Re: [boost] casting shared_ptr to function to void

From: Vladimir Prus (ghost_at_[hidden])
Date: 2004-08-25 07:57:09

Peter Dimov wrote:
> Vladimir Prus wrote:
> >> If I have boost::shared_ptr<some_class>, it's possible to cast it to
> >> shared_ptr<void>. But it does not appear to be any way to convert
> >> pointer to function to pointer to void -- just because there's no
> >> implicit conversion from function pointer to void*.
> >>
> >> So, when I compile the attached program with g++, I get:
> >>
> >>  sp.cpp:22:   instantiated from here
> >>   /home/ghost/Work/boost/boost/shared_ptr.hpp:163: error: invalid
> >> conversion from
> >>   `void (* const)()' to `void*'
> >>
> >> Is there any way to make it work? What I'm trying to do is:

>
> No, not directly. A pointer to a function cannot be converted to void*, not
> even with a reinterpret_cast, and shared_ptr doesn't support
> reinterpret_casts anyway.

I think if shared_ptr supported reinterpret_cast, I could use shared_ptr<void 
()>.  

> You can, however, hold the shared_ptr<F> itself in a shared_ptr<void>, at
> the expense of an additional memory allocation or two.
>
> shared_ptr<void> pv( new shared_ptr<F>(pf) ); // two allocs

Neat. I think this will work for me. I presume 'pf' is another shared_ptr<F> 
and I can write this:

shared_ptr<void> foo()
{
     shared_ptr<F> pf = ...
     return shared_ptr<void>(new shared_ptr<F>(pf));
}

provides that usage count of 'pf' is exactly 1 when it's initialised?

> shared_ptr<void> pv( static_cast<void*>(0), bind( null_deleter(), pf ) );
> // one

And in this case original shared_ptr<F> will be deleted in destructor of the 
object returned by 'bind'. Even neater.

Thanks!

- Volodya