Question

This question is pure curiosity.

In Xcode, why does this work:

if (view.class == [UITextView class]) {
    UITextView *tview = (UITextView *)view;
    tview.textColor = [UIColor colorWithRed:0.020 green:0.549 blue:0.961 alpha:1.];
}

But the following results in the error message Property 'textColor' not found on object of type 'UIView *':

if (view.class == [UITextView class]) {
    (UITextView *)view.textColor = [UIColor colorWithRed:0.020 green:0.549 blue:0.961 alpha:1.];
}

Intuitively, these should accomplish the exact same thing.

But then if I enclose the inline cast in parentheses, it works fine:

if (view.class == [UITextView class]) {
    ((UITextView *)view).textColor = [UIColor colorWithRed:0.020 green:0.549 blue:0.961 alpha:1.];
}

I suspect it just has to do with how C handles order of operations, but I would be curious to hear an explanation. Thanks!

Answer 1

if (view.class == [UITextView class]) {
    (UITextView *)view.textColor = [UIColor colorWithRed:0.020 green:0.549 blue:0.961 alpha:1.];
}

Due to order of precedence, the (UITextView*) will act as a cast on the result of view.textColor, meaning .textColor will be accessed within the UIView* first before it’s cast to a UITextView*

if (view.class == [UITextView class]) {
    ((UITextView *)view).textColor = [UIColor colorWithRed:0.020 green:0.549 blue:0.961 alpha:1.];
}

In this, the extra parentheses will inform the compiler that sub-expression needs to be computed first, before the rest of the expression. As such, this is casting view to be a UITextView*. The side effect of that expression is a UITextView* instance, meaning that the .textColor property can be found on the instance it’s being used against.