Subclassing NSString

Question

Using WSDL2ObjC I am getting lot of classes which are subclasses of NSString. I am finding it troublesome to initialize the NSString value of any object of those classes.

Say my class looks like this :

@interface mClass ; NSString {
     int value;
}

Now in my code I would like to use objects of mClass as both NSString and also want to use its attribute value which is an integer.

How can I do that?

I am trying to use code like this

 mClass *obj = [[mClass alloc] initWithString:@"Hello"];

But it's showing me an error saying I am using an abstract object of a class , I should use concrete instance instead.

Answer 1

If you really need make NSString subclass you should override 3 methods:

- (instancetype)initWithCharactersNoCopy:(unichar *)characters length:(NSUInteger)length freeWhenDone:(BOOL)freeBuffer;
- (NSUInteger)length;
- (unichar)characterAtIndex:(NSUInteger)index;

For example:

MyString.h

@interface MyString : NSString 

@property (nonatomic, strong) id myProperty;

@end


MyString.m

@interface MyString ()

@property (nonatomic, strong) NSString *stringHolder;

@end

@implemenation MyString

- (instancetype)initWithCharactersNoCopy:(unichar *)characters length:(NSUInteger)length freeWhenDone:(BOOL)freeBuffer {
    self = [super init];
    if (self) {
        self.stringHolder = [[NSString alloc] initWithCharactersNoCopy:characters length:length freeWhenDone:freeBuffer];
    }
    return self;
}

- (NSUInteger)length {
    return self.stringHolder.length;
}

- (unichar)characterAtIndex:(NSUInteger)index {
    return [self.stringHolder characterAtIndex:index];
}

@end

Answer 2

It might be smarter to use a wrapper class that internally uses NSStrings to do whatever operations or manipulations you are trying to do. However this will cause you to need to overload any functionality of NSString you want (such as getting the length of the string).

Or, you could create a category of NSString (found right next to Objective-C class in the new file window). This allows you to add any properties or methods that you wish to be "added" to the NSString class. Now just import this category wherever you wish to use it and you will have all of your custom functions available on any NSStrings objects.

Answer 3

Do you really need to subclass NSString? It’s a class cluster, which (apart from other things) means it’s hard to subclass. There’s a good post by Mike Ash on subclassing class clusters. If you didn’t know that class clusters existed you are probably new to Cocoa and in that case the best short answer is don’t try to subclass class clusters.

There’s also previous questions about subclassing NSString here on Stack Overflow. Next time you might want to search a bit before asking a new question.