[Swig-user] Lua bindings appear to assign metatables statically

[Daniel S. <dsc...@sc...>]

Suppose I have a class hierarchy like:

class XmlNode { public:
  XmlNode* parent();
  XmlNode* firstChild();
  XmlNode* nextSibling();
  enum Type {
    TYPE1,
    TYPE2
  };
};

class XmlNodeType1 : public XmlNode { public:
  int attr1();
};

class XmlNodeType2 : public XmlNode { public:
  int attr2();
};

Then if I want to traverse through that, I might want to have Lua code
something like:

if (node:firstChild():type() == mod.XmlNode_TYPE1) then
  print ("child is type1 node with attr1 = " ..
node:firstChild():attr1())
elseif (node:firstChild():type() == mod.XmlNode_TYPE2) then
  print ("child is type2 node with attr2 = " ..
node:firstChild():attr2())
end

But that doesn't actually work with the current implementation of Lua
bindings (I'm using Swig version 2.0.4).  It seems that the wrapper for
XmlNode::firstChild() statically assigns the XmlNode metatable to the
resulting userdata, instead of looking at the actual return value and
figuring out the correct metatable to use.  It would be ugly, and IMO
contrary to the spirit of Lua, to have to define wrappers for various
dynamic_cast<XmlNodeType1*> etc. functions.

My question: is this a bug in the Lua binding runtime, or was it a
design choice (or an unfortunate design restriction maybe)?
--
Daniel