Virtual Methods

• You can prevent classes from deriving from a given class by adding the keyword final
  • I added this to DoubleSpellT.h
  • See brokenTest.cpp
• To do true function replacement we need the keyword virtual
  • This marks a function for replacement.
• How does virtual work?
• When normal member functions are implmeneted, they are "bound" at compile time.
  • This is called static binding or early binding.
  • The compiler decides, based on the type at compile type what function to bind.
• With virtual functions it works differenty
  • Each instance carries along a vtable or table of where to find the virtual functions.
  • At run time, the run time system determines which function to call based on this table.
  • This is called dynamic or late binding.
• See Figure 10-4, page 347 in the book.
• This type of function
  • Takes a few instructions more to execute.
  • Takes a bit more storage (probably a word per virtual function).
• So any function you wish to overload should be a virtual function.
  • Destructors should be virtual
    • A pointer is of type base class, but is pointing to a derived class.
    • The derived class' destructor needs to be called.
• Constructors should not be virtual.
• Order of construction
  • Base class is constructed
  • Other members are constructed
  • Constructor is called for the derived class.
• Order of destruction
  • Derived class' destructor is called.
  • Data members are destructed
  • Base class' destructor is called.
• Once a function is declared to be virtual, it is virtual, no matter many levels of derivation are applied.
• Take a look at abstract.cpp