add a "+" button in a navigationBar

I try to add a "add button" in the navigationBar, but the button who is created is a rectangle button with no text inside, not a "+" button, why?

UIBarButtonItem *addButton = [[UIBarButtonItem alloc] initWithTitle:@"" style:UIBarButtonSystemItemAdd target:self action:@selector(ajouterItem)];

self.navigationItem.rightBarButtonIte开发者_运维问答m = addButton;

[addButton release];

You're mixing up the initializer with the one for title/style. You want this form:

- (id)initWithBarButtonSystemItem:(UIBarButtonSystemItem)systemItem target:(id)target action:(SEL)action

Call it like this:

UIBarButtonItem *addButton = [[UIBarButtonItem alloc] initWithBarButtonSystemItem:UIBarButtonSystemItemAdd target:self action:@selector(ajouterItem)];