Re: [Boost-users] Use boost::shared_ptr with boost::adaptors

Subject: Re: [Boost-users] Use boost::shared_ptr with boost::adaptors
From: Robert Jones (robertgbjones_at_[hidden])
Date: 2012-04-30 06:05:46

On Sat, Apr 28, 2012 at 9:45 PM, Jeffrey Lee Hellrung, Jr. <
jeffrey.hellrung_at_[hidden]> wrote:

> Is it expected behavior of adaptors ?
>>
>
> I think so. Here's my guess what's going on. Consider what the dereference
> function might look like for an iterator to the range
>
> vec | transformed(bind(&transform, _1)) |  indirected
>
> foo& dereference()
> {
>     int& i = *vec_it; // dereference iterator to vec
>     foo_ptr = transform(i); // pass i through bind(&transform, _1)
>     foo& x = *foo_ptr; // pass foo_ptr through indirected
>     return x;
> }
>
> Sadly, the only foo_ptr "backing" the foo& goes out of scope once the foo&
> is returned, so that foo object is deleted and you get a dangling reference.
>
> I don't think there's an obvious alternate implementation that will allow
> you to get away with the above. When you start chaining together range
> adaptors, especially transforms, you have to consider whether the argument
> to the transformation function needs to persist for the result to be remain
> valid.
>
>
When you put it that way I can see your point, but just from the code the
OP presented the intermediate shared_ptr's should persist until
the end of the expression.

Just musing here, but how about if the dereference function you imagined
returned a proxy reference, to keep the shared_ptr alive? Not
sure how I'd implement that in practise, since it would be required to know
foo_ptr was a shared_ptr, but maybe it could done at the level
of the indirected adaptor rather than the indirection iterator.

- Rob.