Intermediate C++ Game Programming Tutorial 18

Another two-parter here, and we got the real stuff now. Virtual functions allow you to unlock the true potential of inheritance in C++. You need to know this shit.

Topics Covered

Part 1

• How to create a virtual function
• Using the override keyword
• Creating a pure virtual function
• Using a container of pointers to manage a heterogeneous collection of objects
• virtual destructors

Part 2

• Using inheritance and composition together
• Basic idea of a polymorphic state machine and its application to entity behavior

Video Timestamp Index

Tutorial 18.1

• Creating a "Virtual Function" in the MemeFighter class 0:23
  • Create a free function DoSpecials() that calls SpecialMove() on the instances of the derived classes of MemeFighter
  • This requires the member function SpecialMove() to be added to the base class...
  • ... and the signatures of these member functions in the derived classes to match up
• Enabling "Dynamic Dispatch" by using virtual on the member function in the base class 3:17
  • This enables the overridden functions in the child classes to be called
  • When you refer to a derived class object using a pointer or a reference to the base class, you can call a virtual function for that object and execute the derived class's version of the function
• Applying the override keyword to increase code safety 5:17
  • Tells the compiler that you are intending to override something virtual in a base class
• Making a base class member function "Pure Virtual" using virtual void Func(...) = 0; 7:08
  • This makes the base class an Abstract Class which cannot be instantiated
  • It enforces that the virtual function is overridden in all sub-classes
  • Whether you make a pure virtual function or whether you supply a default implementation depends on your situation 8:47
• We have now created "Polymorphism" 9:16
  • C++ polymorphism means that a call to a member function will cause a different function to be executed depending on the underlying type of object that invokes the function
• Applying Polymorphism to a container of pointers to the shared base type of derived class objects 9:46
  • In this case, we create two std::vector<MemeFighter*>s to enable team fights
  • These vectors contain pointers to the base type, this allows us to have a single vector that can manage objects of different types, as long as they all fall within the same hierarchy
  • To manage gameplay, we #include <algorithm>
  • Use std::any_of() with a predicate (Lambda function) that tests if team members are still alive 11:26
  • Use std::random_shuffle() to get different match-ups in every round of gameplay 11:46
  • Use std::partition() to make sure only live players engage 11:57

• WORK-IN-PROGRESS

Tutorial 18.2

• WORK-IN-PROGRESS

Source Code

Inheritance Github Repository

Errata

• Forgot the virtual destructor for class Weapon! (this one hurts)
• In the children, the function signatures should be: int CalculateDamage( const Attributes& attr,Dice& d ) const override
• Though not technically an error, it might have been a better decision to make Weapon::GetName() and Weapon::GetRank() (pure) virtual functions (this would reduce the amount of per-instance data to just the vtable ptr)

See also

• Next in series (Tutorial 19)
• Intermediate C++ Game Programming Series