CODEWITHSUNDEEP
iphoneobjective-cfor-loopnsarray
esqew
esqew

Reputation: 44691

for loops - Object type disregarded?

I sometimes like to organize IB elements into NSArrays primarily to help me organize my elements. Most often, different classes of objects make it into the same array with each other. While this is a convenient way of organization, I can't seem to wrap my head around why if I have an array like this:

NSArray *array = [NSArray arrayWithObjects:((UITextField *)textField), ((UISegmentedController *)segmentedController), nil];

Why I get "Does not respond to selector" messages when I put a for loop like this:

for (UITextField *text in array) {
    [text setText:@""];
}

The for loop seems to be passed objects that are not of class UITextField.

What is the point of declaring the object's class if all objects in the specified array are passed through the loop?

EDIT Just for reference, this is how I'm handling it as of now:

for (id *object in array) {
    if ([object isMemberOfClass:[UITextField class]]) {
        foo();
    } else if ([object isMemberOfClass:[UISegmentedController class]) {
        bar();
    }
 }

Upvotes: 7

Views: 165

Answers (3)

Luke
Luke

Reputation: 7210

What is the point of declaring the object's class if all objects in the specified array are passed through the loop?

The class name is just there to let the compiler know what it should expect to find. This allows it to try to figure out what methods it should expect you to call and how you might treat the object. It's the same idea as passing in an int to a method that takes float. The method will not ignore ints - it's assuming you're passing the correct type. You're just giving this construct a little more credit than it's due:

for (UITextField *text in array)

It just doesn't have that functionality. How you're handling it now is the correct way.

Upvotes: 5

SVD
SVD

Reputation: 4753

When you do

for (UITextField *text in...

the object pointers from the array are cast to UITextField* type - so if the object isn't actually a UITextField, all sorts of weird things may happen if you try to call UITextField methods.

So instead use the id type (no * needed, btw):

for (id obj in array)

Then check the type as you do and call the appropriate methods. Or, filter the array to get only objects of a certain type, then go though that type only:

for (UITextField* text in [array filteredArrayUsingPredicate:...])

Edit: here's how to build class filter predicates:

Is it possible to filter an NSArray by class?

Upvotes: 10

Caleb
Caleb

Reputation: 124997

Are you sure you don't get an error when you run that code? The "does not respond to selector" message is a runtime error, not a compile time error. The compiler has no idea whether the objects in the array implement -setText:, but you should certainly get an error when you actually send that message to an instance of UISegmentedControl.

Another possibility is that you've got a class called UISegmentedController that does have a -setText: method. The name of the class that implements the multi-part bar-graph-looking user interface widget is UISegmentedControl. So either the code you're showing isn't real, tested code, or you've got a class that we don't know about.

Upvotes: 0

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