Subject: Re: [boost] [smart_ptr] enable_shared_from_this and multiple inheritance solution
From: Ansel Sermersheim (agserm_at_[hidden])
Date: 2010-11-30 12:05:12
Frank Mori Hess <frank.hess_at_[hidden]> writes:
> On Monday 29 November 2010, Ansel Sermersheim wrote:
>> After mulling this over for some time, I _think_ I have come up with a
>> solution that mitigates all these problems.
>> I have a new patch up, at the same URL. In order to make this
>> mechanism work, I had to add a new constructor to
>> boost::shared_ptr<T>, which I am loath to do. Given that downside the
>> patch appears to work properly. It also adds a free function
>> boost::shared_from<T>(T*) which returns a boost::shared_ptr<T>.
>> The only cast that occurs in this code is a static_cast that downcasts
>> from enable_shared_from_polymorphic<T> to T. I have read the relevant
>> portions of the standard several times and I beleive this is valid,
>> but I would definitely be curious if wiser heads agree with my reading.
> If you use a free function only (see enable_shared_from_raw.hpp in trunk),
> the enabling base class doesn't have to use the CRTP.
That is true. However, part of my goal was to create a drop-in
replacement for enable_shared_from_this which will work even when
multiple classes in the inheritance heirarchy inherit from it. Thus,
shared_from_this() needs to know what type to return.
It certainly would be possible to have another type that inherits from
enable_shared_from_polymorphic_base, which would then allow use of the
free function. Or the free function may not be the most desirable
interface. An earlier version had shared_from as a method, so that one
could only call it as shared_from(this). I haven't really thought
through the pros and cons of free function vs method at this point.
> Why did you need another shared_ptr constructor (sorry, I don't have
> time to actually read the code)? It's hard to imagine, given the
> presence of the shared_ptr aliasing constructor.
Now why on earth didn't I see that the last time I looked at the
available shared_ptr constructors? That is exactly what was needed.
This removes one of my major dislikes about this approach. I will
respin a new patch later today and clean up this area.
Thank you very much for the critique,
Boost list run by bdawes at acm.org, gregod at cs.rpi.edu, cpdaniel at pacbell.net, john at johnmaddock.co.uk