Question

I have a simple program that plays a song. It’s in the inherited awakeFromNib method. So..

-(void)awakeFromNib {
NSSound *song = [NSSound soundNamed:@"MyTune.mp3"];
[song play];
}

My question is, why does this work. How come I don’t have to do this

NSSound *song = [[NSSound alloc]init];
song = [NSSound soundNamed:@"MyTune.mp3"];
[song play];
}

It also seems to work with strings too.. I have a NSTextView variable set up and I can do the following

-(void)awakeFromNib {
NSString *str = [NSString stringWithFormat:@"Hello there!"];
[myTextVariable insertText:str];
}

Why didn’t I have to alloc and init the objects.. I am so lost..
Please help.

Answer 1

Many of Apple’s classes have helper functions, declared at the class level which do the alloc and init for you, inside the helper function. They return a ready to use object. You can tell if yu see the doc for the method and it says something like “Returns the NSSound instance associated with a given name.”

Your first example is therefore good code:

-(void)awakeFromNib {
NSSound *song = [NSSound soundNamed:@"MyTune.mp3"];
[song play];
}

Your second example leaks memory because you alloc and then overwrite your pointer with a new object returned by [NSSound soundNamed:@"MyTune.mp3"]:

  -(void)awakeFromNib {

    // Create an NSSound object in memory and store the address in song.
    NSSound *song = [[NSSound alloc]init]; 

    // If you don't want a memory leak this is your last chance to [song release]

    // Create a NSSound object using a helper function and place its address 
    // in song, over writing the previous address.
    song = [NSSound soundNamed:@"MyTune.mp3"];

    // We now lost track of the first NSSound object and can't release it because 
    // we overwrote the address.

    [song play];
    }

From the documentation you can see that this method is doing the alloc and init inside it and returning the instance to you:

soundNamed:

Returns the NSSound instance associated with a given name.

+ (id)soundNamed:(NSString *)soundName

Parameters

soundName
Name that identifies sound data.

Return Value

NSSound instance initialized with the sound data identified by soundName.