1 Object Oriented Programming Development - Multi File Development z By: Marc Conrad & Rob Manton University of Luton z z Room: D104

2 Module Outline zIntroduction zNon object oriented basics zClasses z Inheritance z Aggregation z Polymorphism z Multifile Development

3 Today’s lecture zPolymorphism II recap yPolymorphic Pointers yOverriding Methods yStatic typing & Dynamic binding zMulti File Development yReferring to classes not yet defined y.H and.CPP files yLibraries

4 Polymorphic Pointers zIn C++ a pointer of a parent class is allowed to point to an object of the child class. E.g. class Vehicle { //... }; class Car : public Vehicle { //... }; //... Vehicle * vp = new Car(); Valid C++ syntax!

5 Overriding Methods Three ways for a derived class to implement a polymorphic method yInherit it unchanged yReplace with a different implementation yAdd to the existing implementation

6 Overriding Methods zMethods in the parent class can be redefined in the child class. zclass Vehicle { void move(int i); }; class Car : public Vehicle { void move(int i); }; //... Vehicle * vp = new Car(); vp->move(100); Valid C++ syntax!

7 Overriding Methods zMethods in the parent class can be redifined in the child class. zclass Vehicle { void move(int i); }; class Car : public Vehicle { void move(int i); }; //... Vehicle * vp = new Car(); vp->move(100); BUT: z Which of these two move() methods will be called?

8 Overriding Methods zMethods in the parent class can be redifined in the child class. zclass Vehicle { void move(int i); }; class Car : public Vehicle { void move(int i); }; //... Vehicle * vp = new Car(); vp->move(100); static typing! z In C++, static typing is the default behaviour. As vp is of type pointer to a Vehicle, the method of the Vehicle is called.

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10 As vp is of type pointer to a Vehicle, the method of the Vehicle is called.

11 Overriding Methods - The keyword virtual zMethods in the parent class can be redefined in the child class. zclass Vehicle { virtual void move(int i); }; class Car : public Vehicle { virtual void move(int i); }; //... Vehicle * vp = new Car(); vp->move(100); dynamic binding! The keyword virtual allows the use of dynamic binding. As vp points to a Car object the method of the Car is called

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13 Dynamic Binding: As vp points to a Car object the method of the Car is called

14 Abstract Methods & Classes zAbstract methods are methods without any implementation (pure virtual methods). zclass Vehicle { virtual void move(int i) = 0; }; class Car : public Vehicle { virtual void move(int i); }; //... Vehicle *vp = new Car(); vp->move(100); Syntax for declaring abstract methods. Note that Vehicle objects cannot be instantiated (but Car objects can).

15 Syntax for declaring abstract methods. We are not allowed to declare an object of an abstract type...

16 Static typing & Dynamic binding zStatic typing means that the legality of a member function invocation is checked at the earliest possible moment: by the compiler at compile time. The compiler uses the static type of the pointer to determine whether the member function invocation is legal. z Dynamic binding means that the address of the code in a member function invocation is determined at the last possible moment: based on the dynamic type of the object at run time. It is called "dynamic binding" because the binding to the code that actually gets called is accomplished dynamically (at run time).

17 Why is dynamic binding useful? zIf we have a variety of classes based on the same base class then each type can be pointed to by a pointer to the base class animal * pAnimal; pAnimal=new sheep(); pAnimal=new crocodile(); Animal mammalreptile humansheepcrocodile

18 Why is dynamic binding useful? zAny method declared in the base class.. Animal mammalreptile humansheepcrocodile MakeNoise()

19 MakeNoise() Why is dynamic binding useful? zAny method declared in the base class.....can be overridden by any derived class Animal mammalreptile humansheepcrocodile MakeNoise()

20 MakeNoise() Why is dynamic binding useful? zWith static binding, the version of the method that gets called is the one defined in the base class (not so useful!) animal * pAnimal; pAnimal=new sheep(); pAnimal->MakeNoise(); Animal mammalreptile humansheepcrocodile MakeNoise()

21 MakeNoise() Why is dynamic binding useful? zWith dynamic binding, the version of the method that gets called is the one defined in the class of the object that the pointer points to animal * pAnimal; pAnimal=new sheep(); pAnimal->MakeNoise(); Animal mammalreptile humansheepcrocodile virtual MakeNoise() MakeNoise()

22 Why is dynamic binding useful? zAt compile time we can create a pointer to the base class and call a method defined in the base class. This should compile properly. zAt run time we can make that pointer point to an object of any class in the same inheritance hierarchy and the correct version of the method gets called. zVery useful for handling situations where the number or composition or lifespan of objects cannot be predicted at compile time

23 Practical example zIn a computer game we might have an inheritance hierarchy containing many different characters (old man, woman, alien, spider, goblin, martial arts expert etc etc) all derived from a base character class. Character Update() Draw()

24 Practical example Character HumanAnimal Old man Martial arts expert Alienspider Each derived class can override the base class methods and provide its own update() and draw() methods

25 Practical example zWe don’t need to know at compile time how many of each type of character will exist at any time during the game zWe can have an array of pointers or a linked list or other container object to store a variable number of character objects zCharacter * characters[50];

26 Practical example zAt run time we can allocate dynamically created objects of any of the classes in the hierarchy to the pointers zeg characters[1]=new Alien(); zcharacters[2]=new Spider(); zcharacters[3]=new OldMan();

27 Practical example zBecause they are all derived from a standard base class we can call the same methods of each derived character for (i=0;i<50;i++) { if (Characters[i]!=NULL) { Character[i]->Update(); Character[i]->Draw(); }

28 Practical example zPolymorphism in action za single method name being called from different types of object (old man, spider, martial arts expert, alien etc) zCaters well with situations where number or composition or life time of objects can’t be predicted at compile time..

29 Important stuff to remember zOverriding methods zPolymorphic pointers zStatic binding zDynamic binding - the virtual keyword zAbstract methods

30 Referring to classes not yet defined a)If a class contains or references another class (aggregation or association) then the compiler needs the enclosed class to be defined first - what happens if we have a circular relationship?

31 Referring to classes not yet defined zwhat happens if we have a circular relationship - which class goes first? class Car { private: Person * myDriver; }; class Person { private: Car * myCar; }; Car has a pointer to person and person has a pointer to car

32 Referring to classes not yet defined Person object not defined yet so we get an error message

33 Referring to classes not yet defined zSolution is to provide a ‘forward declaration’ of the class which is being used but hasn’t been declared yet class Person; class Car { private: Person * myDriver; }; class Person { private: Car * myCar; }; Forward declaration of Person class means that compiler lets you refer to Person class even though Person class hasn’t been declared yet

34 Referring to classes not yet defined Car has a pointer to person and person has a pointer to car forward declaration makes this legal!

35.H and.CPP files zOn larger projects class definitions and implementations are usually split up zdefinitions go into a.H file zimplementations into a.CPP file zcan do this automatically in the compiler with Insert/New Class

36.H and.CPP files Name of class Optional base class Insert/New Class

37.H and.CPP files.h and.cpp files now appear in project Skeleton for class definition Skeleton for class implementation

38.H and.CPP files If we refer to a class defined in separate file we get an error

39.H and.CPP files Need to #include the.h header file then all is OK!

40 Using libraries zRapid application development - using existing modules or libraries rather than writing your own zReady made libraries like DirectX, OpenGL, Renderware etc comprise of a set of Header files (.H) and a set of library files (.LIB) zA static library (.LIB) is a file containing objects that is linked into your program when the executable file is built.

41 Using libraries zDon’t need to recompile the library files each time - saves time zPreserves security of source code (commercial reasons) zUser only needs to know about the interface to the classes which is defined in the header file zfollows C++ notion of separation of interface from implementation

42 Using libraries zHow do you make a library file? Static library is an option in the usual File/New Dialog

43 Using libraries How do you use an existing library file? zInsert both the.lib file and the.h header file into your project using Project/Add to project/Files zAdd the folder that contains the library/header files to the list of paths in Tools/Options/Directories One list of folders relates to include files (.h) another to library (.lib) files

44 That’s all for today zFirst session after Christmas will be revision for the exam zSee you then and have a good one..