holds two kinds of objects obj1 and obj2. obj2 is a subclass of obj1. I wrote a method to sum the value of all occurances of obj1 which includes:

    int total = 0;
    for (obj1 *t in arry1){
        total += t.value;
    }

The problem is it totals both obj1 and obj2 items. It does the same if I change the for loop to be obj2 *t. So I have two questions:

  1. Is there a way to determine the actual class of the current instance inside the for loop?

  2. Is there a way to differentiate the two object instances in the for declaration?

John Morrison
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4 Answers4

2

There is

[obj1 isMemberOfClass: [whateverObj1Is class]];

But that would likely be true for obj2 since it is a subclass

A good solution would be to have a member function called value or something that class 2 would override returning 0 or something along those lines.

0xFADE
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    `isMemberOfClass:` is likely the correct solution here, because it tests for membership of a specific class, not whether the object inherits from the given class. In other words, your example would **not** be true for obj2. – NSAdam Jan 02 '14 at 20:38
1

You could try something like this:

int total = 0;
for (Obj1 *t in arry1){
    if([t isMemberOfClass:[Obj1 class]]) total += t.value;
}

All objects implement the NSObject protocol. This code uses two methods in that protocol. isMemberOfClass: checks if the object is of the class that is passed as an argument. The class method returns the class object for the receiver’s class.

Source/ more info

Also, if you want to check for objects that are of the Obj1 class or any of its sublasses you can use isKindOfClass: instead.

Connor Pearson
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  • There's not sense in declaring the loop variable `NSObject` instead of `obj1`. – Nikolai Ruhe Jan 02 '14 at 20:48
  • I chose to use NSObject because his array contains both obj1 and obj2 objects. In this code sample it doesn't really matter because he only cares about the obj1 objects, but if he wants to do different things to the objects depending on whether they are obj1 or obj2 in the loop, it would make more sense to me to declare the loop variable as NSObject. – Connor Pearson Jan 02 '14 at 20:54
  • The objects in the array are either `obj1` or `obj2`, which is a subclass of `obj1`. It's always best to use the most specific type available to get the benefits of static typing. In this case it removes the need for the ugly cast. – Nikolai Ruhe Jan 02 '14 at 20:57
  • I missed that part. You're right, I'll change that. – Connor Pearson Jan 02 '14 at 20:58
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You have to check each object and only add its value if is is of obj1 class.

int total = 0;
for (obj1 *t in arry1) {
    if ([t class] == [obj1 class])
        total += t.value;
}

Please note that it common to start class names with a capital letter. Also Obj1 would be a misleading name as it implies instance, not class.

Nikolai Ruhe
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  • this will work for most instances, but it is like asking isMemberOfClass, as in wont work for clusters / subclasses – Grady Player Jan 02 '14 at 20:34
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    True. Yet the question seems to imply that the classes are well known. Also it is especially asked for the sum of objects of the obj1 class. If a class cluster silently adds unknown classes, one could argue that they should not be added. – Nikolai Ruhe Jan 02 '14 at 20:38
  • Thank you. I tested this and it works for both the main class and the subclass. – John Morrison Jan 03 '14 at 00:48
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You could use introspection to determine which class each of the objects in the array belong to. There are two useful methods to determine the class of an object. isKindOfClass: and isMemberOfClass:. isKindOfClass: returns a Boolean value that indicates whether the receiver is an instance of given class or an instance of any class that inherits from that class. Whereas, isMemberOfClass: returns a Boolean value that indicates if the object is a member of that specific class.

You could therefore do this:

int total = 0;
obj1 *myObj1;
for (NSObject *object in arry1)
{
    if([object isMemberOfClass:[obj1 class]])
    { 
        myObj1 = (obj1 *)object;
        total += myObj1.value;
    }
}
Tyler Cloutier
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  • `object.value` only works when you declare the loop variable `obj1 object`. – Nikolai Ruhe Jan 02 '14 at 20:43
  • Ah yes, sorry about that I'll update. – Tyler Cloutier Jan 02 '14 at 20:49
  • There's no sense in declaring the loop variable `NSObject` instead of `obj1`. (`obj1` seems to be the name of the class. You are calling it `Obj1ClassName`). You can drop the secondary variable then. – Nikolai Ruhe Jan 02 '14 at 20:52
  • There is sense if you want to do other things with the other objects in the array depending on their class. Although I suppose that wasn't necessary in this case. Still good practice IMHO. Also, you're right, let me fix that as well. – Tyler Cloutier Jan 02 '14 at 20:55
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    I don't see the problem with keeping the secondary variable if it clarifies the code. It would also allow you to access other properties on that object without typecasting each one. – Tyler Cloutier Jan 02 '14 at 21:04