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llewelly
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 Posted: Sun Aug 10, 2003 1:45 am Post subject: Re: reference compatibility |
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[cc'd to comp.std.c++ , and followups set.]
[email]kanze (AT) gabi-soft (DOT) fr[/email] writes:
| Quote: | Andrey Tarasevich <andreytarasevich (AT) hotmail (DOT) com> wrote in message
news:<3F2FD305.7090200 (AT) hotmail (DOT) com>...
Andrey Tarasevich wrote:
...
On the Polish-language pl.comp.lang.c we are having an
interesting discussion on reference compatibility. As I'm the
only one who still seems unconvinced, I thought I'd try this
broader forum to settle it.
The issue seems to be:
given
int *p2i;
int const * const &cref_2_p2i = p2i;
is the temporary going to be created ?
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[snip]
| Quote: | So, in practice you might see that '&p2i' and '&cref_2_p2i' point to
the same location (that's what I observe with GCC 3.2).
If that is true, the compiler is broken. The objects designated by the
symbols p2i and cref_2_p2i have different types, unrelated by
inheritance. Unless the objects are in a union, or one is a base class
of the other, or one is a member of a class, and the other is a base
class of the same class, they must have different addresses.
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Is that really the intent of the standard? 8.5.3/4 does seem to say
that, but IMO that's inconsistent with 4.4/4, and not desireable.
http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/cwg_active.html#233
is the issue for it. (Status: drafting.) It talks mostly about
overloading, but I feel the problem is the same.
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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James Kanze
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| Posted: Tue Aug 12, 2003 6:40 pm Post subject: Re: reference compatibility |
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llewelly <llewelly.at (AT) xmission (DOT) dot.com> writes:
| Quote: | kanze (AT) gabi-soft (DOT) fr writes:
Andrey Tarasevich <andreytarasevich (AT) hotmail (DOT) com> wrote in message
news:<3F2FD305.7090200 (AT) hotmail (DOT) com>...
Andrey Tarasevich wrote:
...
On the Polish-language pl.comp.lang.c we are having an
interesting discussion on reference compatibility. As I'm the
only one who still seems unconvinced, I thought I'd try this
broader forum to settle it.
The issue seems to be:
given
int *p2i;
int const * const &cref_2_p2i = p2i;
is the temporary going to be created ?
[snip]
So, in practice you might see that '&p2i' and '&cref_2_p2i'
point to the same location (that's what I observe with GCC
3.2).
If that is true, the compiler is broken. The objects designated
by the symbols p2i and cref_2_p2i have different types,
unrelated by inheritance. Unless the objects are in a union, or
one is a base class of the other, or one is a member of a class,
and the other is a base class of the same class, they must have
different addresses.
Is that really the intent of the standard? 8.5.3/4 does seem to
say that, but IMO that's inconsistent with 4.4/4, and not
desireable.
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Where is the inconsistence with 4.4/4? You have, in every case, a
conversion. The results of a conversion are an rvalue. When you bind
the address of an rvalue to a reference to const, you are *always*
binding a temporary -- one can almost consider the fact of obtaining
an rvalue the equivalent of using a temporary.
Not really.
The fact is, a reference is not a pointer. Particularly not at
initialization. In the pointer example in this DR, there is a user
defined conversion resulting in an rvalue, which is then used to
initialize a pointer. In the reference example, there is a user
defined conversion resulting in an lvalue, and there is nothing
afterwards which would require an lvalue to rvalue conversion, so the
lvalue-ness is, and must be, respected. In the question we are
concerned with, there is a built-in conversion to an *rvalue*, a new
object. The results of the conversion is not an int const*& lvalue,
but an int const* rvalue.
--
James Kanze mailto:kanze (AT) gabi-soft (DOT) fr
Conseils en informatique orientée objet/
Beratung in objektorientierter Datenverarbeitung
11 rue de Rambouillet, 78460 Chevreuse, France +33 1 41 89 80 93
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