Russell Yanofsky wrote:
> The proposed "reference collapsing" solution to CWG Defect 106 [1]
> treats cv-qualifiers on reference types differently than other parts of
> the language. Consider the following function declaration:
> typedef int &int_ref;
> void foo(int_ref const); // equivalent to void foo(int &);
> The current C++ standard in 8.3.2p1 requires that the const qualifier
> on the int_ref type be ignored. Now consider a similar declaration
> which would be allowed under DR106:
> typedef int &int_ref;
> void foo(int_ref const &); // equivalent to void foo(int const &);
> In this case, DR106 requires that the const qualifier be applied. So,
> in one case, the const qualifier is applied, in the other it isn't,
> even though the two declarations are identical except for a redundant &
> operator.
This issue is now under active discussion on the Standard
Committee's core working group email reflector, and it will be
discussed at next week's Committee meeting.
Personally, I don't see an inconsistency here. The difference
between your examples is the difference between "...& const" and
"...const &". Adding a top-level reference to a parameter type
prevents other kinds of type adjustments (e.g., dropping a
top-level const from a parameter type when creating the function
type, array-to-pointer and function-to-pointer decay), so it
seems reasonable (to me, at least) that adding a top-level
reference to a "const int_ref" would also prevent the discarding
of the const qualifier.
Furthermore, it seems reasonable (to me) that a person who
declares a "const T& t" should be able to rely on const semantics
for "t", that you should be able to distinguish overloaded
functions with "T&" and "const T&" parameters, etc., without
having to worry about whether T is a reference type or not.
There are different views represented on the Committee, so I'm
sure we'll have an interesting discussion next week. It's
already been interesting on the core reflector.
> I don't see why it needs to be this way. It'd be better if DR-106 were
> either scaled back so cv-qualifiers on references were consistently
> ignored throughout the language, or broadened so they could be
> consistently applied.
Again, from my perspective, cv-qualifiers on references _are_
consistently ignored throughout the language; according to the
issue 106 resolution, "const int_ref&" is a reference to a
cv-qualified type, not a cv-qualified reference.
We will be discussing the other alternative you mentioned,
namely, not ignoring cv-qualification on top-level reference
types. We have something of a parallel situation already in the
language with the way cv-qualified array types qualify the
element type as well, so something like that might be reasonable
for reference types, too. However, that kind of change clearly
would break some code, although it's not clear how much, and no
one has done much analysis yet on how such a change might be
defined nor on its impact.
> I wonder if the author/authors of this
> proposal were aware of 8.3.2p1 and how it conflicts (conceptually) with
> the behavior they specify for reference collapsing.
Aware of 8.3.2p1, yes; viewing it as a conflict, no. 8.3.2p1
deals with cv-qualification of top-level references, which is not
what happens in the issue 106 resolution.
--
William M. (Mike) Miller | Edison Design Group, Inc.
w...@world.std.com
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