Objective C - 子类中的窄实例变量类型？ [英] Objective C -- narrow instance variable types in subclasses?

问题描述

是否可以缩小子类中允许的ivar类型。这样的事情：

Is it possible to narrow the allowed type of an ivar in a subclass. Something like this:

@interface person: NSObject {
  NSArray *friendArray;
}

@interface mutablePerson: person {
  NSMutableArray *friendArray;
}

我只是尝试了那个确切的代码，Xcode给了我一个编译错误。我想知道是否有办法绕过它。

I just tried that exact code, and Xcode gave me a compile error. I'm wondering if there is a way around it.

我正在研究的项目会有很多这种情况。我知道我可以使用强制转换来使代码工作。但如果我这样做，我会做很多演员表，我想知道是否有更好的方法。

The project I am working on is going to have a lot of this sort of situation. I understand that I can use casts to make the code work. But I will be making an awful lot of casts if I do that, and I'm wondering if there is a better way.

推荐答案

我结束了覆盖setter以声明正在设置的对象是合适的类型，并创建一个新的只读getter，如下所示：

I ended overriding the setter to assert that the object being set is of the appropriate type, and creating a new read-only getter, like this:

@interface MutablePerson {
}

@property (readonly) NSMutableArray *mutableFriendArray;

@implementation MutablePerson

-(NSMutableArray *) mutableFriendArray {
  NSMutableArray *ret = (NSMutableArray *)[super friendArray];
  NSAssert ([ret isKindOfClass: [NSMutableArray class]], @"array should be mutable");
  return ret;
}

-(void) setFriendArray: (NSMutableArray *) array {
  NSAssert ([array isKindOfClass: [NSMutableArray class]], @"array should be mutable");
  [super setFriendArray: array];
}

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