copy/move ctor

[tvaneerd]

It should probably be mentioned that the copy/move ctor should never be explicit.

And by 'never' does mean 'rarely' (as usual) or does it really mean never?

A heavy class, ie an Image class should probably not have a copy-ctor.
Typically a copy() or clone() function makes it more clear than there is no free lunch.
Would an explicit copy-ctor actually make sense here?