Re: Good style for memory management?

[Paul Davis <paul linuxaudiosystems com>]

On Sun, 2007-03-18 at 16:40 -0500, one of a multiplicity of Paul Davis's
wrote:

> And yes, there are times where I do throw from constructors.

i do so often, despite what i wrote.

> There are other
> > cases where it doesn't fit either (for example, deserializing from
> > something like XML which tends to promote HAS-A relationships but your
> > objects are all inheritance based (IS-A)).
> >
> 
> I disagree.  Its just a matter of the engine you build to create your
> objects.  And the first two thoughts that came to my mind were either
> 
> 1) Make every object have a constructor that can accept a XML tree.
> 2) Create a set of factory objects that produce the objects using
> their 'usual' constructors from the parsed XML.

nuh-uh :)

my objects all have XML-based constructors, and many have factories but
you run into problems nevertheless. consider this:

class A { public: class A(const XMLNode&); }
class B : public class A { public: class B(const XMLNode&); }

now consider an XML tree that says "this describes a B object". so the
factory looks superficially at the XMLNode, sees this, and goes to call
B::B (const XML&Node). so far so good.

but now B::B (const XMLNode&) has to call A:A (const XMLNode&). not so
bad. but wait a minute. what node will it pass in? if it passes the same
node that it was handed, that means that A's XML parser has to
understand how to get to "the A node inside any derived class's XML
node". if it doesn't pass in the same node, it means that B has to
figure out the right node before we've even gotten inside the body of
its constructor (something like A::A (B::find_A_node (node)))

this happens because XML encourages a heirarchy where a parent node
HAS-A child node, but in C++, the relationship tends towards IS-A.

i do have solutions for this type of design problem, but i can't say i
like them very much. most of them involve have a virtual set_state
(const XMLNode&) method that is called explicitly by the derived class.