Re: Copy vector in Ada

[Maciej Sobczak <see.my.homepage@gmail.com>]

W dniu sobota, 3 listopada 2012 17:03:33 UTC+1 użytkownik Hibou57 (Yannick Duchêne) napisał:

> Some other words: I hardly believe an assignment operator could ever be OK  
> within a context where the meaning of what an object is depends on whether  
> it is used by value or reference (the truncation case Peter made me  
> recall), and where there's nothing like proper class wide type, as C++  
> class wide behaviour also depends on whether an instance is used by value  
> vs by reference (always class wide with reference). Can you make an  
> assignment operator works fine and uniformly in such circumstances?

No and I don't even intend to do it. Mixing assignment with polymorphism is evil and cannot be implemented reasonably anyway. In the case of derivation hierarchy I would rather introduce the "clone" operation or something equivalent. Interestingly, in the hierarchy I would also ban the copying entirely, which means that pass by copy becomes impossible and the whole problem of uniformity just disappears.

Note that Ada does not provide any universal solution to this problem, either - assignment cannot be reasonably implemented apart from the case of matching tags, which introduces a run-time aspect to the whole; this is in contrast to banning assignment, which in terms of limitedness is always resolved statically.
It's a bit messy.

> The
> orthogonality C++ lacks here is a fact to be added to the particular point  
> you raised.

Not much. In the case of derivation hierarchy, neither Ada nor C++ offer anything useful (and that's OK as the concept is broken anyway). C++ does provide a cleaner solution for the case of non-dispatching assignment.

-- 
Maciej Sobczak * http://www.msobczak.com * http://www.inspirel.com