Help understanding class method returning singleton [duplicate]

Answer 1

I would also override ’mutableCopyWithZone:’ and ’copyWithZone:’ methods in order to avoid that the singleton get copied:

// copy cannot be done
- (Game *)copyWithZone:(NSZone *)zone {
    return self;
}
// mutablecopy cannot be done
- (Game *)mutableCopyWithZone:(NSZone *)zone {
    return [self copyWithZone:zone];
}

Since ’copy’ and ’mutableCopy’ (assuming the singleton class doesn't inherit from another class which has overridden default NSObject implementations) just call ’copyWithZone:’ and ’mutableCopyWithZone:’ there will be not need to override those too.

To also avoid other developers from breaking the singleton pattern by using ’init’ or ’new’, the two methods can be declared unavailable:

- (instancetype)init NS_UNAVAILABLE;
+ (instancetype)new NS_UNAVAILABLE;

In this way the compiler won't allow the use of the two methods, and other developers will be forced to use the documented method for obtaining the shared instance. Another —more strict— technique is to override ’init’ and ’new’ and raise an exception.