Polymorphism

• having many forms.
• But most likely for us
  • A base class
  • Several derived classes.
• Let's think about a simple unit for the stompy robot game.
  • We probably have different classes of .
    • Robots
    • Tanks/trucks/APC
    • Infantry/civilians/robot-pilots
    • Airplanes
    • Ships
    • Buildings (Forts/walls/...)
    • Mines/traps/...
  • There are a set of common methods we will want each to implement
    • Given the problems with casting, we should have the same methods in everyting.
    • These probably include
      • Move
      • Attack
      • Defend
      • TakeDamage
      • Plus a set of observers like
        • Name
        • Description
        • Facton
        • Reset
        • CanAttack
        • ...
  • Much of the actual logic of attack will be part of the weapon class.
    • It will need to determine cover and range.
    • It will determine damage
    • But these will be called by the unit class.
      • A unit has-a weapon.
  • I am not the happiest with this example.
  • What should the move function do?
    • Move:
      • It most likely takes a destination.
      • And returns a bool if the move were successful
  • How would the move function work?
  • In a robot without jump jets
    • Ask the map for a path from source to destination.
    • Make sure that the path is traversable, or not blocked.
      • This is specialized by unit type.
    • Compute the total cost of moving along the path.
    • If everything is ok,
    • For each sector along the path
      • Move robot to new location.
      • Check for encounter in new location.
  • In a robot with jump jets
    • Ask the map for a path from source to destination.
    • No obsticles.
    • Compute the total cost of moving along the path.
    • If everything is ok, move driectly to the new location.
    • Check for encounter in the new location.
  • An airpane:
    • Basically the same, but it would not land
  • A ship:
    • Would only be able to move in water sectors.
  • What should we do about the move function in the base class?
    • We could just return true
    • But that would cause a purity of design problem.
    • This assums everything was successful.
    • We could return false
    • But this has the same problem.
• Pure virtual method/Abstract classes.
  • When there is no clear implementation of a method in a base class.
  • But you want to have it present to specify the interface for polymophism
  • vitrual type function(params) specifiers = 0
  • This is a pure virtual method.
  • It is explicitly undefined in the base class.
• A class that contains a pure virtual methods
  • Is called an abstract class
  • An abstract class can not be constructed.
• By declaring a pure virtual method we are
  • refusing to commit to how it will be implmented in any derived class.
  • But forcing it to be implemented in derived classes.