Question

The pointer passed into the “menuItem” methode is alloc and for the “Pointer Inside” a value is registered. But for the “Pointer Outside” the value is “null” … why? I passed in the pointer and it should had been modified in the method?

In the header file:

UIButton *bMenu_time;
UILabel *lMenu_time;

Implementation:

- (void) menuItem: (UIView*)vMenu  menuButton:(UIButton*)bMenu menuLabel: (UILabel*)lMenu  menuPosX: (double)posX   menuLenX: (double)lenX  menuTagNum: (int)tagNum menuText: (NSString*)txtMenu{

    bMenu = [UIButton buttonWithType:UIButtonTypeRoundedRect];
    [bMenu setFrame:CGRectMake(posX,0,lenX,25)];
    [bMenu setTag: tagNum];
    [bMenu addTarget:pSelf action:@selector(NewNumber:) forControlEvents:UIControlEventTouchUpInside];
    [vMenu addSubview:bMenu];


    lMenu = [[UILabel alloc] initWithFrame:CGRectMake(posX,0,lenX,25)];
    [lMenu setBackgroundColor:[UIColor lightGrayColor]];
    [lMenu setText:[NSString stringWithFormat:  txtMenu]];
    [lMenu setFont:[UIFont systemFontOfSize:14 ]];
    [lMenu setTextAlignment:UITextAlignmentCenter];
    [vMenu addSubview: lMenu];

    NSLog(@"\nPointer Inside: %@\n", lMenu); // <--------- INSIDE WORKS
}



- (void) menuBuild{     
    pSelf = self;
    theString = @"";


    vMenu = [[UIView alloc] initWithFrame:CGRectMake(0,0,320,25)];
    [pSelf.view addSubview:vMenu];
    [vMenu setBackgroundColor:[UIColor grayColor]]; 


    iTime = 2;
    [self menuItem:vMenu menuButton:bMenu_time menuLabel:lMenu_time menuPosX:240+20 menuLenX:60 menuTagNum:102  menuText:[NSString stringWithFormat:  @"Hold: %d", iTime]];

    NSLog(@"\nPointer Outside: %@\n", lMenu_time); // <----- OUTSIDE is NULL ??
}

Answer 1

paxdiablo is absolutely right, but in case you’re uncertain of the syntax for pass-by-reference, here is what your code should be:

- (void) menuItem: (UIView*)vMenu  menuButton:(UIButton **)bMenu menuLabel: (UILabel **)lMenu  menuPosX: (double)posX   menuLenX: (double)lenX  menuTagNum: (int)tagNum menuText: (NSString*)txtMenu{

    *bMenu = [UIButton buttonWithType:UIButtonTypeRoundedRect];
    [*bMenu setFrame:CGRectMake(posX,0,lenX,25)];
    [*bMenu setTag: tagNum];
    [*bMenu addTarget:pSelf action:@selector(NewNumber:) forControlEvents:UIControlEventTouchUpInside];
    [vMenu addSubview:*bMenu];


    *lMenu = [[UILabel alloc] initWithFrame:CGRectMake(posX,0,lenX,25)];
    [*lMenu setBackgroundColor:[UIColor lightGrayColor]];
    [*lMenu setText:[NSString stringWithFormat:  txtMenu]];
    [*lMenu setFont:[UIFont systemFontOfSize:14 ]];
    [*lMenu setTextAlignment:UITextAlignmentCenter];
    [vMenu addSubview: *lMenu];

    NSLog(@"\nPointer Inside: %@\n", *lMenu); // <--------- INSIDE WORKS
}



- (void) menuBuild{     
    pSelf = self;
    theString = @"";


    vMenu = [[UIView alloc] initWithFrame:CGRectMake(0,0,320,25)];
    [pSelf.view addSubview:vMenu];
    [vMenu setBackgroundColor:[UIColor grayColor]]; 


    iTime = 2;
    [self menuItem:vMenu menuButton:&bMenu_time menuLabel:&lMenu_time menuPosX:240+20 menuLenX:60 menuTagNum:102  menuText:[NSString stringWithFormat:  @"Hold: %d", iTime]];

    NSLog(@"\nPointer Outside: %@\n", lMenu_time); // <----- OUTSIDE is NULL ??
}

Things to note:

1. objects passed by reference in the method parameters use **
2. to pass the address of an object to one of these parameters, use &object when calling the method
3. to refer to an object that is passed by reference, prefix the variable with *

Have a look at how iOS uses error parameters in methods like NSString initWithContentsOfFile:error: for more examples of how this approach is used.