Re: [Boost-users] [SmartPtr] enable_shared_from_raw

Subject: Re: [Boost-users] [SmartPtr] enable_shared_from_raw
From: Gavin Lambert (gavinl_at_[hidden])
Date: 2014-07-27 18:47:50

On 28/07/2014 01:03, Bjorn Reese wrote:
> On 07/24/2014 06:53 AM, Gavin Lambert wrote:
>
>> 1. shared_from_this can't be used from the constructor (requiring
>> factory-method-init shenanigans); reportedly
>
> If you are using C++11, then your factory can be made generic by using
> perfect forwarding:

While that's somewhat an improvement on manually specifying parameters, 
it still counts as factory-method-init shenanigans.  (Only somewhat, 
because this sort of thing tends to confuse IDE-provided parameter-info 
hints, which makes it harder to call.)

>> And it should be possible to obtain that weak_ptr before any shared_ptr
>> exists, and "fix" it later to point at the real shared_ptr, especially
>> if it is known that the weak_ptr will never actually be used until that
>> real shared_ptr exists.
>
> This sounds a bit risky if the last assumption does not hold.

It's not any more risky than the current behaviour of shared_from_raw.

There's nothing incomplete or invalid about the weak_ptr thus obtained, 
and there's no reason why other code couldn't actually use it or even 
lock it back to a shared_ptr and save that somewhere.  It just points at 
the internal self-cyclic pointer, which will create a memory leak if 
it's not cleared by having code outside of the constructor assign the 
raw pointer to a "real" shared_ptr.  As soon as that occurs, everything 
is happy again.  But at no point is anything invalid or unusable.