Re: [Boost-users] revisiting casting a boost::shared_ptr to void*

Subject: Re: [Boost-users] revisiting casting a boost::shared_ptr to void*
From: Edward Diener (eldiener_at_[hidden])
Date: 2014-07-29 00:46:45

On 7/28/2014 11:22 PM, Chris Cleeland wrote:
> I'm resurrecting this topic:
>
> https://groups.google.com/forum/#!searchin/boost-list/shared_ptr$20void*/boost-list/QE_VCTen1YQ/styxqKB2wbQJ
>
> because there is an unaddressed issue in it that I'm about to run into:
>
> "... beware the pointee may be deleted if the last copy of the shared
> pointer
> goes out of scope, as the raw pointer won't have incremented the
> reference count."
>
> I cannot be the first person to run into this--a common issue when
> crossing an API boundary.  Has anyone come up with a good, safe solution
> wherein the reference count gets properly incremented?

The issue has nothing to do with casting a shared pointer to a void *. 
If you need to pass a shared pointer as any raw pointer, if the last use 
of the shared pointer ends the encapsulated raw pointer is deleted. If 
someone is still holding on to that raw pointer after that happens and 
tries to use it you will almost cetainly get a crash.

There is no good solution to this other than not using a shared pointer 
in the first place in your particular situation. Think about your 
problem. You want to be able to share this pointer among other objects 
but then you want one or more objects to share this pointer as a raw 
pointer. Given what a shared pointer is, doing this just does not make 
sense.

If you are just passing your shared pointer to a function as a raw T * 
you have no problem if the function is a synchronous function which does 
not hold on to the raw pointer past the functions exit. This is because 
the shared pointer from which the raw pointer is passed will still be in 
scope when your function returns.