什么是 NSConcreteData，它在哪里定义？
以下代码：
[[NSData alloc]initWithContentsOfURL:[NSURL URLWithString:mapURL]]
返回 NSConcreteData 的实例，而不是 NSData （这是我所期望的！）。 NSConcreteData 到底代表什么？为什么返回它而不是 NSData 的实例？ 此外，这个类是在哪里定义的？ XCode 抱怨它尚未定义，但我不知道应该包含什么才能获取该类。
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The following code:
[[NSData alloc]initWithContentsOfURL:[NSURL URLWithString:mapURL]]
returns an instance of NSConcreteData as opposed to NSData (which I expected!). What does NSConcreteData represent, exactly, and why is it being returned instead of an instance of NSData? Further, where is this class defined? XCode is complaining that it hasn't been defined, but I don't know what I should include in order to get the class.
Any help appreciated!
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Foundation框架在某些领域使用类簇来为各种类提供公共接口。 这意味着，当您使用
NSDataAPI 创建NSData对象时，实例化并返回的实际类与NSData不同>，但可以而且应该被代码视为NSData对象并引用。归根结底，简单的答案是：忽略
NSConcreteData的存在并将其视为NSData。有关类簇的更多信息，请参阅 Apple Cocoa 基础指南。
The Foundation framework uses class clusters in certain areas to provide a common interface to various classes. What this means is that, when you use an
NSDataAPI to create anNSDataobject, the actual class that is instantiated and returned is different fromNSData, but can and should be treated and referred to as anNSDataobject by your code.At the end of the day, the simple answer is: ignore the existence of
NSConcreteDataand treat it likeNSData.For more information on class clusters, please see the Class Clusters section of Apple's Cocoa Fundamentals Guide.