大多数语言使用 true/false 关键字作为布尔值.我发现即使 Smalltalk 也在使用 true/false.我知道 Objective-C 只是借用 Smalltalk 的概念，而不是语言本身，但我很好奇它为什么使用 YES/NO 而不是更广泛使用的 true/false.有什么历史原因吗?
Most languages use the true/false keywords for boolean values. I found that even Smalltalk is using true/false. I know Objective-C is just borrowing concepts from Smalltalk, not the language itself, but I'm curious why it's using YES/NO instead of the more widely-used true/false. Is there any historical reason?
Objective-C 被设计为(并且仍然是)C 的严格超集.创建者非常努力地确保他们不会破坏与 C 的兼容性以任何方式.他们还试图让他们的修改更加明显，以便很容易分辨出代码的哪些部分使用了 Objective-C，哪些部分使用了普通 C.例如，用于表示 NSStrings 的 @而不仅仅是使用引号.这允许纯 C 字符串与新字符串共存.
Objective-C was designed to be (and still is) a strict superset of C. The creators worked very hard to ensure that they did not break compatibility with C in any way. They also tried to make their modifications somewhat obvious so that it would be easy to tell which parts of the code use Objective-C and which parts use plain C. Case in point, the @ used to denote NSStrings rather than just using quotes. This allows plain C strings to coexist with the new ones.
C 已经有一个非正式的 TRUE/FALSE 宏系统.我怀疑 Objective-C 的设计者选择 YES/NO 宏是为了避免冲突，并明确代码实际上是 Objective-C.还要注意空"对象的 nil 用法，而不是仅仅修改旧 NULL 的行为.
C already had an informal system of TRUE/FALSE macros. I suspect the designers of Objective-C chose the YES/NO macros to avoid conflict and to make it obvious that the code is actually Objective-C. Notice also the usage nil for the 'empty' object rather than just modifying the behavior of good old NULL.
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