Re: Generic default parameters

[Stephen Leake <stephen.a.leake.1@gsfc.nasa.gov>]

Thomas Wolf <t_wolf@angelfire.com> writes:

> 5. Linked to (3) above: some way to specify a storage pool that
>    is equal to whatever pool the compiler would use if no
>    "for Some_Access'Storage_Pool use ..." clause was present, i.e.
>    a generic way to refer to the standard storage pool of a type
>    without referring to the type. Something like
> 
>    generic
>       type Something is private;
>       Pool : in out System.Storage_Pools.Root_Storage_Pool'Class := <>;
>    package X is
>       type Some_Access is access all Something;
>       for Some_Access'Storage_Pool use Pool;
>       ...
> 
>    and if an instantiation provides an actual for 'Pool', that will
>    be taken as the storage pool of type 'Some_Access', but if an
>    instantiation doesn't provide an actual, 'Some_Access' will use
>    a standard storage pool.
> 
>    Not sure if point (5) makes sense, especially since it would be
>    useful only for storage pools, but make no sense at all for other
>    types...

I think a cleaner solution to 5 is a standard name for the standard
storage pool, combined with 3 (default object for "in out"). So we'd
have:

  generic
     type Something is private;
     Pool : in out System.Storage_Pools.Root_Storage_Pool'Class := 
         System.Storage_Pools.Default_Storage_Pool;
  package X is
     type Some_Access is access all Something;
     for Some_Access'Storage_Pool use Pool;

There may have to be more than one default storage pool; that may be
why there is no standard name now.
  
> Comments, anyone? Would these things be worth to consider for inclusion
> in the next Ada revision? 

I agree they would all be useful, but I have personally only seen the
need for 5 and 3.

-- 
-- Stephe