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From: stt@inmet
Newsgroups: comp.lang.ada
Subject: Re: limited private types
Message-ID: <124000023@inmet>
Date: 6 Dec 88 20:09:00 GMT
References: <8197@nsc.nsc.com>
Nf-ID: #R:nsc.nsc.com:-819700:inmet:124000023:000:1813
Nf-From: inmet!stt    Dec  6 15:09:00 1988
List-Id: <comp.lang.ada>


All Ada compilers already deal with "stack" objects
of size unknown at compile time (e.g. x : string(1..f(y));).
Some compilers do this more efficiently than others,
but the big advantage is that the compiler takes care
of freeing the associated storage when the object goes away.
When access types are used, it is always a challenge to
figure out how to ensure that the storage gets efficiently
reclaimed.  (This is one of the big reasons why
I believe user-defined "finalization" should be added into the language.)

Nevertheless, access types with a deferred incomplete designated
type are still used in complex systems, because they break
recompilation dependencies.  It seems like allowing the
deferral of any private type to a package body would be little or no
additional burden on compiler writers, and would provide
the same nice break in recompilation dependency when used.  
For non-discriminated types,
the size would be an elaboration-time constant.  For discriminated types,
the size would be determined by a function call (many
compilers already create size functions to save code space
for complex variant record size calculations).
However, it would be appropriate to require that the full
definition of the private type be a record type, so that
pass-by-reference semantics would be allowed.  

The syntax for such a deferred private record type might be:
     . . .
  private
      type Priv is record;
     . . .

I fully agree with the importance of pass-by-value semantics
for all scalars/access-values.  It is easy enough to wrap a scalar
into a record if pass-by-reference semantics are preferred for some
particular application.  The improved speed possibilities for pass-by-value
scalars far outweigh any abstraction disadvantage.

S. Tucker Taft
Intermetrics, Inc.
Cambridge, MA  02138