I have a c++ class derived from a base class in a framework.

The derived class doesn't have any data members because I need it to be freely convertible into a base class and back - the framework is responsible for loading and saving the objects and I can't change it. My derived class just has functions for accessing the data.

But there are a couple of places where I need to store some temporary local variables to speed up access to data in the base class.

mydata* MyClass::getData() {
   if ( !m_mydata ) { // set to NULL in the constructor
      m_mydata = some_long_and complex_operation_to_get_the_data_in_the_base()
   }  
   return m_mydata;
}

The problem is if I just access the object by casting the base class pointer returned from the framework to MyClass* the ctor for MyClass is never called and m_mydata is junk.
Is there a way of only initializing the m_mydata pointer once?