Why do common collection classes in Objective C like NSString, NSArray, NSDictionary etc have a mutable as well as an immutable version. What is the logic behind defining them separately? Performance, memory management or anything else? Why do common collection classes in Objective C
阅读背景：
在Objective C中有一个类似NSArray，NSDictionary等类的可变和不可变版本的逻辑是什么？
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