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

From: Anthony Liguori (anthony_at_[hidden])
Date: 2003-05-18 19:50:41

>
>
>It's really interesting! :)  I don't understand it, though :(
>  
>
:-)

>I've never understood what people meant by "virtual constructor"
>anyway.  The closest thing I can imagine is "clone() member function",
>but that doesn't appear to be quite what you're doing.
>  
>
Well, I've always taken a virtual constructor just to mean a method to 
allocate objects whereas the type isn't known at compile time (you can 
think of virtual methods in the same light).

It's not a clone() function because a clone() function duplicates an 
object.  This is more similar to the java Constructor class [the return 
of Class.getConstructors()].  Of course, this is a type safe version of 
that concept :-)

>Can this thing handle construction in raw memory, or does it always
>do a dynamic allocation?
>
Sort of.  Right now it uses std::allocators.  One could create a 
std::allocator that was a small object allocator that used one big blob 
of memory.

Since the C++ standard does not allow allocators to have any state, one 
couldn't use it as a replacement for the placement new operator.  Of 
course, as I said in the original message, I would rather use a more 
powerful allocator (and have the ctor_fn function objects have an 
allocator object as state).  Of course the trade off for this is that it 
is no longer conformant to standard allocators.

I'm looking for feedback as to what folks would think the best 
allocation strategy is so if you have any recommendations, I'd be happy 
to hear them.

Best Regards,
Anthony Liguori