Question

I want to implement a class which can be used by two classes of my project.

One is manipulating ‘NewsRecord’ objects.
One is manipulating ‘GalleriesRecord’ objects.

In another class, I may use one of the two objects so i do something like that :

// header class
id myNewsRecordOrGalleriesRecord;

// class.m
// NewsRecord and GalleriesRecord have both the title property
NSLog(myNewsRecordOrGalleriesRecord.title);

and i get :

error : request for member 'title' in something not a structure or union

any ideas 😀 ?

Thanks.

Gotye

How am I supposed to do it ?

Answer 1

You can’t use dot syntax on id types because the compiler cannot know what x.foo means (the declared property may make the getter a different name, e.g. view.enabled -> [view isEnabled]).

Therefore, you need to use

[myNewsRecordOrGalleriesRecord title]

or

((NewsRecord*)myNewsRecordOrGalleriesRecord).title

If title and more stuffs are common properties of those two classes, you may want to declare a protocol.

@protocol Record
@property(retain,nonatomic) NSString* title;
...
@end

@interface NewsRecord : NSObject<Record> {  ... }
...
@end

@interface GalleriesRecord : NSObject<Record> {  ... }
...
@end

...

id<Record> myNewsRecordOrGalleriesRecord;
...

myNewsRecordOrGalleriesRecord.title;  // fine, compiler knows the title property exists.

---

BTW, don’t use NSLog(xxx);, which is prone to format-string attack and you can’t be certain xxx is really an NSString. Use NSLog(@"%@", xxx); instead.