NS_DESIGNATED_INITIALIZER.m

// Subclass -> Superclass -> NSObject
// 
// NSObject's designated initializer is -init
// Superclass's designated initializer is -initWithSomething:
// Subclass's designated initializer is -initWithSomethingElse:
// 
// Superclass cannot be initialized with -init
// Subclass cannot be initialized with -init
// Subclass cannot be initialized with -initWithSomething:
// 
// How do we correctly use NS_DESIGNATED_INITIALIZER and NS_UNAVAILABLE?
// 


// ------------------------------------------------------------
// First pass:
// ------------------------------------------------------------

@interface NSObject
- (instancetype)init NS_DESIGNATED_INITIALIZER;
@end

@interface Superclass : NSObject
- (instancetype)initWithSomething:(id)x;
@end

@interface Subclass : Superclass
- (instancetype)initWithSomethingElse:(id)x;
@end


@implementation Superclass

- (instancetype)initWithSomething:(id)x
{
	return (self = [super init]);
}

@end


@implementation Subclass

- (instancetype)initWithSomethingElse:(id)x
{
	return (self = [super initWithSomething:x]);
}

@end



// ------------------------------------------------------------
// Adding designated to our headers:
// ------------------------------------------------------------


@interface Superclass : NSObject
- (instancetype)initWithSomething:(id)x NS_DESIGNATED_INITIALIZER;
@end


@interface Subclass : Superclass
- (instancetype)initWithSomethingElse:(id)x NS_DESIGNATED_INITIALIZER;
@end


@implementation Superclass

// WARNING: Method override for the designated initializer of the superclass '-init' not found
- (instancetype)initWithSomething:(id)x 
{
	return (self = [super init]);
}

@end


@implementation Subclass

// WARNING: Method override for the designated initializer of the superclass '-initWithSomething:' not found
- (instancetype)initWithSomethingElse:(id)x
{
	return (self = [super initWithSomething:x]);
}

@end




// ------------------------------------------------------------
// Adding unavailable to our headers:
// ------------------------------------------------------------


@interface Superclass : NSObject
- (instancetype)init NS_UNAVAILABLE;
- (instancetype)initWithSomething:(id)x NS_DESIGNATED_INITIALIZER;
@end


@interface Subclass : Superclass
- (instancetype)initWithSomething:(id)x NS_UNAVAILABLE;
- (instancetype)initWithSomethingElse:(id)x NS_DESIGNATED_INITIALIZER;
@end


@implementation Superclass

// WARNING: Method override for the designated initializer of the superclass '-init' not found
- (instancetype)initWithSomething:(id)x 
{
	return (self = [super init]);
}

@end


@implementation Subclass

// WARNING: Method override for the designated initializer of the superclass '-initWithSomething:' not found
- (instancetype)initWithSomethingElse:(id)x
{
	return (self = [super initWithSomething:x]);
}

@end




// ------------------------------------------------------------
// Adding overrides to our implementations:
// ------------------------------------------------------------


@interface Superclass : NSObject
- (instancetype)init NS_UNAVAILABLE;
- (instancetype)initWithSomething:(id)x NS_DESIGNATED_INITIALIZER;
@end


@interface Subclass : Superclass
- (instancetype)initWithSomething:(id)x NS_UNAVAILABLE;
- (instancetype)initWithSomethingElse:(id)x NS_DESIGNATED_INITIALIZER;
@end


@implementation Superclass

- (instancetype)init
{
	assert(false);
	return nil;
}

- (instancetype)initWithSomething:(id)x 
{
	return (self = [super init]);
}

@end


@implementation Subclass

- (instancetype)initWithSomething:(id)x
{
	assert(false);
	return nil;
}


- (instancetype)initWithSomethingElse:(id)x
{
	return (self = [super initWithSomething:x]);
}

@end




//
// ------------------------------------------------------------
// Conclusion:
// ------------------------------------------------------------
// 
// Correct usage:
// 
// 	- If a subclass does not support being initialized via a 
// 	  superclass's designated initializer then:
// 	  
// 	  	1) Mark it as uninitialized in the subclass's header
// 		2) Override it in the subclass's implementation to simply return nil.
// 	
// The purpose of requiring the override is not clear to me.
//