[boost] Re: [Review] Named parameters library

From: David Abrahams (dave_at_[hidden])
Date: 2004-11-17 18:45:32

"Thorsten Ottosen" <nesotto_at_[hidden]> writes:

> "David Abrahams" <dave_at_[hidden]> wrote in message
> news:uwtwlb7yh.fsf_at_boost-consulting.com...
> | "Thorsten Ottosen" <nesotto_at_[hidden]> writes:
> |
> | > "David Abrahams" <dave_at_[hidden]> wrote in message
> | > news:u4qjqq4is.fsf_at_boost-consulting.com...
> | > | Thorsten Ottosen <nesotto_at_[hidden]> writes:
> | >
>
> | > | > why can't you store a reference?
> | > |
> | > | Because you'd be storing a reference to a temporary, which would have
> | > | evaporated by the time you get to actually initiate the call.
> | >
> | > you must be confusing move-semantics with this situation, or I'm
> | > missing something.
> |
> | You're missing something.
> |
> | > there should be no difference between storing a temporary implicitly
> | > or explicitly...it's still there and still takes up stack-space no
> | > matter what.
> |
> | Yes, but your scheme requires copying that temporary into a new
> | object, where it is stored again.  The proposed library allows a
> | reference to be bound to the temporary with no copying.  And I
> | seriously doubt there is a compiler in existence today that can elide
> | that copy, because of the restrictions on where the object must be
> | stored.
>
> ah, yes I see it now. That is indeed a major benefit.
>
> Where in the standard are those restriction described?

It's described by omission.  The standard only gives permission to
elide copies and not assignments.  Therefore if you construct a
subobject and then you assign into it, you have to do the assignment.

> I mean I would have suspected that RVO could be extended to support classes
> like in this:
>
> struct X
> {
>     Foo f;
> };
>
> Foo foo();
>
> X x;
> x.f = foo();

Forget it; it's an assignment.

> If I change the interface to set_foo( const Foo& r ) { f = r; } instead of a
> public member, it
> is slower, but otherwise not AFAICT (see attachment).

It's still an assignment.

-- 
Dave Abrahams
Boost Consulting
http://www.boost-consulting.com