[libxml++] Re: [Libxmlplusplus-general] Node ownership

[murrayc t-online de (Murray Cumming)]

On Thu, 2003-02-06 at 18:23, Stefan Seefeld wrote:
> > Murray Cumming wrote:
> > Clearly libxml++ objects need to be able to create nodes. Maybe 
> > people
> > want to create nodes with normal C++ memory management too.

> Well, that's not so clear to me.

But that's what you've been telling us that you want.

> And even if it was, there are multiple
> ways to achieve this.

That's what I'm trying to discuss.

> I was discussing this with DV, and he suggested to
> make something simple, even if it is a bit restrictive. It's more
> important to have a non-leaking library (by that I also mean an API
> that is clear about the semantics) than something that is flexible but
> is an invitation for bugs.

That is the intention at the moment. Nodes own nodes.

> > So, let's list some possible solutions. For instance,
> > 
> > 1. A boolean owned_by_libxml member variable.
> >   The libxml destroy callback would not destroy the C++ wrapper instance
> > if owned_by_libxml is false.
> 
> but what would it do instead ? The impl object is already destroyed, so
> the wrapper is invalid anyways.

True. Does libxml provide any way to detach a node from it's parent? If
not then we probably can't provide functionality via a wrapper that's
not in the libxml itself.

> I'd suggest
> 
> 4. add a clipboard-like API, i.e. a place where nodes that aren't
>     currently part of a document are helt. This could be hidden:
> 
>     We don't need an extra flag to specify ownership. If the document
>     points to the node (and, the node to the document), it is owned. The
>     question arises only if we take the node out of the document.
> 
>     So, the question is how to implement that 'remove node from tree'
>     method. It could work like this:
> 
>     void Node::remove_child(Node *node)
>     {
>        Clipboard::store(node); // unlink the node from the current doc
>     }
>     void Node::insert_child(iterator i, Node *node)
>     {
>        Clipboard::restore(node);       // remove the node from the
>                                        // clipboard, if it was stored,
>                                        // else unlink it from the old
>                                        // document
>        xmlAddPrevSibling(node->_impl); // link the node into the new doc
>     }
> 
> 
>     the Clipboard would be hidden, only accessible by xmlpp objects.
>     Oh, and we may add a static method to Node:
> 
>     void Node::destroy(Node *node); that really deletes a subtree.
> 
>     The advantage here is that nodes are always owned by *exactly* one
>     container. Either a document or a clipboard. That's simple to keep
>     in mind by users, and it is simple to implement correctly.

This doesn't show how we would detach the node in C. If there is a way
to do that then 1. would also seem to be possible, and simpler.

-- 
Murray Cumming
murray usa net
www.murrayc.com