Casting About

Notes

• We need to examine casting a bit.
• static_cast
  • Used when there are built in routines (int to float)
  • Or when classes provide constructors (RationalT(int real))
  • Or when changing pointers
  • Casting from a derived class to a more general (towards base) is called upcasting

    • BarbarianT barbarian;
      FighterT * fp = & barbarian;
                

      • This is not dangerous in that data is lost, but the base class does not expect for there to be data.
      • And virtual functions will still work because the vtable/ run time system.
  • Casting from a general class to a derived class is called downcasting
    • This is truly a problem
    • The specialized (derived) class potentially has more members than the base class.
    • But these are not present in the base class that was cast to it.
    • So there will most likely be errors.
    • The methods will exist, but the data will not
  • static_cast, as we said is safe for built in values.
    • But does not checking for classes in a hierarchy.
    • And is not the way to go.
  • dynamic_cast is for this purpose
    • It does a run time check to see if the proposed cast is safe
    • It returns a pointer to the class instance if it is
    • It returns a null pointer if it is not.
• Take a look at slice.cpp.