Skip to main content

8. Inheritance and Virtual Functions

28/02/23

Inheritance​

  • Models the 'is-a' relationship. The sub-class object is-a type of base class object
  • Be sure that inheritance really is what you want before use it
  • Define a new class in terms of a current class
  • Uses the : notation. Equivalent of Javas extends
class MyClass : public MySuperClass
{

}
  • Can have different access rights; public -> protected -> private

Virtual Functions​

Cost of flexibility​

  • Adding a virtual function to a class may make the objects of that class bigger

  • Looking up which function to call at runtime is slower when functions are virtual

    • Why the default is to not have virtual functions

  • Even if a function is virtual, you can still call the base class version from the sub-class version.

  • Can use the scoping operator to call global functions or access global variables :: with nothing before it

  • Also used to denote a function is a class member in a definition

  • Left of scoping operator is

    • blank - to access a global variable/function
    • class name - to access member of that class
    • namespace name - to use that namespace

Inheritance and constructors​

  • Construction occurs in the order; base class first, then derived class
  • Destruction occurs in the order; derived class first, then base class
  • Derived classes part of the object can always assume that base class part exists
  • They will NOT exist/be initialised when the base class constructor/destructor is called
  • Do not call virtual functions from the constructor or destructor

Virtual Destructors​

  • If destructor is NOT virtual then it will NOT be called if the object is destroyed through a base class pointer, reference or function
  • But if you make destructor virtual then the objects of that class will have a (hidden) vtable pointer