VALUE Data_Wrap_Struct(VALUE klass, RUBY_DATA_FUNC mark, RUBY_DATA_FUNC free, void *sval)

#define Data_Wrap_Struct(mrb,klass,type,ptr) mrb_data_object_alloc(mrb,klass,ptr,type)

class A
{
    mrb_value mrb_object;
};

class B
{
    A* a;
    mrb_value mrb_object;
};

class C
{
    B* b;
    mrb_value mrb_object;
}

b = B.new
c = C.new
c.b = b

GC.start

p c.b.a