Re: point by point advantages of Ada

[Mark Biggar <mark.a.biggar@attbi.com>]

Randy Brukardt wrote:
> Warren W. Gay VE3WWG wrote in message <3E1B4C4C.7050205@cogeco.ca>...
> 
>>Randy Brukardt wrote:
> 
> 
>>I thought that this might work:
>>
>>   declare
>>      V : T'Class := ...
>>   begin
>>      ...
>>      V := Some_Other_Func;  - Dispatching
>>   end;
>>
>>But GNAT (at least) reports that this assignment is "ambiguous"
>>and will not compile it. I would have expected that it would
>>resolve it at runtime, knowing the current value of V and its
>>current tag. Is this consistent with the LRM (IANAL)?
> 
> 
> Unfortunately, the LRM has no position on this. This case is not covered
> at all by the standard. The ARG has been working (slowly) on an AI to
> fix this, and it finally was finished in October and approved at the
> recent ARG and WG9 meetings. It's AI-196.
>     http://www.ada-auth.org/cgi-bin/cvsweb.cgi/AIs/AI-00196.TXT
> 
> The solution is essentially to add a paragraph saying that your example
> works as you expect. Now, of course, you'll have to wait for Ada
> compilers to conform to this ruling. (Which will be helped when an ACATS
> test gets added for it.)

This isn't the interesting case anyway.  The interesting case
is where the function is used as a parameter to another subroutine
which has additional dispatching operands.  For example:

Type Set is tagged ...;

function Union(L, R: Set) return Set;
function Empty return Set;

Type Ordered_Set is new Set with ... ;

function Union(L, R: Ordered_Set) return Ordered_Set;
function Empty return Ordered_Set;

S: Ordered_Set'class := ...;

S := Union(S, Empty);

Now, which version of Empty is called is determined by run-time
dispatch depending of whether S is a Set or an Ordered_Set.

-- 
Mark Biggar
mark.a.biggar@attbi.com