Re: [boost] [smart ptr] Any interest in copy-on-write pointerforC++11?

Subject: Re: [boost] [smart ptr] Any interest in copy-on-write pointerforC++11?
From: Jeffrey Lee Hellrung, Jr. (jeffrey.hellrung_at_[hidden])
Date: 2013-02-11 17:34:37

On Mon, Feb 11, 2013 at 1:47 PM, Peter Dimov <lists_at_[hidden]> wrote:

> read_ptr<T> pr;
>>
>> if( write_ptr<T> pw = pr.write() )
>> {
>>     pw->mutate();
>>     pr = std::move( pw );
>> }
>>
>> I don't, however, particularly like the semantics of the case in which pr
>> is already unique.
>>
>
> In fact, now that I think of it, .write should be a move, too.

In the case that pr has a use_count of 1, definitely. I thought that's what
you had been proposing all along :) Deep copy the pointed-to object only if
the use_count >= 2. Whatever you get back, do your mutations, then pass it
back to the read_ptr.

In this case, it's wouldn't be surprising to sometimes find pr holding a
> NULL after the move.
>
>    write_ptr<T> pw = std::move( pr );
>    pw->mutate();
>    pr - std::move( pw );
>
> The advantage here is that (1) in
>
>    read_ptr<T> get_value();
>    write_ptr<T> p = get_value();
>
> you don't need a write() call,

I would expect most of the time you're not going to be immediately
modifying a read_ptr rvalue like above. The typical use case I'd expect is
you have existing read_ptr lvalue which points to an object you want to
mutate. So whether you write .write() or move(pr) is not a huge syntactic
difference.

and (2) this avoids the mistake of doing
>
>    pr.write();
>
> without storing the return value and as a result, losing *pr when it's
> unique. Just doing
>
>    std::move( pr );
>
> would do nothing, so it's safer.
>

That's a fair point, but I think this is really an abuse of rvalue
reference syntax :) I guess one concern I have is if you're moving a smart
ptr, I'd expect it to be as fast as a couple pointer assignments, but here
it could incur a deep copy, and that behavior is entirely runtime-dependent.

- Jeff