Re: Operator visibility/inheritance for derived private type? (Repost)

[Tucker Taft <stt@averstar.com>]

Rod Chapman wrote:
> 
> Confused of Bath writes:
> 
>   Imagine I have a package exporting a private type with an
> explicitly redeclared "=" operator - Ada.Task_Identification.Task_ID
> for instance.  I want to derive another private type from
> this, exporting only a subset of the operations of the original,
> so I write
> 
> with Ada.Task_Identification;
> package Rod.TI is
>    type Task_ID is private;
>    function Current_Task return Boolean;
> private
>    type Task_ID is new Ada.Task_Identification.Task_ID;
> end Rod.TI;
> 
> If a user of this package does
> 
>    A, B : Rod.TI.Task_ID;
>  begin
>    if A = B then ...
> 
> which "=" is called?  Do I get some simple comparison of
> A and B or should I get a call of
> Ada.Task_Identification."="?

Semantically, you end up with a "call" on Ada.Task_Identification."=", but 
that might of course be a built-in "simple comparison" operation.
It certainly doesn't need to be an out-of-line function call.

> GNAT (3.12a3 on NT) seems to be generating the former,
> but I'm not sure if the language requires the latter,
> and GNAT is simply doing an optimisation.  If
> I _definintely_ want the call to Ada.Task_Identification."=",
> so I need to explicitly redeclare my own "=" for
> Rod.TI.Task_ID?  Which language rule(s)
> am I missing to explain what going on?

How do you know that GNAT is generating a "simple comparison" but
not a "call" on Task_Identification."=", since they very well might
be one and the same thing?

You could check GNAT by writing your own package, and making your
"=" a user-defined function.  In that case, GNAT should generate
an out-of-line call on your function.  However, since Ada.Task_Identification
is provided by the GNAT implementators, it seems quite likely that
the "=" on task IDs uses the built-in equality operation, presuming
it does the "right thing."   I wouldn't really call this an "optimization,"
but rather just the expected way of doing things.

> Cheers,
>  Rod Chapman
>  Praxis Critical Systems

-- 
-Tucker Taft   stt@averstar.com   http://www.averstar.com/~stt/
Technical Director, Distributed IT Solutions  (www.averstar.com/tools)
AverStar (formerly Intermetrics, Inc.)   Burlington, MA  USA