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| | ** The SpriteEffect system (see [[Intermediate C++ Game Programming Tutorial 14|Intermediate Tutorial 14]]) uses functors to define drawing behavior. These functors are called on heavily as they contain PutPixel calls | | ** The SpriteEffect system (see [[Intermediate C++ Game Programming Tutorial 14|Intermediate Tutorial 14]]) uses functors to define drawing behavior. These functors are called on heavily as they contain PutPixel calls |
| − | ** In theory, virtual functions would be nicer, as you could swap out different effects dynamically at runtime (can't do that with functors). | + | ** In theory, virtual functions would be nicer, as you could swap out different effects dynamically at runtime (can't do that with a functor). |
| | ** However, in that scenario the use of virtual functions would introduce performance issues | | ** However, in that scenario the use of virtual functions would introduce performance issues |
| | ** The choice depends on the estimated frequency of function calls (relevant when greater than O(1E6 calls/second) | | ** The choice depends on the estimated frequency of function calls (relevant when greater than O(1E6 calls/second) |
Revision as of 00:41, 14 October 2019
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
[Expand]
- 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 applied the concept of "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
- Polymorphism creates the power to manage different types in a single container
- Applying Polymorphism to a container of pointers to the shared base type of different 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 ensure the maximum number of live matchups 11:57
- Implementing the game battle loops 12:34
- Applying Polymorphism to objects created on the heap (dynamic memory management) 13:46
- Create vectors with pointers to dynamically created objects on the heap
- When done, free the memory by calling
delete on the pointers in the containers 14:18
- "Virtual Destructors": Managing destructors in base and derived classes 14:48
- Making the destructor in the parent class
virtual ensures that the destructors of the children objects are called (it enables dynamic dispatch so the appropriate child class destructor is called)
- The Construction and Destruction procedure and the order in which ojects are created/destroyed (RAII) 16:31
- Review of termonology and concepts learned 19:49
Tutorial 18.2
[Expand]
- Design choices in class hierarchy: inheritance vs. composition 0:43
- The MemeFighter game is expanded such that fighters can hold weapons
- Chili argues why this situation best lends itself to a composition relationship: the abstract fighter class can have a pointer to an abstract weapon class (which has derived classes for various types of weapons)
- Adjusting the MemeFighter code to include the
Weapon class 4:20
- Adding MemeFighter member functions GiveWeapon(), PilferWeapon() 8:03
- Adding derived classes of the Weapon class to repepresent Fist, Knife, Bat 9:34
- Add feature where one fighter can take the other fighter's weapon 11:36
- We add a free function
TakeWeaponIfDead() that handles the weapon exchange
- Recap of advantage of the inheritance-composition construct 14:00
- It avoids having to change a fighter into a different type when exchanging weaponry
- A State Machine models patterns of behavior (like attack, evade, wander, find aid, etc.)
- You can use virtual functions that control the boss (depending on type of behavior) and transition from one behavior to another, based on some condition
- Very common pattern in game programming, e.g. using AI to model the behavior of enemies
- Presenting the template class
Behavior within the class Entity 15:22
- How to transtion between behavioral states? 16:35
- Associate each state with a stack of successor states that are consecutively called
- The last state on this stack can then lead into a new chain (fill up another stack)
- This again represents a combination of inheritance and composition: states are composed of other states as part of their "stack of states"...
- ... avoiding an explosion of (derived) classes that would be needed to define this behavior with just inheritance
- The performance impact of using Virtual Functions 19:36
- The direct impact: If you use virtual functions in a class, all objects of that class will have to store a (hidden) pointer to a virtual function table ("v-table")
- The v-table holds pointers to code for each virtual function supported by the object. It is used as a lookup table to determine which functions are being called at runtime
- An indirect impact: because the compiler does not know which function will be called at runtime, it can't inline any virtual functions that are being dynamically bound
- The difference in performance is generally not problematic, except for specific (rare) cases
- An example of where performance actually matters: The SpriteEffect system 21:07
- The SpriteEffect system (see Intermediate Tutorial 14) uses functors to define drawing behavior. These functors are called on heavily as they contain PutPixel calls
- In theory, virtual functions would be nicer, as you could swap out different effects dynamically at runtime (can't do that with a functor).
- However, in that scenario the use of virtual functions would introduce performance issues
- The choice depends on the estimated frequency of function calls (relevant when greater than O(1E6 calls/second)
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
