Re: [boost] Re: Virtual Constructor submission (a.k.a. ctor_fn())

From: Anthony Liguori (anthony_at_[hidden])
Date: 2003-05-19 19:59:22

>
>
>That's not what I mean.  I mean, "I give you the blob of memory, you
>put an object in it", or "if the blob is big enough, put the object
>there".  I guess you can do that with allocators, too.
>  
>
Out-of-the-box, it is possible to do with the class I have now although 
it would require creating a type with a static member that stored where 
the preallocated memory.

What I'd rather to though is specialize the class to a placement 
allocator and add a constructor that took a void * argument.  The usage 
would look something like this:

    void *my_custom_address = malloc(sizeof(A));
    function<A *(int, int, int)> func = ctor_fn<A *(int, int, int), 
ctor::placement>(my_custom_address);

    A *new_a = func(1, 2, 3); // create object in my_custom_address

Another possible usage could be this:

    void *my_custom_address = malloc(sizeof(A));
    function<function<A *(int, int, int)> (void *)> func = ctor_fn<A 
*(int, int, int), ctor::placement>();

    A *new_a = func(my_custom_address)(1, 2, 3);

>>Since the C++ standard does not allow allocators to have any state,
>>    
>>
>
>Oh, but it does.  They're even encouraged to have state!  
>  
>
I knew I'd get the standard quoted to me when I said that :-)

>It's just that it also gives the containers permission to treat them
>as though they have no state. Twisty, eh? ;-)
>  
>
Yeah, what I meant is that right now, I'm relying on STL-container 
allocator semantatics.

>Actually, the containers may only assume there's "one state per
>allocator type", but not that there's no state whatsoever.
>
Sure, I should have said no object-state as class-state is surely allowed.

I never thought of providing placement semantic to this library.  I'm 
leaning toward my second example to provide this mechanism.  Another 
thing I began to ponder is giving array semantics so that one could do 
the following:

    boost::ctor_fn<A *(int, int, int)> func;
    A *new_a = func[20](1, 2, 3); // new_a is now an array of 20 elements

Of course, if doing this, there needs to be a corresponding dtor_fn 
class so that objects can safely be destroyed...

Regards,
Anthony