Question

I saw someone passing arguments by a button’s tag as follow.
how could this work? and is it safe to do like this ?

NSCustomClass* cls = [[NSCustomClass alloc] init:@"",@"",@""];
[button setTag:(int) cls];



- (void)OnClick:(id)sender
{
    NSCustomClass* cls = (NSCustomClass*)[sender tag];
    // to do something with the "cls".
    [cls release];
}

In fact,I didn’t get weird results.

It works fine by passing arguments like ‘NSString’,’NSArray’ in this way.

Can you tell me why it is a bad idea?

Answer 1

In general you never want to abuse the frameworks. The tag is intended to store an integer and is used mainly to access a view with viewWithTag:, which can be useful in some cases if your view was built in Interface Builder but a referencing IBOutlet is inappropriate. Stuffing a pointer into an int can give unpredictable results, as others have indicated.

On top of that there’s a memory management issue. Even if the pointer survives the cast you aren’t retaining the object being pointed to. This in effect is a weak pointer but without any management by the run-time. Attempts to use the pointer will likely lead to the dreaded EXC_BAD_ACCESS. And who knows what ARC might do with this mess – blow up, I would expect.

To attach data to a button (which in and of itself sounds like a possible design flaw) you should either subclass or leverage the power of the run-time with objc_setAssociatedObject() and friends.