Designated initializer should only invoke a designated initializer on 'super'

Question

This code is a modified version of Ordered Dictionary implemented here. https://github.com/nicklockwood/OrderedDictionary/tree/master/OrderedDictionary

Interface -> OrderedDictionary.h

 @interface OrderedDictionary : NSMutableDictionary
{
}

Implementation -> OrderedDictionary.m

// This is the new method added.
- (instancetype)init
{
    return [self initWithCapacity:0];
}

- (instancetype)initWithCapacity:(NSUInteger)capacity
{
    self = [super init];
    if (self != nil)
    {
       // Allocate here.
    }
    return self;
}

The code works fine but I get following warnings in "- (instancetype)init".

1. Designated initializer should only invoke a designated initializer on 'super'
2. Designated initializer missing a 'super' call to a designated initializer of the super class

What am I doing wrong and how do I fix it?

Made following changes to the code to fix the problem

 // This is the new method added.

- (instancetype)init
{
    self = [super init];
    if (self != nil)
    {
       // Allocate here.
    }
    return self;
}

- (instancetype)initWithCapacity:(NSUInteger)capacity
{
    self = [super initWithCapacity:capacity];
    if (self != nil)
    {
       // Allocate here.
    }
    return self;
}

Answer 1

try adding - (instancetype)initWithCapacity:(NSUInteger)capacity NS_DESIGNATED_INITIALIZER;

To the @interface block. All initialization paths should flow through the designated initializer - in your case initWithCapacity: is the obvious choice since init calls it. You may still run into problems, the designated initializer must call the super class's designated initializer. If the super class defines designated initializers those must be implement by your class as well...

In general subclassing NSMutableDictionary is considered bad practice because NSMutableDictionary is the public facing piece of a class cluster. This may be part of the cause of the compiler warnings.

Answer 2

As you can read in NSMutableDictionary documentation, there are two designated initializer for this class:

• initWithCapacity: Designated Initializer
• init Designated Initializer

Here you are calling from initWithCapacity in your class to super.init. That's the reason the compiler warns you.

This code maybe is better:

// This is the new method added.
- (instancetype)init
{
    return [self initWithCapacity:0];
}

- (instancetype)initWithCapacity:(NSUInteger)capacity
{
    self = [super initWithCapacity:capacity];
    if (self != nil)
    {
       // Allocate here.
    }
    return self;
}