xcode 使用[self class]有什么意义
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What's the Point of Using [self class]
提问by user4951
Is this code correct
这段代码正确吗
@implementation Vehicle
+(id) vehicleWithColor:(NSColor*)color {
id newInstance = [[[self class] alloc] init]; // PERFECT, the class is // dynamically identified
[newInstance setColor:color];
return [newInstance autorelease];
}
@end
Why use [self class]
为什么使用[自类]
I thought self already points to the class on static methods (the ones with +)
我认为 self 已经指向静态方法的类（带有 + 的那些）
回答by Sherm Pendley
It's to support subclassing. If you hard-coded the class name, as in [[Vehicle alloc] init], then a subclass of Vehicle would have to override +vehicleWithColor: to make it do the right thing. With [self class], you could create a subclass HarleyDavidson, and [HarleyDavidson vehicleWithColor:[NSColor blackColor]]would do the right thing automatically, creating an instance of HarleyDavidson instead of an instance of Vehicle.
它是为了支持子类化。如果您对类名进行了硬编码，如[[Vehicle alloc] init]，则 Vehicle 的子类必须覆盖 +vehicleWithColor: 以使其执行正确的操作。使用[self class]，您可以创建一个子类 HarleyDavidson，并且[HarleyDavidson vehicleWithColor:[NSColor blackColor]]会自动做正确的事情，创建 HarleyDavidson 的实例而不是 Vehicle 的实例。
(Edit:)
（编辑:)
See Joe's comment below concerning selfvs. [self class]in class methods - In class methods, it doesn't make a difference. But there isa situation where it can. Classes can respond to instance methods that are defined in a root class - -classitself is just such a method, defined as an instance method in the NSObject protocol. So if you extend a root class such as (for example) NSObject by adding an instance method, that method should always use [self class]if it needs to refer to its own Class object.
请参阅下面关于self与[self class]类方法中的Joe 的评论- 在类方法中，它没有区别。但是，是的情况下就可以了。类可以响应定义在根类中的实例方法——-class本身就是这样一种方法，在 NSObject 协议中定义为实例方法。因此，如果您通过添加实例方法扩展根类（例如）NSObject，则该方法应始终使用，[self class]如果它需要引用其自己的 Class 对象。
回答by Daniel Dickison
You're right: [self class]is unnecessary in a class method (it's more commonly called that in Objective-C rather than "static" method), because selfis already a class, and [self class]returns itself.
你是对的：[self class]在类方法中是不必要的（在 Objective-C 中更常称为它而不是“静态”方法），因为self它已经是一个类，并[self class]返回自身。
But it gets a bit more interesting. In Objective-C, class objects are technically instances of metaclasses. So [self class]in a class method ought to return the metaclass instead of the class itself. But for practical purposes, Objective-C hides the metaclass so it handles this case specially.
但它变得更有趣了。在 Objective-C 中，类对象在技术上是元类的实例。所以[self class]在类方法中应该返回元类而不是类本身。但出于实际目的，Objective-C 隐藏了元类，因此它专门处理这种情况。
Some good reading on this topic:
关于这个主题的一些很好的阅读：
