Objective-C中的typeof，__ typeof和__typeof__之间的区别
在Objective-C中，我经常在处理块等时使用__typeof__(obj).为什么不使用__typeof(obj)或typeof(obj).
In Objective-C I often use __typeof__(obj) when dealing with blocks etc. Why not __typeof(obj) or typeof(obj).
何时使用哪个?
__typeof__()和__typeof()是C语言特定于编译器的扩展，因为标准C不包含此类运算符.标准C要求编译器在语言扩展之前加上双下划线(这也是为什么您永远不要为自己的函数，变量等使用前缀的原因)
__typeof__() and __typeof() are compiler-specific extensions to the C language, because standard C does not include such an operator. Standard C requires compilers to prefix language extensions with a double-underscore (which is also why you should never do so for your own functions, variables, etc.)
typeof()完全相同，但是在理解每个现代编译器都支持的情况下，将下划线放在了窗口之外. (实际上，既然考虑到这一点，Visual C ++可能不会.尽管它确实支持decltype()，但通常提供与typeof()相同的行为.)
typeof() is exactly the same, but throws the underscores out the window with the understanding that every modern compiler supports it. (Actually, now that I think about it, Visual C++ might not. It does support decltype() though, which generally provides the same behaviour as typeof().)
这三者均表示同一件事，但都不是标准C语言，因此合格的编译器可能会选择使任何意思有所不同.
All three mean the same thing, but none are standard C so a conforming compiler may choose to make any mean something different.