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Object Oriented ProgrammingDevelopment - Multi File Development
By: Marc Conrad & Rob MantonUniversity of Luton
Email: Marc.Conrad@luton.ac.uk Rob.Manton@luton.ac.uk Room: D104
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Module Outline
IntroductionNon object
oriented basicsClasses
InheritanceAggregationPolymorphismMultifile
Development
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Today’s lecture
Polymorphism II recap Polymorphic Pointers Overriding Methods Static typing & Dynamic binding
Multi File Development Referring to classes not yet defined .H and .CPP files Libraries
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Polymorphic Pointers
In 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!
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Overriding Methods
Three ways for a derived class to implement a polymorphic method
Inherit it unchanged Replace with a different implementation Add to the existing implementation
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Overriding Methods
Methods in the parent class can be redefined in the child class.
class Vehicle { void move(int i);};class Car : public Vehicle { void move(int i);}; // ... Vehicle * vp = new Car(); vp->move(100);
Valid C++ syntax!
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Overriding Methods
Methods in the parent class can be redifined in the child class.
class Vehicle { void move(int i);};class Car : public Vehicle { void move(int i);}; // ... Vehicle * vp = new Car(); vp->move(100);
BUT: Which of
these two move() methods will be called?
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Overriding Methods
Methods in the parent class can be redifined in the child class.
class Vehicle { void move(int i);};class Car : public Vehicle { void move(int i);}; // ... Vehicle * vp = new Car(); vp->move(100);
static typing! 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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As vp is of type pointer to a Vehicle, the method of the Vehicle is called.
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Overriding Methods - The keyword virtual
Methods in the parent class can be redefined in the child class.
class 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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Dynamic Binding:As vp points to a Car object the
method of the Car is called
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Abstract Methods & Classes
Abstract methods are methods without any implementation (pure virtual methods).
class 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).
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Syntax for declaring abstract methods.
We are not allowed to declare an object of an abstract type...
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Static typing & Dynamic binding
Static 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.
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).
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Why is dynamic binding useful?
If 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
mammal reptile
human sheep crocodile
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Why is dynamic binding useful?
Any method declared in the base class..
Animal
mammal reptile
human sheep crocodile
MakeNoise()
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MakeNoise()
Why is dynamic binding useful?
Any method declared in the base class... ..can be overridden by any derived class Anim
al
mammal reptile
human
sheep crocodile
MakeNoise()
MakeNoise() MakeNoise() MakeNoise()
MakeNoise()
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MakeNoise()
Why is dynamic binding useful?
With 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
mammal reptile
human
sheep crocodile
MakeNoise()
MakeNoise() MakeNoise() MakeNoise()
MakeNoise()
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MakeNoise()
Why is dynamic binding useful?
With 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
mammal reptile
human
sheep crocodile
virtual MakeNoise()
MakeNoise() MakeNoise() MakeNoise()
MakeNoise()
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Why is dynamic binding useful?
At compile time we can create a pointer to the base class and call a method defined in the base class. This should compile properly.
At 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.
Very useful for handling situations where the number or composition or lifespan of objects cannot be predicted at compile time
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Practical example
In 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.
CharacterUpdate()
Draw()
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Practical example
Character
Human Animal
Old man Martial arts expert
Alien spider
Each derived class can override the base class methods and provide its own update() and draw() methods
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Practical example
We don’t need to know at compile time how many of each type of character will exist at any time during the game
We can have an array of pointers or a linked list or other container object to store a variable number of character objects
Character * characters[50];
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Practical example
At run time we can allocate dynamically created objects of any of the classes in the hierarchy to the pointers
eg characters[1]=new Alien();characters[2]=new Spider();characters[3]=new OldMan();
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Practical example
Because 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();}
}
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Practical example
Polymorphism in actiona single method name being called
from different types of object (old man, spider, martial arts expert, alien etc)
Caters well with situations where number or composition or life time of objects can’t be predicted at compile time..
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Important stuff to remember
Overriding methodsPolymorphic pointersStatic bindingDynamic binding - the virtual
keywordAbstract methods
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Referring to classes not yet defined
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?
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Referring to classes not yet defined
what 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
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Referring to classes not yet defined
Person object not defined yet so
we get an error message
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Referring to classes not yet defined
Solution 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
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Referring to classes not yet defined
Car has a pointer to person
and person has a pointer to car
forward declaration makes this
legal!
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.H and .CPP files
On larger projects class definitions and implementations are usually split up
definitions go into a .H file implementations into a .CPP file can do this automatically in the
compiler with Insert/New Class
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.H and .CPP filesName of class
Optional base class
Insert/New Class
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.H and .CPP files
.h and .cpp files
now appear in project
Skeleton for class definition
Skeleton for class implementation
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.H and .CPP files
If we refer to a class defined in
separate file we get an error
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.H and .CPP files
Need to #include the .h
header file then all is OK!
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Using libraries
Rapid application development - using existing modules or libraries rather than writing your own
Ready made libraries like DirectX, OpenGL, Renderware etc comprise of a set of Header files (.H) and a set of library files (.LIB)
A static library (.LIB) is a file containing objects that is linked into your program when the executable file is built.
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Using libraries
Don’t need to recompile the library files each time - saves time
Preserves security of source code (commercial reasons)
User only needs to know about the interface to the classes which is defined in the header file
follows C++ notion of separation of interface from implementation
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Using libraries
How do you make a library file?
Static library is
an option in the usual
File/New Dialog
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Using libraries
How do you use an existing library file? Insert both the .lib file and the.h header file into your project using Project/Add to project/Files Add 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
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That’s all for today
First session after Christmas will be revision for the exam
See you then and have a good one..
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