在抽象的Objective-C类中使用常量？

Question

首先,让我澄清一下,我知道你不能在Objective-C中拥有实际的抽象类.当我说抽象时,我的意思是我没有创建该类的任何实例,而只是将它用作继承的基类.

我的抽象类包含一个NSInteger.从这个抽象类继承的所有类都需要使用这个整数.一旦设置,NSInteger将永远不会被更改.NSInteger的值虽然会因派生类而异.因为它永远不会被改变,所以我想制作变量const.但事情是,如果我将它设为const,我将不得不在实例化时设置值.因此,我必须在基本抽象类中设置值 - 但是我不能根据它存在的派生类来调整值.

我意识到这很难理解所以我创建了一些基本的伪代码.我使用NSString作为示例,但在我的程序中它是一个NSInteger:

Class animal: const NSString soundItMakes = "Sound" //the constant integer
//since soundItMakes is const I have to set it upon instantiation even though it doesn't make sense in the base class

Class dog: animal //derives from animal. Needs to change the const soundItMakes to fit a dog however since it was already set in animal, the base class, I can't really change it

Class cat: animal //derives from animal. Needs to change the const soundItMakes to fit cat however since it was already set in animal, the base class, I can't really change it.

我不想从基类中删除变量,而是将它放在每个派生类中,也不想让它变为非常量.有没有人看到这个解决方案？我意识到这是令人困惑的,所以如果需要更多的信息,请随时问.

我在Mac OS X Snowleopard中使用Xcode 3.2.6中的Cocoa.

Answer 1

通常我会用方法而不是ivar来解决这个问题.

@interface Animal : NSObject
+ (NSString *)soundItMakes;
@end

@implementation Animal
+ (NSString *)soundItMakes {
  NSAssert(NO, @"Abstract");
}
@end

@implementation Dog
+ (NSString *)soundItMakes {
  return @"bark";
}