[boost] Re: Perfect moving (almost)

From: Rani Sharoni (rani_sharoni_at_[hidden])
Date: 2004-02-10 10:08:55

David Abrahams wrote:
> "Peter Dimov" <pdimov_at_[hidden]> writes:
>> That's because your "move" constructor is not a move constructor from
>> language point of view (i.e. it has no special status). As I
>> understand it, the compiler's thought process goes somewhat like:
>> need to create a temporary, but it can be elided, so we'll just
>> access check the copy constructor. The compiler doesn't want to
>> backtrack and try not to elide the temporary, which would've
>> succeeded.
>
> OK, so 8.5.3/5 says:
>
>   A temporary of type ``cv1 T2'' [sic] is created, and a constructor
>   is called to copy the entire rvalue object into the temporary. The
>   reference is bound to the temporary or to a sub-object within the
>   temporary.*
>
>   [Footnote: Clearly, if the reference initialization being processed
>   is one for the first argument of a copy constructor call, an
>   implementation must eventually choose the first alternative (binding
>   without copying) to avoid infinite recursion. --- end foonote]
>
>   The constructor that would be used to make the copy shall be
>   callable whether or not the copy is actually done.
>
> The problem here is, IIUC, that the implementors read that as saying
> that, because "a constructor is called to copy..." that the
> constructor used must be a copy constructor?
>
> I don't understand why the converting constructor fails to be
> considered when the templated SFINAE'd constructor *will* be used to
> copy const lvalues.  Templated constructors are never "copy ctors"
> either, IIUC.

(As I already mentioned)

http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/cwg_active.html#291:
<Q>
Another detail worth of note is that in the *draft* version of the Standard
as of 2 December 1996 the second bullet read:
A temporary of type "cv1 T2" [sic] is created, and a *copy constructor* is
called to copy the entire rvalue object into the temporary...

J. Stephen Adamczyk replied that the reason for changing "a copy
constructor" to "a constructor" was to allow for member template converting
constructors.
</Q>

This relaxation makes any viable converting constructor legitimate for
constructing this temporary and therefore the syntactic constraint is met
which makes your (and Andrei's) suggestion perfectly functional in the
boundaries of the current standard.

Regards,
Rani