Overview. Game Programming in C++ Inheritance. Inheritance. Inheritance. Inheritance. Inheritance. Polymorphism

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1 Overview Game Programming in C++ Arjan Egges Lecture #4: Advanced OO programming Overview, examples When (not) to use it Polymorphism Virtual members Abstract base classes Problems with multiple inheritance = classes inherit data and functionalities from parent classes This allows to create classes that are variations of an existing class Syntax: Example: CPolygon int width, height class derived_class: public base_class { // class declaration ; CRectangle CTriangle class CPolygon { void set_values(const double& a, const double& b); protected: double width_, height_; ; CPolygon.h CRectangle.h class CRectangle: public CPolygon { double area() const; ; CPolygon.cpp void CPolygon::set_values(const double& a, const double& b) { width_ = a; height_ = b; CRectangle.cpp double CRectangle::area() const { return width_*height_; 1

2 CTriangle.h class CTriangle: public CPolygon { double area() const; ; CTriangle.cpp double CTriangle::area() const { return width_*height_/2.0; int main () { CRectangle rect; CTriangle trgl; rect.set_values (4,5); trgl.set_values (4,5); cout << rect.area() << endl; cout << trgl.area() << endl; Main.cpp Member access: Inherited members keep the same access: Access Members of the same class Members of derived classes Not members public protected no private no no CPolygon::width_ // protected access CRectangle::width_ // protected access CPolygon::set_values() // public access CRectangle::set_values() // public access Because: class CRectangle: public CPolygon { ; What is inherited from the base class? Everything, except constructors and destructors Calling the right base constructor from the derived class Syntax: derived_constr(parameters) : base_constr(parameters) { class Tree { Tree(); Tree(int a); ; Tree.h Tree::Tree() { Tree.cpp cout << Tree: no parameters" << endl; Tree::Tree(int a) { cout << Tree: int parameter" << endl; 2

3 class Oak : public Tree { Oak.h class Willow : public Tree { Willow.h ; Oak(int a); ; Willow(int a); Oak::Oak(int a) { Oak.cpp cout << Oak: int parameter" << endl; Willow::Willow(int a) : Tree(a) { Willow.cpp cout << Willow: int parameter" << endl; Main program: int main () { Oak tree1(0); Willow tree2(0); Main.cpp Output: Tree: no parameters Oak: int parameter Tree: int parameter Willow: int parameter Because: Oak(int a) // nothing specified: call default Willow(int a) : Tree(a) // also calls parent constructor When to use inheritance? Rule 1: Containment vs. : Is each D a special kind of B? Is every D automatically a B? Is it impossible to have a D without also having a B? Are sure that the B is not really a part of D? Are they created and destroyed simultaneously? Rule 2: Behaviour vs. Data Is the behaviour of D different from B? Is the difference between D and B only in data values? (B = base, D = derived) CPolygon* createrandompolygon() { int main() { CPolygon* p1 = createrandompolygon(); CPolygon* p2 = createrandompolygon(); // print out the area of both polygons cout << "Area of p1:" << p1->area() << endl; cout << "Area of p2:" << p2->area() << endl; 3

4 In order for this to work we should: Add the area() method to the CPolygon class But CPolygon and CTriangle use different area() methods! Dynamic binding Virtual members Virtual members class CPolygon { protected: double width_, height_; void set_values(const double& a, const double& b); virtual double area() const; ; double CPolygon::area() const { return 0.0; CPolygon.h CPolygon.cpp Virtual members int main () { CPolygon * ppoly1 = new CRectangle(); CPolygon * ppoly2 = new CTriangle(); CPolygon * ppoly3 = new CPolygon(); ppoly1->set_values(4,5); ppoly2->set_values(4,5); ppoly3->set_values(4,5); cout << ppoly1->area() << endl; cout << ppoly2->area() << endl; cout << ppoly3->area() << endl; Main.cpp Resulting output: Virtual members If area() was not declared virtual : Virtual members C++ virtual is the opposite of Java final In Java, all methods are virtual by default The effect of automatically calling the method from the derived class is called polymorphism If a function could be overridden, it should be declared as virtual When to use virtual members? Virtual methods add a small performance overhead because of extra lookup table (vtable) Example adapted from the book (p. 13): class GraphicsRenderer { virtual void setrenderstate(); virtual void settexturestate(); virtual void setlight(); virtual void drawtriangle(); ; 4

5 Virtual destructor The destructor should be virtual if a delete operation can be performed on a base class pointer: int main () { CPolygon * ppoly1 = new CRectangle(); // do some stuff delete ppoly1; Virtual destructor Proper destructor declaration: class CPolygon { CPolygon(); virtual ~CPolygon(); ; class A { A(); virtual ~A(); ; Virtual destructor class B : public A { B(); virtual ~B(); ; A* myinstance = new B(); // first A() is called,then B() delete myinstance; // first ~B() is called, then ~A() Abstract base classes class CPolygon { protected: double width_, height_; void set_values(const double& a, const double& b); virtual double area() const = 0; ; area() is a pure virtual method CPolygon is an abstract base class and cannot be instantiated Abstract base classes Not possible: CPolygon p; CPolygon* p2 = new CPolygon(); Possible: CPolygon* p; CPolygon * ppoly1; CPolygon * ppoly2; Abstract base classes Add the following method to CPolygon: void printarea() const; Definition: void CPolygon::printArea() const { cout << this->area() << endl; 5

6 int main () { Abstract base classes CPolygon * ppoly1 = new CRectangle(); CPolygon * ppoly2 = new CTriangle(); ppoly1->set_values(4,5); ppoly2->set_values(4,5); ppoly1->printarea(); ppoly2->printarea(); Suppose we want to add an output mechanism: class COutput { void output(const double& i); ; COutput.h void COutput::output(const double& i){ COutput.cpp cout << "Outputting: " << i << endl; ; Our new CRectangle class: class CRectangle: public CPolygon, COutput { double area() const; ; is often used to inherit from multiple abstract base classes Similar to usage of interfaces in Java But, there are some problems: Ambiguity Topography Program architectures PROBLEM 1: Ambiguity Suppose both COutput and CPolygon classes have a method bool isinit() : if (myrectangle->isinit()) { // compiler error! Solution: prefix the class name: void CRectangle::someFunction() { if (COutput::isInit() && CPolygon::isInit()) { Even worse outside the derived class: bool initialized = myrectangle.coutput::isinit() && myrectangle.cpolygon::isinit(); So now the calling code needs to know about the parent classes of CRectangle! 6

7 PROBLEM 2: Topography MovingAI int counter; LandAI FlyingAI HybridAI Diamond of Death (DOD) Result: Contents of MovingAI appears twice in HybridAI Using a member variable of MovingAI is ambiguous We need to specify the inheritance path C++ offers so-called virtual inheritance but this adds a space & runtime performance cost PROBLEM 3: Program architectures Deep inheritance hierarchies Too tight coupling between classes Unusable interfaces Result: Difficult to maintain and add features Difficult code reuse Increased compile and link times A few final words: Consider using containment instead Abstract interfaces in combination with multiple inheritance works without problems Avoid multiple inheritance in already complex hierarchies C style casting: // Automatic conversion from int to float. // This will generate a compiler warning. int a = 200; float b = a; // Explicit C-style casting (no warning) float c = float(a); float d = (float)a; // Another possible syntax An example where casting is useful: void GameEntity::onCollision(GameEntity& e) { if (e.istype(gameentity::projectile)) { GameProjectile& p = (GameProjectile&) e; p.blowup(); else if GameProjectile is a subclass of GameEntity 7

8 C++ style casting: static_cast, const_cast, reinterpret_cast and dynamic_cast Syntax of new cast style: static_cast<type>(expression); For example: // C++ style cast float f2 = static_cast<float>(n)/100.0f; static_cast Restricted version of C style cast Allows to convert between two data types, but only if they are related Possible to cast points up and down the inheritance hierarchy but not outside Cannot involve a change in constness const_cast Can change the constness of the expression it applies to Const non-const and non-const const (the latter is normally not needed) Only use if absolutely necessary If used in your own code, probably a reason to rethink your design reinterpret_cast Has the same power as the C-style cast Does not look at relation between pointers or constness Results are implementation dependent Use sparingly and only when other types of casts are not enough dynamic_cast Can be applied only to pointers or references Checks at run-time whether or not conversion is possible Checks the actual type of the objects, and evaluates whether or not conversion is possible and if so, returns a new pointer Also accounts for multiple inheritance If conversion is not possible -> returns NULL Example of using dynamic_cast: void GameEntity::onCollision(GameEntity* e) { GameProjectile* p = dynamic_cast<gameprojectile*>(e); if (p!= NULL) { p->blowup(); 8

9 Summary & polymorphism How/when to use it Advantages/disadvantages How/when to use it When to avoid it Next course: Programming tips! 9

17. Friendship and Inheritance

- 117 - Object Oriented Programming: 17. Friendship and Inheritance Friend functions In principle, private and protected members of a class cannot be accessed from outside the same class in which they