Unsmart Pointers: Part II

Listing 3

class any_ptr
{
public:
  template <class T>
  struct functions;

  template <class T>
  any_ptr(T* x)
    : m_a(x), m_t(&functions<T>::table)
  {}
  any_ptr()
    : m_a(NULL), m_t(NULL)
  {}
  void Delete() {
    assert(m_a != NULL);
    m_t->Delete(m_a);
    m_a = NULL;
  }
  const std::type_info& GetType() const {
    return m_t->GetType(m_a);
  }
  any_ptr Clone() const {
    any_ptr o(*this);
    o.m_a = m_t->Clone(m_a);
    return o;
  }
  template<typename ValueType>
  ValueType* PtrCast() {
    if (!(typeid(ValueType) == GetType())) {
      throw BadObjectCast();
    }
    return static_cast<ValueType*>(m_a);
  }
  // Function table type order is important
 // It must match all other lists of functions
  struct table {
    void (*Delete)(void*);
    const std::type_info& (*GetType)(void*);
    void* (*Clone)(void*);
  };
  // For a given referenced type T, generates functions for the
  // function table and a static instance of the table.
  template<class T>
  struct functions
  {
    static typename any_ptr::table table;
    static void Delete(void* p) {
      delete static_cast<T*>(p);
    }
    static const std::type_info& GetType(void* p) {
      return typeid(T);
    }
    static void* Clone(void* p) {
      return new T(*static_cast<T*>(p));
    }
  };
private:
  void* m_a;
  table* m_t;
};
template<class T>
typename any_ptr::table
any_ptr::functions<T>::table = {
  &any_ptr::template functions<T>::Delete
  ,&any_ptr::template functions<T>::GetType
  ,&any_ptr::template functions<T>::Clone
};