NICE Language Committee: ms-pointer
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Key : MS-POINTER
Title : Type POINTER - conformance.
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Proposal (Michael Schweitzer, late March 1994)
Discussion:
As far as I can see, the conformance rules for type POINTER
are not clearly defined. I always thought that POINTER con-
forms only to itself and nothing but POINTER conforms to it.
But recently (comp.lang.eiffel - ISE bug report) Bertrand
said that POINTER should conform to ANY. If this is the
case then an assignment of the form
a := p -- `a' of type ANY, `p' of type POINTER
requires a type promotion, similar to the case
a := i -- `a' of type ANY, `i' of type INTEGER.
In the latter example `i' is first promoted to INTEGER_REF.
Proposals (two alternatives):
(1)
Type POINTER conforms only to itself and no type other than
POINTER conforms to it.
(2)
We introduce a class POINTER_REF. It has only one feature
`item' of type POINTER. POINTER is an expanded class which
inherits from POINTER_REF. POINTER conforms directly to
POINTER_REF (and therefore indirectly to ANY). In a reattachment
of the form
r := p
where `r' is a reference type and `p' a POINTER, `p' is first
converted into a POINTER_REF and a reference to that object
is attached to r.
Comment:
I don't have a clear preference for one of the two proposals.
The only thing that matters for me is that there is a clear
and definite rule. Neither of the two proposals requires significant
changes in our new implementation. Currently we've implemented
version (2).
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Agreement (Steve Tynor)
This is what TowerEiffel does. I think that it's the correct choice:
without a POINTER_REF-like conversion, POINTER's can't be reverse
assigned, and thus can't be read from a generalized persistent store.
Note that the Tower POINTER_REF has more than just `item':
class POINTER_REF
ancestors
POINTER_REF
creation features
make
feature specification
set_item,
make (p: POINTER)
-- Make a POINTER_REF from `p'.
item : POINTER
-- The value of this reference
end
The `set_item' and `make' features are for consistency with other *_REF
classes.
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Agreement (Bertrand Meyer)
This is a bit of overkill but I agree it is consistent with
the other basic types so I support the proposal, alternative (2).
I do not support Steve's `make', however, since it
does not exist in other classes.
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