Re: [boost] [variant] Please vote for behavior

Subject: Re: [boost] [variant] Please vote for behavior
From: Paul Smith (pl.smith.mail_at_[hidden])
Date: 2013-01-30 10:52:01

On Wed, Jan 30, 2013 at 12:58 PM, Antony Polukhin <antoshkka_at_[hidden]> wrote:
> I think that this example is perfect for showing the benefits of
> nulled recursive wrapper.
> Watch the hands:
>
> union my_union {
>     my_union u_; // Impossible to do
>     int i_;
> };
>
> Because we can not use field of type my_union in my_union we need to wrap it:
>
> union my_union {
>     recursive_wrapper<my_union> u_; // Ok
>     int i_;
> };
>
> Now we just need to decide, what a recursive_wrapper is! Is it behave
> like a reference:
>
> // # 1
> union my_union {
>     my_union& u_;
>     int i_;
> };
>
> Or is it behave like a pointer:
>
> // # 2
> union my_union {
>     unique_ptr<my_union> u_;
>     int i_;
> };
>
> #1 is not what a recursive_wrapper is, because recursive_wrapper OWNS a value
> #2 is much closer to functionality of recursive_wrapper
>

Neither of these are models of what recursive_wrapper is. It's not a
pointer and it's not a reference. A unique_ptr owns an object, not a
value. It can own different objects or nothing at all during its
lifetime. A reference is an alias for an object. A recursive_wrapper
*is* the object it wraps and its value *is* the object's value - it
doesn't own anything else or have any other visible state. The fact
that it has its storage allocated on the heap is an implementation
detail. The point is simply to allow recursive_wrapper<T> to be a
complete type even when T isn't.

A variant that contains a recursive_wrapper<T> behaves exactly like a
variant that contains a T, except that T can be a recursive type -
that's all. And this is exactly why it shouldn't have different
semantics - move or otherwise. If you want the semantics offered by
unique_ptr, shared_ptr, "copy_ptr", reference_wrapper, optional or
anything else - by all means, use that!

--
Paul Smith