Getting an error on my (first ever) CardGameViewController - objective-c

Question

I'm getting the error incompatible pointer types assigning to Deck *__strong from PlayCards *
And i'm not sure why is that. Its in the first method implemented (deck):
#import "CardGameViewController.h"
#import "PlayingCards.h"
#interface CardGameViewController ()
#property (weak, nonatomic) IBOutlet UILabel *cardLabel;
#property (nonatomic) NSUInteger flipsCount;
#property (strong, nonatomic) Deck *deck;
#end
#implementation CardGameViewController
-(Deck *) deck {
if (!_deck) _deck = [[PlayingCards alloc] init];
return _deck;
}
-(void) setFlipsCount:(NSUInteger)flipsCount {
_flipsCount = flipsCount;
self.cardLabel.text = [NSString stringWithFormat:#"Flips:%d", self.flipsCount];
}
- (IBAction)flipCard:(UIButton *)sender {
sender.selected = !sender.isSelected;
self.flipsCount++;
}
#end
This is the header file(nothing going on here):
#import <UIKit/UIKit.h>
//#import "Card.h"
//#import "Deck.h"
//#import "PlayingCards.h"
#interface CardGameViewController : UIViewController
#end
And the PlayingCard class inheriting from Deck class..
this is the PlayingCards.m
#import "PlayingCards.h"
#implementation PlayingCards
#synthesize suit = _suit;
//modifying the contents getter so it will return array with the ranks and rank+suit
-(NSString *) contents {
NSArray *cardsRank = [PlayingCards rankStrings];
return [cardsRank[self.rank] stringByAppendingString:self.suit];
}
//creating a method to make sure we get validated suits
+(NSArray *) validSuit {
return #[#"♠",#"♣",#"♥",#"♦"];
}
//creating calss method to validate the rank
+(NSArray *) rankStrings {
return #[#"?",#"A",#"2",#"3",#"4",#"5",#"6",#"7",#"8",#"9",#"10",#"J",#"Q",#"K"];
}
//creating a new setter for suit to make sure we get the valitated suits, uding the validateSuit method
-(void) setSuit:(NSString *)suit {
if ([[PlayingCards validSuit] containsObject:suit]) {
_suit = suit;
}
}
//creating new getter for suit to make sure its not empty
-(NSString *) suit {
return _suit? _suit: #"?";
}
//creating a class method to make sure when user set the rank he will will
+(NSUInteger) maxRank {
return [self rankStrings].count - 1;
}
//creating a new setter to the renk to make sure the rank is validates
-(void) setRank:(NSUInteger)rank {
if (rank <= [PlayingCards maxRank]) {
_rank = rank;
}
}
#end
PlayingCards.h
#import "Card.h"
#import "Deck.h"
#interface PlayingCards : Card
#property (strong, nonatomic) NSString *suit;
#property (nonatomic) NSUInteger rank;
+(NSArray *) validSuit;
+(NSUInteger) maxRank;
#end

Answer 1

This line:
if (!_deck) _deck = [[PlayingCards alloc] init];
Should be:
if (!_deck) _deck = [[PlayingCardDeck alloc] init];

Answer 2

If the parent for Card is of class NSObject as you say, and given that PlayingCards inherits from Card, then you can't assign an instance of PlayingCards to a variable of type Deck*. That's what the compiler is telling you.
If you really need to do it, you have to write:
if (!_deck) _deck = (Deck*)[[PlayingCards alloc] init];
It would only be valid because in Objective-C the implementation is given at runtime and which method of which class is called is only decided at runtime when the message is dispatched. However, this pattern is very unusual and you better be certain that PlayingCards is implementing all the selectors that might be called on a Deck instance. A better way would be to use protocols.
You can define a protocol and then use:
id <myProtocol> deck = [[PlayingCards alloc] init];
Put in the protocol all the selectors you need.
Why can't you use this ?
PlayingCards* deck = [[PlayingCards alloc] init];