The format is the same. I'm going to ask for some input, and then I am going to use that procedure to check, is this the right type of value. And I'm trying to use that to do the coercion if you like. Same thing if it works, I'm going to skip that, if it not, it's going to throw the exception.

格式是一样的，我需要一些输入,然后我要用一个过程来检查,是否是正确的值,然后你就可以做强制转换了,如果能运行，那是一样的，我打算跳过这段，如果不正确，那就会抛出一个异常。

麻省理工公开课 - 计算机科学及编程导论课程节选

The NSTextAlignmentNatural value is unsupported and will throw an exception when used with the textAlignment property of UILabel or supplied as the alignment parameter to the drawInRect:withFont:lineBreakMode:alignment: method of NSString.

ENGADGET: Apple seeds iOS 6 beta 4 to developers: here's the changelog