Why does NSError need double indirection? (pointer to a pointer)

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Quite simply:

if you pass a pointer to an object to your function, the function can only modify what the pointer is pointing to.

if you pass a pointer to a pointer to an object then the function can modify the pointer to point to another object.

In the case of NSError, the function might want to create a new NSError object and pass you back a pointer to that NSError object. Thus, you need double indirection so that the pointer can be modified.

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