[boost] Re: [Review] Named parameters library

From: Thorsten Ottosen (nesotto_at_[hidden])
Date: 2004-11-17 14:16:54

"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?

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();

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).

-Thorsten

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`
end