Question

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?

Answer 1

It doesn’t have members and you must maintain bit-for-bit memory layout compatibility… except it does and C++ doesn’t have a concept of freely-convertible.

If the existing framework allocates the base objects, you really can’t derive from it. In that case, I can think of two options:

• Define your own class Cached which links to Base by reference. Make the reference public and/or duplicate Base‘s interface without inheritance.
• Use a hash table, unordered_map< Base *, mydata > mydata_cache;. This seems most appropriate to me. Use free functions to look up cache data before delegating to the Base *.