NSError

虽然标题是NSError，但是我们不打算讲NSError怎么用的，因为它是一个很纯粹的模型类，无需赘述其用法。

问题的关键是，我们要考虑一些场合：

参考http://amattn.com/2011/12/

NSErrorThe ubiquitous NSError is a little bit tricky. Typical Cocoa convention is that they are implemented via out-parameters (aka indirect pointers).In ARC, out-parameters are by default __autoreleasing and should be implemented like this:- (BOOL)performWithError:(__autoreleasing NSError **)error{    // ... some error occurs ...    if (error)    {        // write to the out-parameter, ARC will autorelease it        *error = [[NSError alloc] initWithDomain:@""                                             code:-1                                         userInfo:nil];        return NO;    }    else    {        return YES;    }}When using an out-parameter, you will usually use __autoreleasing on your *error object like so:NSError __autoreleasing *error = error;BOOL OK = [myObject performOperationWithError:&error];if (!OK){    // handle the error.}If you forget the __autoreleasing, the compiler will simply insert a temporary, intermediate autoreleasing object for you. This was a compromise made under the oppressive regime of backwards compatibility. I have seen some compiler configurations which do not automatically make them __autoreleasing. It is safer to include the __autoreleasing for all new code.

一般NSError用作out参数，在方法内部产生。在ARC中，它的释放时机应该是自动释放的。在外部，为确保类型一致，应该使用一个自动释放的指针来接收。

__autoreleasing NSString *retStr和 NSString * __autoreleasing retStr都是正确的写法。现在变成二维指针时，需要把*retStr看成一个整体，代表reStr的上一级指针，所以就变成了NSString * __autoreleasing *retStr。

http://stackoverflow.com/questions/11678528/ios-autoreleasing-const-error 中提到的问题和block有关系，用于接收的指针在block之外声明，在内部是const的，需要加上__block提供修改的机会。