Question

I am new to ios, coming from heavy UI work in C#.
I have an app that I wanna control which button are enabled disabled based on some logic in my code.
to do this I created a small method to handle the state of the buttons, like so :

-(void)activateUI:(BOOL *)activate {

    [ validateDataBtn setEnabled: *activate ] ;
    [ modifyCompDataBtn setEnabled: *activate ] ;
    [ saveCompDataBtn setEnabled: *activate ] ;
}

Where all of those IBOutlets are bound to UI buttons like so :

__weak IBOutlet UIButton *saveCompDataBtn;
__weak IBOutlet UIButton *modifyCompDataBtn;
__weak IBOutlet UIButton *validateDataBtn;

when I run the app I get an exception on the first line of my method :

[ validateDataBtn setEnabled: *activate ] ;

the error is EXC_BAD_ACCESS (code = 2, address =0X0)
what am I doing wrong?

Answer 1

EDIT: My, my. I just realized how badly I messed this one up.

Incorrect: First off, you shouldn’t be using BOOL *, and if you ever did use BOOL * you would reference it as &activate, not *activate. The * in C/Objective-C means the variable is a pointer; it contains an address to a block of memory. & dereferences pointers to access the value on the other side.

Correct: An * indicates that the argument is a pointer, and is also used to deference a pointer. An & in front of a variable indicates that you want to use the address of the variable in memory. I mixed them up. This means your original code is correct; the problem is that you probably call it using [self activateUI:someBOOL], which passes a BOOL, then tries to deference it like a pointer due to the *.

You might be getting confused since objects are passed as <object type> * because all variables used to handle instances of objects are pointers. BOOL, however, is typedefed to int, which is primitive, thus the * is not needed. Your method should be:

- (void) activateUI:(BOOL)activate {
    [validateDataBtn setEnabled: activate];
    [modifyCompDataBtn setEnabled: activate];
    [saveCompDataBtn setEnabled: activate];
}

//called like this: [self activateUI: someBOOL];

If you really DID mean to pass a pointer to a BOOL as an argument use what you originally wrote, but you need to call it like this:

[self activateUI: &someBOOL];

If you declare the method to take a pointer to a BOOL (BOOL *), and dereference it inside the method body (*someBOOL), you need to pass the address to the BOOL (&someBOOL), not the BOOL itself. If you declare the method to take a BOOL itself (just BOOL), and use it directly in the method body (someBOOL), you need to pass the BOOL itself (someBOOL).