Question

Why can’t I use [y release] at the end of function?

- (void)functionused:(NSTimer *)timer
{   
    [password resignFirstResponder]; 
    mainViewController* y=[[mainViewController alloc]initWithNibName:@"mainViewController" bundle:nil];
    UIView *currentView = (UIView *)[timer userInfo];

    for ( UIView *subview in [self.view.superview subviews] ) {

        if ((subview.tag != 14)) {

            [subview removeFromSuperview];
        }
    }
    [currentView.superview addSubview:[y view]];

    //[y release];
}

Answer 1

Because that will bring the reference count of mainViewController to zero and it will be deallocated. You need to keep a reference to it around somewhere.

The line [currentView.superview addSubview:[y view]]; only adds it’s view to the superview. The view will be retained, but the controller itself won’t.