@throw([NSException exceptionWith…])

Xcode将@throw语句识别为函数出口点，就像return语句一样。使用@throw语法可以避免错误的“Control may reach end of non-void function”警告，你可能会从[NSException raise:…]得到这种警告。

同样，@throw也可以用来抛出不属于NSException类的对象。

这里有一个警告。在Objective-C中，与许多类似的语言不同，您通常应该尽量避免在正常操作中可能发生的常见错误情况下使用异常。

Apple的Obj-C 2.0文档中写道:“重要的是:异常在Objective-C中是资源密集型的。您不应将异常用于一般的流控制，或仅用于表示错误(例如文件不可访问)”

Apple's conceptual Exception handling documentation explains the same, but with more words: "Important: You should reserve the use of exceptions for programming or unexpected runtime errors such as out-of-bounds collection access, attempts to mutate immutable objects, sending an invalid message, and losing the connection to the window server. You usually take care of these sorts of errors with exceptions when an application is being created rather than at runtime. [.....] Instead of exceptions, error objects (NSError) and the Cocoa error-delivery mechanism are the recommended way to communicate expected errors in Cocoa applications."

这样做的部分原因是坚持Objective-C中的编程习惯(在简单的情况下使用返回值，在更复杂的情况下使用按引用参数(通常是NSError类))，部分原因是抛出和捕获异常的代价要高得多，最后(可能是最重要的)Objective-C异常是C的setjmp()和longjmp()函数的一个瘦包装，本质上混乱了你仔细的内存处理，参见下面的解释。

自从ObjC 2.0以来，Objective-C异常不再是C的setjmp() longjmp()的包装器，并且与c++异常兼容，@try是“免费的”，但抛出和捕获异常的代价要高得多。

无论如何，断言(使用NSAssert和NSCAssert宏家族)抛出NSException，这是明智的使用它们作为Ries状态。

可以使用两种方法在try catch块中引发异常

@throw[NSException exceptionWithName];

或者第二种方法

NSException e;
[e raise];

我使用[NSException raise:format:]如下所示:

[NSException raise:@"Invalid foo value" format:@"foo of %d is invalid", foo];

Regarding [NSException raise:format:]. For those coming from a Java background, you will recall that Java distinguishes between Exception and RuntimeException. Exception is a checked exception, and RuntimeException is unchecked. In particular, Java suggests using checked exceptions for "normal error conditions" and unchecked exceptions for "runtime errors caused by a programmer error." It seems that Objective-C exceptions should be used in the same places you would use an unchecked exception, and error code return values or NSError values are preferred in places where you would use a checked exception.