On Mar 27, 2008, at 3:44 PM, Justin Giboney wrote:
I need to create a series of classes that implement the Singleton
design pattern. These classes have a lot of similar methods (I am
trying to create a series of DAOs see: http://en.wikipedia.org/wiki/
Data_Access_Object).
I was thinking that it would be best to create a super class, and a
series of subclasses to that super class. The problem I am running
upon is that the Singleton pattern requires a static variable.
How can I get a variable that is static to each subclass, but that
is declared in the super class?
Objective-C doesn't support the notion of class variables. The
convention is to use static variables, which have their usual C
semantics. That is, they are not "static to a class", they are
static to the compilation unit.
You're going to need a static variable in the implementation file of
each subclass. It may be possible to pass a pointer to that variable
to the superclass implementation at a strategic point so you can
still share code, but this strikes me as more complicated than
necessary.
Also, I'm aware that there are some implementations of the basic
Singleton functionality in a reusable forms. You can search the list
archives for them. I don't remember them off-hand. Here's one:
http://code.google.com/p/google-toolbox-for-mac/source/browse/trunk/
Foundation/GTMObjectSingleton.h
You're probably familiar with this, but I thought I'd add it for
completeness: http://developer.apple.com/documentation/Cocoa/
Conceptual/CocoaFundamentals/CocoaObjects/chapter_3_section_10.html
Cheers,
Ken
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