smart pointers, slicing, and double deletion

#include <iostream>
#include <lua.hpp>
#include <luabind/luabind.hpp>
#include <boost/intrusive_ptr.hpp>
#include <string>
#include <list>
#include <map>

using namespace luabind;
using namespace std;

lua_State *L;

class widget;

int use_count( widget* ptr);

class widget
{
public:
    widget( string caption ) : name(caption), childIter(children.end())
    {
        cout << "widget constructed: " << this << " " << name << endl;
    }
    virtual ~widget()
    {
        while( !children.empty() )
            children.pop_front();

        cout << "widget destructed: " << this << " " << name << endl;
    }
    virtual void show()
    {
        cout << name << endl;
    }
    void run()
    {
        boost::intrusive_ptr<widget> ptr;

        for( reset(), ptr = next(); ptr; ptr = next() )
        {
            ptr->show();
            cout << "use count:" << use_count(ptr.get()) << endl;
        }
    }
    void addChild( boost::intrusive_ptr<widget> w )
    {
        children.push_back(w);
    }
    boost::intrusive_ptr<widget> next()
    {
        if( childIter == children.end() )
            childIter = children.begin();
        else
            childIter++;

        if( childIter != children.end() )
        {
            cout << "next = " << (*childIter)->name << endl;
            return boost::intrusive_ptr<widget>( *childIter );
        }

        cout << "next = nil" << endl;
        return boost::intrusive_ptr<widget>();
    }
    void reset()
    {
        childIter = children.end();
    }
protected:
    string name;
    list<boost::intrusive_ptr<widget> > children;
    list<boost::intrusive_ptr<widget> >::iterator childIter;
};

struct reference
{
	int refCount;
	int luaRef;

	reference() : refCount(0), luaRef(LUA_NOREF)
	{}
};

class widget_wrapper : public widget, public wrap_base
{
    public:
    widget_wrapper( string caption ) : widget( caption )
    {
    }
};

typedef map<widget *, reference> WidgetRefList;

WidgetRefList widgetReferences;

void intrusive_ptr_add_ref( widget* ptr )
{
	reference ref = widgetReferences[ptr];

	ref.refCount++;

	if( ref.luaRef == LUA_NOREF )
	{
		// This object has just been created.  Create a Lua reference to keep
		// it alive in Lua, and the Lua part not garbage collected.
		// This will stop the object getting 'sliced' when the only ptr to it
		// is on the C++ side.
		widget_wrapper* wrapper = dynamic_cast<widget_wrapper*>(ptr);
		assert(wrapper);
		detail::wrap_access::ref(*wrapper).get(L);
		ref.luaRef = lua_ref( L, -1 );
	}
	cout << "refCount for " << ptr << " is now " << ref.refCount << endl;

	widgetReferences[ptr] = ref;
}

void intrusive_ptr_release( widget* ptr )
{
	reference ref = widgetReferences[ptr];

	ref.refCount--;

	cout << "refCount for " << ptr << " is now " << ref.refCount << endl;

	widgetReferences[ptr] = ref;

	if( ref.refCount == 0 )
	{
		lua_unref( L, ref.luaRef );
		widgetReferences.erase( ptr );
		delete ptr;
	}
}

int use_count( widget* ptr )
{
	return widgetReferences[ptr].refCount;
}

int main()
{
    //lua_State *L;
    cout << "Hello from C++" << endl;

    L = lua_open();

    luaL_openlibs( L );

    luabind::open( L );

    module(L)
    [
        class_<
			widget,
			widget_wrapper,
			boost::intrusive_ptr<widget>
			>("widget")
        .def(constructor<string>())
        .def("show", &widget::show )
        .def("run", &widget::run )
        .def("addChild", &widget::addChild )
        .def("next", &widget::next)
        .def("reset", &widget::reset)
    ];

    if( luaL_dofile( L, "test.lua" ) )
    {
        object error( from_stack( L, 1 ) );
        cout << error << endl;
    }

    lua_close(L);

    return 0;
}

print 'Hello from Lua!'

class 'mywidget' (widget)

function mywidget:__init( name )
    widget.__init(self, name)
end

function mywidget:test()
end

function test()

    print '--- Start list ---'

    top:reset()

    c = top:next()
    if c==nil then print( 'got nil' ) end
    while c do
        c:show()
        if c.test then print 'is a mywidget - not sliced!' end
        c = top:next()
        if c==nil then print( 'got nil' ) end
    end

    print '---  End list  ---'
end

top=widget('top')
t={}
for i=1,5 do
    t[i]=mywidget('widget '..i)
    top:addChild(t[i])
    super=nil
end

test()
--print '--- C++ listing ---'
--top:run()
--print '--- end C++ listing ---'

print'Zap t'
t=nil
collectgarbage()

test()
--print '--- C++ listing ---'
--top:run()
--print '--- end C++ listing ---'