Pointer to a pointer in objective-c?

Question

I would like to declare a pointer to a pointer in objective-c.

I have an instance variable (primaryConnection) that should be updated dynamically to point to a local variable when it changes.

NSURLConnection *primaryConnection;

-(void) doSomething
{
  NSURLConnection *conn;
  primaryConnection = conn;

  conn = // set by something else

  // primaryConnection should now reflect the pointer that conn is pointing to, set by something else... but it doesn't?
}

Is it possible to declare a pointer to a pointer somehow? Or am I missing something?

Answer 1

You pretty much never ever want to do this, the one typical exception is when you want to pass a reference as an argument that may be filled in. See the use of (NSError **) throughout the APIs.

In particular, you do not want to declare an instance variable as NSConnection ** and then set it somewhere else; somewhere outside the object. That totally breaks encapsulation and is a sure sign that your code is poorly or, at the least, oddly designed.

Try this instead:

@interface MyClass:NSObject
{
    NSConnection *connection;
}

@property(retain) NSConnection *connection;
@end

@implementation MyClass
@synthesize connection;
@end

And in whatever class / code needs to set the connection:

@implementation SomeOtherClass
- (void) configureConnection: (MyClass *) anObject
{
    NSConnection *aConnection;

    aConnection = ... initialize the connection ...

    anObject.connection = aConnection;
}
@end

That will preserve encapsulation and allow something else to set up the connection for MyClass. If this doesn't solve your problem, you will need to tell us what it is you are really trying to do.