Objective C: warning on overriding init

Question

I have a class 'DOInstance' which I inherit later on. Here's its declaration:

@interface DOInstance : NSObject {

}

- (DOInstance *) initWithSynckey:(NSString *)_synckey;
@end

Then I have a subclass of DOInstance:

@interface Workflow_Workitem_Header_1px: DOInstance {

}
//- (Workflow_Workitem_Header_1px *) initWithSynckey:(NSString *)_synckey;
@end

I go ahead and implement it in the implementation file:

- (Workflow_Workitem_Header_1px *) initWithSynckey:(NSString *)_synckey {

    [super initWithSynckey:_synckey];
    //..
    //..
    return self;
}

Now, If I do not declare initWithSynckey: (the commented declaration above) in my subclass declaration, I get a warning at the implementation: "warning: initialization from distinct Objective-C type". If I declare it, this warning goes away. Okay.

Moving on:

I later do an instantiation of my subclass:

Workflow_Workitem_Header_1px *instance;
instance = [[Workflow_Workitem_Header_1px alloc] initWithSynckey:@"xxxx"];

Now, this gives me the same warning (irrespective of whether or not I declare the corresponding initWithSynckey: selector in my subclass. Namely, "warning: initialization from distinct Objective-C type".

What am I doing wrong?

Answer 1

Methods named init... should have return type (id), not the type of the class. Check out NSString.h and NSArray.h (among other classes) for examples. That may be what is causing your problem.

Answer 2

In this case, the overriding method must return the same type as the superclass' declaration.

DOInstance defines this:

- (DOInstance *) initWithSynckey:(NSString *)_synckey;

so Workflow_Workitem_Header_1px must look like this:

@interface Workflow_Workitem_Header_1px: DOInstance {
}
- (DOInstance *) initWithSynckey:(NSString *)_synckey;
@end

Any time you get the warning "warning: initialization from distinct Objective-C type" you're doing something in contravention of your typing: changing a method signature, and the like.