Question

if header file declares

@interface SomeClass: NSObject {
  Data* d;
}

@property (nonatomic, retain) Data* d;

Why is the following line in the implementation file giving me a warning (and init method does not get called?)

[[[self d] alloc] init];

The warning i get is

Instance method ‘-alloc’ not found (return type defaults to ‘id’)

Meanwhile, Data has
- (id) init method, that is not being called.

Please help me understand why.

Answer 1

alloc should be invoked on a class, not on an instance.

interface SomeClass : NSObject
{
    Data *d;
}

Declare an init method on SomeClass and make it look like:

- (id) init
{
    self = [super init]; 
    if (self)
    {
        d = [[Data alloc] init];
    }

    return self;
}

- (void) dealloc
{
    [d release];
    [super dealloc];
}  

Now you do:

SomeClass *c = [[SomeClass init] alloc];

And you can use the class. Note that you should probably read a little more on classes and objects and about memory management too (when you should release c, etc.).

If, by any chance, you have the possibility to use ARC (automatic reference counting), you won’t need to take care of releasing stuff. But that doesn’t come with Xcode 4.1, only with 4.2 which is not publicly accessible, apparently.