CODEWITHSUNDEEP
objective-c
James Raitsev
James Raitsev

Reputation: 96391

Initializing objects using Objective-C, clarification needed

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.

Upvotes: 0

Views: 1261

Answers (3)

Rudy Velthuis
Rudy Velthuis

Reputation: 28806

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.

Upvotes: 5

adamrothman
adamrothman

Reputation: 956

You should be doing

self.d = [[Data alloc] init];

As Matt says, alloc is a class method, and must be called on the class itself.

Upvotes: 1

Matt Wilding
Matt Wilding

Reputation: 20153

The problem isn't -(id)init, it's -(id)alloc. alloc is a class method of NSObject, which means you send it to the class itself and not to an instance of that class, i.e.:

[Data alloc]; // Correct
[someDataInstance alloc]; // Method not found

When you call [self d], you're given an instance of a Data, which you're then sending a -(id)alloc message to. Since NSObject doesn't have a -(id)alloc (only a +(id)alloc), you get the warning.

Upvotes: 2

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