为什么使用[ClassName alloc]而不是[[self class] alloc]？ [英] Why use [ClassName alloc] instead of [[self class] alloc]?

问题描述

我正在阅读Mark Dalrymple的在Mac上学习Objective-C （仅在协议一章中，所以还是相对较新），并试图找出一些答案：

I'm reading through Mark Dalrymple's Learn Objective-C on the Mac (only at the chapter on Protocols, so still relatively newbish) and trying to figure something out:

为什么要用自己的名字引用一个类？如果我有一个名为 Foo 的类，为什么我要写

Why would you ever reference a class by its own name? If I had a class called Foo, why would I ever want to write, say,

[[Foo alloc] init]

而不是

[[[self class] alloc] init]

如果我有Bar的子类，第一个选择不会使我无法写作

If I had a subclass Bar, wouldn't the first option invalidate me from writing

[[Bar alloc] init]

而第二种选择允许吗？第一种选择何时会更好？

whereas the second option would allow it? When would the first option be better?

推荐答案

通常，在类方法中，您确实使用 [ [self alloc] init] 。例如，为类编写便捷方法的规范方法是：

Generally, within a class method, you do use [[self alloc] init]. For example, the canonical way to write a convenience method for a class is:

+ (id)fooWithBar:(Bar *)aBar
{
    return [[[self alloc] initWithBar:aBar] autorelease];
}

（请注意，在类方法中， self 引用类对象。）

(Note that in a class method, self refers to the class object.)

但是，您将使用 [[Foo alloc] init] （即显式类名），如果您实际上想要 Foo 类的实例（而不是子类）。

However, you would use [[Foo alloc] init] (that is, an explicit class name) if you actually want an instance of the Foo class (and not a subclass).

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