Question

I’m seeing two problems in a setup like this:

namespace ns1
    {
     class ParentClass
     {
      protected:
       void callback();
     };
    }

    namespace ns1
    {
     namespace ns2
     {
      class ChildClass : public ParentClass
      {
        public:
        void method()
        {
         registerCallback(&ParentClass::callback);
        }
      };
     }
    }

1. ChildClass::method() gives a compile error: “‘ns1::ParentClass::callback’ : cannot access protected member declared in class ‘ns1::ParentClass’“
2. ParentClass *pObj = new ChildClass() gives an error, that it can’t do the conversion without a cast. C++ can down-cast happily, no?

Answer 1

Change:

registerCallback(&ParentClass::callback);

…to:

registerCallback(&ChildClass::callback);

The reason is because &ParentClass::callback is a fully-qualified typename, not resolved from the context of ChildClass but from global context. In other words, it is the same problem as this:

class Thingy
{
protected:
    virtual int Foo() {};
};


int main() 
{
    Thingy t;
    t.Foo();
    return 0;

}