Re: [Boost-users] [Interprocess] shared_ptr in shared memory and inheritance

Subject: Re: [Boost-users] [Interprocess] shared_ptr in shared memory and inheritance
From: Ion Gaztañaga (igaztanaga_at_[hidden])
Date: 2009-02-10 12:08:45

Gaetan Gaumer wrote:
> In boost::shared_ptr doc
> http://www.boost.org/doc/libs/1_38_0/libs/smart_ptr/shared_ptr.htm#Members 
> It's said :
> "This constructor has been changed to a template in order to remember 
> the actual pointer type passed. The destructor will call *delete* with 
> the same pointer, complete with its original type, even when *T* does 
> not have a virtual destructor, or is *void*."

Sorry, I spoke too fast. I was trying to say that if you cast your type 
to the base class and they construct a shared_ptr<Base>, then you loose 
all your polymorphism. shared_ptr's template constructor does this for 
you: shared_ptr ptr only knows the real type of the object in that 
constructor and that compile-time information is going to disappear when 
the templated constructor ends, so shared_ptr constructs a polymorphic 
shared_count that will call the correct deleter when needed:

template<class Y> explicit shared_count( Y * p ): pi_( 0 )
{
    //sp_counted_impl_p has VIRTUAL functions
    pi_ = new sp_counted_impl_p<Y>( p );

    //...
}

sp_counted_impl_p has type Y so it calls Y's constructor and 
sp_counted_impl_p is polymorphic so it can be casted to sp_counted_base 
without loosing functionality. So you always need virtual functions to 
get type erasure with shared_ptr. And that's why you can't do this with 
interprocess's shared_ptr.

> Is it really impossible to achieve the same behavior in 
> boost::interprocess::shared_ptr than in boost::shared_ptr ?

I afraid you can't. shared memory forbids any type of run-time 
polymorphism and that includes shared_ptr.

> PS : By the way, thanks Ion and others for this great work on boost in 
> general and the Interprocess library in particular.

Thanks for using Boost in general and Interprocess in particular ;-)

Regards,

Ion