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iOS Build a Simple iPhone App (iOS7) Refactoring into a Model Understanding @property

Shaun Johnson
Shaun Johnson
1,214 Points
Posted by Shaun Johnson
Shaun Johnson
1,214 Points

NSArray *_predictions;

The video is unclear as to the definition of the array pointer NSArray *_predictions; and accessing the instance variable _predictions. _predictions is accessed directly, "_predictions", and is not a pointer. So why the pointer notation in the explicit declaration?

3 Answers

Stone Preston
Stone Preston
42,016 Points
Stone Preston
Stone Preston
42,016 Points

_predictions is of type NSArray, which means it is an object. All objects in objective c are declared as pointers.

Shaun Johnson
Shaun Johnson
1,214 Points
Shaun Johnson
Shaun Johnson
1,214 Points

Thanks, that makes sense. The code goes on to access "_predictions" (if (_predictions == nil)) directly. Can you lend some insight into how this works, given _predictions is a pointer to an object?

Stone Preston
Stone Preston
42,016 Points
Stone Preston
Stone Preston
42,016 Points

are you asking why you didnt have to dereference the pointer like:

if (*_predictions == nil)
Shaun Johnson
Shaun Johnson
1,214 Points
Shaun Johnson
Shaun Johnson
1,214 Points

Yes. I NSLog'd _predictions and it is in fact the array I defined. I was expecting the address of the object.

Stone Preston
Stone Preston
42,016 Points
Stone Preston
Stone Preston
42,016 Points

ok thats because when using NSLog and passing it a %@ specifier, what that really does is send the description message to that object. the description message (and most other messages) accepts a pointer to an objects not the object itself. So thats why

 printf(@"Predictions: %@", _predictions)

doesnt print the memory address of _predictions since the description method accepts a pointer as its argument, not actual objects themselves. Objective c methods do all that stuff for you behind the scenes so you dont usually have to dereference object pointers because of the way the language is designed to work. Its a lot more complicated than that simple explanantion, but you get the idea.