为什么在AFNetworking中使用点对点类型（NSProgress * __autoreleasing *）而不是仅使用点类型（NSProgress * __autoreleasing）？

Question

In AFNetworking I find this function:

- (NSURLSessionUploadTask *)uploadTaskWithRequest:(NSURLRequest *)request
                                         fromFile:(NSURL *)fileURL
                                         progress:(NSProgress * __autoreleasing *)progress
                                completionHandler:(void (^)(NSURLResponse *response, id responseObject, NSError *error))completionHandler;

Here the progress type is NSProgress * __autoreleasing *.

I do not why here a point to point type is used rather than just a point type. The usage of progress parameter is as follows in this function:

if (progress) {
    *progress = delegate.uploadProgress;
}

In my mind, if declare:

NSProgress *progress = nil;

passing:

progress:(NSProgress * __autoreleasing *)progress

and use it as:

*progress = delegate.uploadProgress;

is just the same as passing

progress:(__autoreleasing NSProgress *)progress

and use it as:

progress  = delegate.uploadProgress;

Could any one help explain why a point to point type is used here?

Answer 1

The purpose of that parameter is to let the method pass back a pointer to an NSProgress object. To do that, the method needs to assign into the caller's variable.

Functions receive a copy of the passed value. If the parameter were just __autoreleasing NSProgress* , then the function would receive a copy of the passed pointer. Both the caller and the method would have variables containing a pointer to an NSProgress object, but they would be separate variables. When the method assigned to its variable using progress = delegate.uploadProgress; it would only change its copy. The assignment would not affect the caller's variable.

When the parameter is NSProgress * __autoreleasing * and the caller passes &callersProgress , the function receives a copy of a pointer to the caller's variable. When the method uses *progress (as in *progress = delegate.uploadProgress; ), it dereferences that pointer. That yields a reference to the caller's variable. So, the method is assigning to the caller's variable, not just a local variable.