Re: [boost] [ptr_container] ptr_vector across DLL boundaries

Subject: Re: [boost] [ptr_container] ptr_vector across DLL boundaries
From: Christian Schladetsch (christian.schladetsch_at_[hidden])
Date: 2009-06-25 16:46:08

How does a clone_allocator make a new clone, if it needs access to the
allocator in the parent ptr_container? I had this problem when I tried to
write a clone allocator for monotonic to support ptr_containers:

        struct inline_clone_allocator
        {
            template< class U >
            static U* allocate_clone( const U& r )
            {
                // can't allocate clone without access to the allocator??
                return 0;
            }

What am I missing?

On Fri, Jun 26, 2009 at 6:05 AM, Thorsten Ottosen <
thorsten.ottosen_at_[hidden]> wrote:

> Robert Dailey skrev:
>
>> On Thu, Jun 25, 2009 at 9:41 AM, Thorsten Ottosen <
>> thorsten.ottosen_at_[hidden]> wrote:
>>
>>>
>>> Do you think a stateful custom clone_allocator would allow you to do what
>>> you want?
>>>
>>
>>
>> Could you elaborate a bit on what you mean? What state would you give to
>> clone_allocator?
>>
>
> That is up to you. The important fact is that allocator is stored as an
> object, and/or that you can access and modify that object to your wishes.
>
>  Keep in mind that clone_allocator needs to, at some point,
>> be implemented IN the DLL itself, otherwise the correct 'delete' will not
>> be
>> called. Some sort of polymorphism would solve this, that's the most
>> obvious
>> solution I can think of. Either that, or instead of a functor, take a
>> boost::function, and that way I can pass you a function pointer to call
>> for
>> clone allocation/deallocation. There's several easy ways of solving this
>> particular issue that I can see.
>>
>> But I would be interested in hearing more about the idea you have. I'm not
>> sure I fully understand just yet.
>>
>
> Let me give you an example of what I was trying to do. To avoid storing
> duplicates of NullObjects, I want to let my clone_allocator
>
> 1. don't delete a null object
>
> 2. clone a null object by returning just a pointer (i.e. don't allocate
> anything)
>
> This is how I could do that:
>
> template<class NullObject>
> class null_object_clone_allocator
> {
> public:
>     typedef NullObject null_object_type;
>
> private:
>     const NullObject* null_object_;
>
> public:
>     explicit null_object_clone_allocator( const NullObject& x )
>      : null_object(&r)
>     { }
>
>     template< class U >
>     U* allocate_clone( const U& r ) const
>     {
>         if( &r == null_object_ )
>           return null_object_;
>         return new_clone( r );
>     }
>
>     template< class U >
>     void deallocate_clone( const U* r ) const
>     {
>         if( r != null_object_ )
>            delete_clone( r );
>     }
> };
>
> boost::ptr_vector<base,null_object_clone_allocator<null_base>> vec(
> my_alloc) ;
>
> -Thorsten
>
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