在子类的子类中实现NSCopying [英] Implementing NSCopying in Subclass of Subclass

问题描述

我有一个很小的类层次结构，在实现copyWithZone:时遇到了麻烦.我已经阅读了NSCopying文档，但找不到正确的答案.

I have a small class hierarchy that I'm having trouble implementing copyWithZone: for. I've read the NSCopying documentation, and I can't find the correct answer.

参加两个课程:形状和正方形.正方形定义为:

Take two classes: Shape and Square. Square is defined as:

@interface Square : Shape

没有惊喜.每个类具有 one 属性，Shape具有"sides"整数，Square具有"width"整数. copyWithZone:方法如下所示:

No surprise there. Each class has one property, Shape has a "sides" int, and Square has a "width" int. The copyWithZone: methods are seen below:

形状

- (id)copyWithZone:(NSZone *)zone {
    Shape *s = [[Shape alloc] init];
    s.sides = self.sides;
    return s;
}

平方

- (id)copyWithZone:(NSZone *)zone {
    Square *s = (Square *)[super copyWithZone:zone];
    s.width = self.width;
    return s;
}

看看文档，这似乎是做事的正确"方法.

Looking at the documentation, this seems to be the "right" way to do things.

不是.

如果您尝试设置/访问由copyWithZone:方法返回的Square的width属性，则它将失败，并显示类似以下错误:

If you were to try to set/access the width property of a Square returned by the copyWithZone: method, it would fail with an error similar to the one below:

2010-12-17 11:55:35.441 Hierarchy[22617:a0f] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[Shape setWidth:]: unrecognized selector sent to instance 0x10010c970'

在Square方法中调用[super copyWithZone:zone];实际上会返回一个Shape.您甚至可以在该方法中设置width属性，这是一个奇迹.

Calling [super copyWithZone:zone]; in the Square method actually returns a Shape. It's a miracle you're even allowed to set the width property in that method.

话虽这么说，如何以一种对子类实施NSCopying 的方式，使不让它们负责复制其超类的变量?

That having been said, how does one implement NSCopying for subclasses in a way that does not make them responsible for copying the variables of its superclass?

推荐答案

在问完之后您马上意识到的事情之一.

One of those things you realize right after asking...

超类( Shape )中copyWithZone:的实现不应假定它是Shape.因此，不是我上面提到的错误方法:

The implementation of copyWithZone: in the superclass (Shape) shouldn't be assuming it's a Shape. So instead of the wrong way, as I mentioned above:

- (id)copyWithZone:(NSZone *)zone {
    Shape *s = [[Shape allocWithZone:zone] init];
    s.sides = self.sides;
    return s;
}

您应该改用:

- (id)copyWithZone:(NSZone *)zone {
    Shape *s = [[[self class] allocWithZone:zone] init]; // <-- NOTE CHANGE
    s.sides = self.sides;
    return s;
}

这篇关于在子类的子类中实现NSCopying的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！