"Keith" <nono@joimail.net> a �crit dans le message news:
aoo3h9$s7u$1@news.chatlink.com...
> What is the implementation consequence of not following  Ada reference
> manual section 4.6 12/1 "The component subtypes shall statically match"?
>
> What was the rational behind not allowing something like the following:
>
> type some_type is new integer;
> type A is array ( index_type) of integer;
> type B is array ( index_type) of some_type;
> Array_A:A;
> Array_B:B;
>
>
> declare
>
> begin
>     Array_A:=A( Array_B);
>
> end;
>
> Rather than the legal but more cumbersome
>
>    for Index in index_type'range loop
>       Array_A(Index):=A(Array_B(Index));
>    end loop;
>
A derived type needs not have the same representation as its ancestor type.
If the subtypes statically match, the compiler can perform a simple memory
copy. Otherwise, the compiler must in effect generate the exact equivalent
of the loop you give, and there is a deliberate decision in Ada to avoid, as
far as possible, *hidden* inefficiencies.

--
---------------------------------------------------------
           J-P. Rosen (rosen@adalog.fr)
Visit Adalog's web site at http://www.adalog.fr