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9. Function pointers & Virtual/non-virtual functions

02/03/23

Function Pointers​

  • Stored in memory
    • Can ask for the address of them
    • Can store these in function pointers
  • Used a lot in low-level programming
  • Function pointers can be passed into functions
  • Used for 'callback functions'
    • Allows something to call you back
    • 'Call this function when an event happens'
    • Event driven programming is VERY common

v-tables​

Normal/default (non-virtual) functions​

  • Type of pointer determines function to call
  • Use the type of the object the compiler thinks it is:
    • Type of pointer to the object
    • Type of the member function making the call
    • Easier for the compiler, type is known at compile-time

Virtual Function​

  • Finds out the actual function to call based upon the object type AT RUNTIME - much more difficult - slower
  • Works in the same way as Java functions

Special Cases​

  • virtual-ness is inherited
  • If a function is virtual in the base class then the function is virtual in the derived class(es)
  • Even when the keyword virtual is not used in the derived class

Should a function be virtual?​

  • If member function is called from a base class function or through a base class pointer AND the behaviour should depend on class type of the object then the member function has to be virtual
  • Utility functions will often not be virtual