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Message from discussion status of rvalue reference proposal (n1377, n1770, et al)
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Howard Hinnant  
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 More options Jul 18 2005, 12:54 pm
Newsgroups: comp.std.c++
From: Howard Hinnant <hinn...@metrowerks.com>
Date: Mon, 18 Jul 2005 10:54:11 CST
Local: Mon, Jul 18 2005 12:54 pm
Subject: Re: status of rvalue reference proposal (n1377, n1770, et al)
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In article <1121517407.194853.244...@g14g2000cwa.googlegroups.com>,
 "Russell Yanofsky" <russell.yanof...@us.army.mil> wrote:

> Thanks for the detailed replies. That was exactly the kind of
> information I was looking for.

> I've got a separate question on N1770. The proposed clause on
> expressions (5/6) reads:

>   If an expression initially has the type ``reference to T''
>   (dcl.ref, dcl.init.ref), the type is adjusted to ``T'' prior to
>   any further analysis and the expression designates the object or
>   function denoted by the reference. The expression is considered an
>   lvalue after this adjustment if the expression type was
>   ``rvalue-reference to T'' and the expression is a variable
>   reference, parameter reference, class member access operation
>   (expr.ref), or pointer-to-member operation (expr.mptr.oper), or if
>   the expression type was ``lvalue-reference to T''; otherwise, it
>   is considered an rvalue.

> What is the bit about pointer-to-member operations? I'd think there
> could never be a pointer to member operation with the type of
> ``rvalue-reference to T'' since a pointer to member can't point to a
> member with reference type (8.3.3/3).

I believe you are correct (but I could be mistaken).  The intent of 5p6
in N1770 is to implement the general notion of N1377:

http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2002/n1377.htm#Ret...
ng%20A&&

> Earlier this paper proposed that:

>     Named rvalue references are treated as lvalues.

> This thought will now be completed:

>     Unnamed rvalue references are treated as rvalues.

That is, if a function returns an rvalue reference:

A&& foo();

then that function result is treated as an rvalue.

bar(foo()); // foo() is treated as an rvalue when
            // selecting among the overload resolution set

But rvalue references that have an identifier attached to them, such as
a function parameter, are treated as lvalues:

void foo(A&& a)
{
    // a is an lvalue here

}

The standard has no concept of "named and unnamed references", and so
the concept was translated into currently accepted standardeeze.  It is
quite possible that a bug or two crept in during that translation.  
Thanks for bringing this possibility to our attention.

-Howard

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