Can I declare a variable of a 'protocol' type in an Objective-C interface?

My idea is very similar to declare a variable of an interface type in java.

So for example,

header file 1:

@protocol Calculator

@end

I then define an @interface CalculatorImpl which implements the above Calculator protocol.

In header file 2:

@interface SomeViewController : UIViewController {


}

@property (weak, nonatomic) IBOutlet UITextField *txtResult;
@property (weak, nonatomic) Calculator* calculator;

@end

However, the xcode will flag an error at the calculator line

property with 'weak' attribute must be of object type 

Is this usage of protocol disallowed by objective-c?