[NSNull null]和nil有什么区别？ [英] What's the difference between [NSNull null] and nil?

问题描述

这里是一个我看到的上下文：

Here's a context where I have seen that:

NSMutableArray *controllers = [[NSMutableArray alloc] init];
for (unsigned i = 0; i < kNumberOfPages; i++) {
    [controllers addObject:[NSNull null]];
}

为什么不在那个地方？

推荐答案

直接从 Apple ：NSNull类定义了一个单例对象，用于在nil被禁止作为值（通常在诸如数组或字典的集合对象）的情况下用于表示空值。

Directly from Apple: "The NSNull class defines a singleton object you use to represent null values in situations where nil is prohibited as a value (typically in a collection object such as an array or a dictionary)."

所以在你的例子中，这正是发生的事情，程序员正在选择将一个空对象放入控制器数组，其中nil不允许作为值。

So in your example, that's exactly what's happening, the programmer is choosing to put a null object into the controllers array, where nil is not allowed as a value.

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