Difference between revisions of "Intermediate C++ Game Programming Tutorial 18"

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** It avoids having to change a fighter into a different type when exchanging weaponry
 
** It avoids having to change a fighter into a different type when exchanging weaponry
 
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* One more real world example: "State Machine" for a Boss Battle [https://youtu.be/cdOB_gKnJOM?t=14m28s 14:28]
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* One more real world example used in [[RPG Project Twin]]: a finite "State Machine" for a Boss Battle [https://youtu.be/cdOB_gKnJOM?t=14m28s 14:28]
 
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** A state maching models patterns of behavior (like attack, evade, wander, find aid, etc.)
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** 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
 
** 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
 
** Very common pattern in game programming, e.g. using AI to model the behavior of enemies
 
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* Presenting the template class <code>Behavior</code> within the class <code>Entity</code> [https://youtu.be/cdOB_gKnJOM?t=15m22s 15:22]
 
* Presenting the template class <code>Behavior</code> within the class <code>Entity</code> [https://youtu.be/cdOB_gKnJOM?t=15m22s 15:22]
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* WORK-IN-PROGRESS
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* How to transition between behavioral states? [https://youtu.be/cdOB_gKnJOM?t=16m35s 16:35]
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** Associate each state with a stack of successor states that are consecutively called
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** The last state on this stack can then lead into a new chain (fill up another stack)
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** This again represents a combination of inheritance and composition: states are composed of other states as part of their "stack of states"...
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** ... avoiding an explosion of (derived) classes that would be needed to define this behavior with just inheritance
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* The performance impact of using Virtual Functions [https://youtu.be/cdOB_gKnJOM?t=19m36s 19:36]
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** 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 ("vtable")
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** 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
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** 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
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** The difference in performance is generally not problematic, except for specific (rare) cases
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* An example of where performance actually matters: The SpriteEffect system [https://youtu.be/cdOB_gKnJOM?t=21m07s 21:07]
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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
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** In theory, virtual functions would be nicer, as you could swap out different effects dynamically at runtime (can't do that with a functor).
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** However, in that scenario the use of virtual functions would introduce performance issues
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** The choice depends on the estimated frequency of function calls (relevant when greater than O(1E6) calls/second)
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* [[Intermediate C++ Game Programming Tutorial 19|Next in series (Tutorial 19)]]
 
* [[Intermediate C++ Game Programming Tutorial 19|Next in series (Tutorial 19)]]
 
* [[Intermediate C++ Game Programming Series]]
 
* [[Intermediate C++ Game Programming Series]]
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* [[RPG Project Twin]]

Latest revision as of 03:55, 15 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
  • Enabling "Dynamic Dispatch" by using virtual on the member function in the base class 3:17
  • Applying the override keyword to increase code safety 5:17
  • Making a base class member function "Pure Virtual" using virtual void Func(...) = 0; 7:08
  • We have now applied the concept of "Polymorphism" 9:16
  • Applying Polymorphism to a container of pointers to the shared base type of different derived class objects 9:46
  • Applying Polymorphism to objects created on the heap (dynamic memory management) 13:46
  • "Virtual Destructors": Managing destructors in base and derived classes 14:48
  • Review of termonology and concepts learned 19:49

Tutorial 18.2

[Expand]
  • Design choices in class hierarchy: inheritance vs. composition 0:43
  • Adjusting the MemeFighter code to include the Weapon class 4:20
  • Add feature where one fighter can take the other fighter's weapon 11:36
  • Recap of advantage of the inheritance-composition construct 14:00
  • Presenting the template class Behavior within the class Entity 15:22
  • How to transition between behavioral states? 16:35
  • The performance impact of using Virtual Functions 19:36
  • An example of where performance actually matters: The SpriteEffect system 21:07

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

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