NSError的正确用法

You Don’t Own Objects Returned by Reference

Some methods in Cocoa specify that an object is returned by reference (that is, they take an argument of type ClassName ** or id *). A common pattern is to use an NSError object that contains information about an error if one occurs, as illustrated by initWithContentsOfURL:options:error: (NSData) and initWithContentsOfFile:encoding:error: (NSString).

In these cases, the same rules apply as have already been described. When you invoke any of these methods, you do not create the NSError object, so you do not own it. There is therefore no need to release it, as illustrated in this example:

NSString *fileName = <#Get a file name#>;
NSError *error = nil;
NSString *string = [[NSString alloc] initWithContentsOfFile:fileName
                        encoding:NSUTF8StringEncoding error:&error];
if (string == nil) 
{
    // Deal with error...
}
// ...
[string release];

之所以要用二重指针作为参数,是因为需要为NSError分配内存,有也就是说error本身存取的内容会改变,而参数传递属于拷贝传值,必须将指针的地址传过去,通过指针的地址访问指针,才改变的是指针的直.