* A question for compiler vendors/lang experts. redefinition of "="
@ 1997-12-16 0:00 Barry and Jackie Schiff
1997-12-16 0:00 ` Tucker Taft
1997-12-16 0:00 ` David Weller
0 siblings, 2 replies; 3+ messages in thread
From: Barry and Jackie Schiff @ 1997-12-16 0:00 UTC (permalink / raw)
Got one answer from a large compiler vendor when another vendor's
implementation implies otherwise. Looked in the Rationale and LRM and it
was hard to find a clear answer. So here is the question for all those
language experts out there:
package Foo is
type objects is abstract tagged record
Some_value : some_type;
end record;
type class_wide_pointers is access all objects'class;
function "="(left, right : access objects) return boolean; --
Legal??
end Foo;
The question: Is the redefinition of "=" shown above legal and if so
what are the semantics. Specifically, is "=" a dispatching primitive
operation or is it ambiguous with the predefined equality defined for
the class_wide_pointers type.
--------------
In my reading the LRM it seems to me that the spirit of access
parameters is that the above operation is a dispatching operation. I
have had a large Ada compiler vendor tell me that the above is
ambiguous. While another compiler implements that above as a dispatching
operation. Can anyone point me to the definitive answer in the LRM.
Thanks in advance. If it is not to much trouble could you email answer.
--Barry Schiff
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: A question for compiler vendors/lang experts. redefinition of "="
1997-12-16 0:00 A question for compiler vendors/lang experts. redefinition of "=" Barry and Jackie Schiff
@ 1997-12-16 0:00 ` Tucker Taft
1997-12-16 0:00 ` David Weller
1 sibling, 0 replies; 3+ messages in thread
From: Tucker Taft @ 1997-12-16 0:00 UTC (permalink / raw)
Barry and Jackie Schiff (schiff@netspace.net.au) wrote:
: Got one answer from a large compiler vendor when another vendor's
: implementation implies otherwise. Looked in the Rationale and LRM and it
: was hard to find a clear answer. So here is the question for all those
: language experts out there:
: package Foo is
: type objects is abstract tagged record
: Some_value : some_type;
: end record;
: type class_wide_pointers is access all objects'class;
: function "="(left, right : access objects) return boolean; --
: Legal??
: end Foo;
: The question: Is the redefinition of "=" shown above legal and if so
: what are the semantics. Specifically, is "=" a dispatching primitive
: operation or is it ambiguous with the predefined equality defined for
: the class_wide_pointers type.
It is a dispatching operation of type Objects. It does not
override the predefined operator for type class_wide_pointers, whose
profile is:
function "="(left, right : class_wide_pointers) return boolean;
However, essentially all cases where you use the function will be
ambiguous. For example, if you "use" package Foo, then:
x, y : class_wide_pointers;
begin
if x = y then ...
is ambiguous, as is:
procedure blah(a, b : objects) is
begin
if a'access = b'access then ...
It is not easy to construct a case where a use of your operator won't be
ambiguous with a use of the predefined "=" operator for a named access type.
: --------------
: In my reading the LRM it seems to me that the spirit of access
: parameters is that the above operation is a dispatching operation. I
: have had a large Ada compiler vendor tell me that the above is
: ambiguous.
These are not incompatible statements. It is a dispatching operation,
and almost all uses of it will be ambiguous.
: ... While another compiler implements that above as a dispatching
: operation. Can anyone point me to the definitive answer in the LRM.
You are both right.
: Thanks in advance. If it is not to much trouble could you email answer.
Will do.
: --Barry Schiff
--
-Tucker Taft stt@inmet.com http://www.inmet.com/~stt/
Intermetrics, Inc. Burlington, MA USA
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: A question for compiler vendors/lang experts. redefinition of "="
1997-12-16 0:00 A question for compiler vendors/lang experts. redefinition of "=" Barry and Jackie Schiff
1997-12-16 0:00 ` Tucker Taft
@ 1997-12-16 0:00 ` David Weller
1 sibling, 0 replies; 3+ messages in thread
From: David Weller @ 1997-12-16 0:00 UTC (permalink / raw)
In article <34964C63.F97005E6@netspace.net.au>,
Barry and Jackie Schiff <schiff@netspace.net.au> wrote:
>Got one answer from a large compiler vendor when another vendor's
>implementation implies otherwise. Looked in the Rationale and LRM and it
>was hard to find a clear answer. So here is the question for all those
>language experts out there:
>
>
>package Foo is
>
> type objects is abstract tagged record
> Some_value : some_type;
> end record;
>
> type class_wide_pointers is access all objects'class;
>
> function "="(left, right : access objects) return boolean; --
>Legal??
>
Yes, it is dispatching, but why not just declare:
function "="(L, R : access Objects'Class) return boolean; ??
Of course, perhaps I've missed a subtlety...it wouldn't be the first
time :-/
>parameters is that the above operation is a dispatching operation. I
>have had a large Ada compiler vendor tell me that the above is
>ambiguous.
Perhaps they've gotten too large? :-) Don't want to fix their
compiler? :-)
>Thanks in advance. If it is not to much trouble could you email answer.
>
Oh, great, complicate things why don't ya?
:-)
--
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