Derive C++ Class in Lua

I am trying to derive my base class in Lua. My code in Lua looks like this:

class 'square' (shape)
function square:__init()
print("Square constructor")
shape:__init()


end



a = shape()
local v = a:area()
print(v)

b = square()
v = b:area()
print(v)

The output is:
Shape Constructor
100
Square Constructor
Shape Constructor
<crash>

Here's my code in C++:

class shape
{
public:
shape(): x(10), y(10) { cout << "Shape constructor.\n"; }

virtual int area() { return (x*y); }


int x, y;
};

class shape_wrapper: public shape,public luabind::wrap_base
{
public:
shape_wrapper(): shape() { }
virtual int area() { return luabind::call_member<int>(this, "area"); }
static int def_area(shape* p) { return p->shape::area(); }
};

...
module( L )
[
class_<shape, shape_wrapper>("shape")
.def(constructor<>())
.def("area", &shape::area, &shape_wrapper::def_area)
.def_readwrite("x", &shape::x)
.def_readwrite("y", &shape::y)
        ];


I am not sure why it is crashing.

-- 
Regards,
Emmanuel Barroga

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