Writing an abstract class custom initializer method

Question

In an abstract class, a custom init method looks like this:

- (id)init
{
    self = [super init];

    if (self)
    {
        //do some initialisations
    }

    return self;
}

This would work perfectly fine when working with abstract classes: no problems at all. However, when creating objects, I prefer Class *object = [Class new]; over Class *object = [[Class alloc]init];. The problem with such a method in my abstract class, is that this doesn't work:

+ (id)new
{
    id object = [super new];

    if (object)
    {
        //do some initialisations
    }

    return object;
}

When trying to access properties of the object, Xcode will bring up an error, saying the corresponding property is not found on object of type '__strong id'. I'm guessing that's because the compiler has no idea what properties my object has, as it's just been created.

Is there any way to get around this limitation? If there's not, I'll just use the init method as it works just fine, but I was just curious about this.

Matt Wilding · Accepted Answer · 2012-09-13 17:58:17Z

3

You don't need to override new. According to the documentation, new...

Allocates a new instance of the receiving class, sends it an init message, and returns the initialized object.

So you only ever override init to perform your initialization. You're then free to create instances via alloc/init or new, depending on your taste.

edited Sep 13, 2012 at 17:58

answered Sep 13, 2012 at 17:52

Matt Wilding

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Comments

Analog File · Accepted Answer · 2012-09-13 17:54:33Z

2

+ (id)new
{
    return [[self alloc] init;
}

Note the + instead of - as this is a class method.

Except you do not need to write that as new is already implemented in NSObject.

answered Sep 13, 2012 at 17:54

Analog File

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1 Comment

Tim Vermeulen Over a year ago

Whoops, knew about that actually ;) Edited it now.

hisdrewness · Accepted Answer · 2012-09-13 18:01:46Z

0

I would not suggest the pattern you're using. It is much more semantic to create custom factory method initializer methods:

- (id) init
{
   // init code
}

- (id) initWithName:(NSString *)name
{
   // init code
}

+ (id) personWithName:(NSString *)name
{
    // add autorelease on non-ARC
    return [[super alloc] initWithName:name];
}

Person *person = [Person personWithName:@"Steve Jobs"];

Notice that it's a class method.

answered Sep 13, 2012 at 18:01

hisdrewness

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3 Comments

hisdrewness Over a year ago

What a silly comment. Of course more semantic implies meaningful, especially in comparison to new. Don't over think it. Look up the definition of semantics..."the study of meaning"

Tim Vermeulen Over a year ago

What do you dislike about my pattern? I know about custom initialisers like your initWithName: method, but I don't really see how your pattern would be different to mine.

hisdrewness Over a year ago

Code is literature homie. Your code should read like sentences.

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