Introduction to Object Oriented Programming Lecture-3

Programming Techniques Unstructured Programming Procedural Programming Modular Programming Object Oriented Programming

Unstructured Programming The first step into programming Hello World program as seen below However it is difficult to manage a program once the program gets large > 100 lines? #include using namespace std; int main(void) { cout << “Hello World” << endl; return 0; }

Procedural Programming Divide program into smaller blocks of code These may then be called more than once Makes program more structured (and less prone to errors) Allows programs to be designed to show a flow of Data where data is passed from the main program into the procedures

Procedural Programming #include using namespace std; double calculate_new(double); int main(void) { double prices[] = 54,312,78,654,65,18; for(i=0; i<6; i++) { double newprice = calculate_new(price[i]); cout << “New price for item “ << i<< “= “ << newprice <<endl; } return 0; } double calculate_new( double oldprice) { return 0.70*oldprice; }

Modular Programming Procedures of common functionality are grouped into separate modules Each module can have it's own data Each module can maintain an internal state However the module may exist at most once in each program Structure of modular programs dictated by functions not data Programs compiled into different modules so we use a makefile to control the process (or ide based system which manages the project)

Why Object Oriented Programming Previously data effectively global to the program  This leads to errors Increasing size of programs make maintenance difficult (sometimes impossible) Inter / intra project communication problems (teams of developers using different modules to do the same thing) Software re-use

But What is OO (the world as objects) Systems are modelled as sets of interacting objects Proponents claim that Object Orientation  Is intuitive and abstract  Manages Complexity  Aids communication  Scales up to large systems  Allows consistent representation throughout the lifecycle phases

What is an Object A tangible or visible 'thing' e.g. a person 'An object represents an individual identifiable item, unit or entity, either real or abstract, with a well defined role in the problem domain' This is good news for graphics programming as most things we want to design are objects or things that act on objects (e.g. a Shape is an abstract object which can be realised into a more concrete objects such as sphere, cube cone etc.)

Object Structure What Defines an Object ?  A boundary e.g. a car body or building exterior  An internal state e.g. Blood pressure, waiting time  Behavior -- what it does  A Unique identity (as with instances, entities or roles)

Objects have boundaries Physical Boundaries Skin, hull, surface, volume Conceptual Boundaries  'Crisp' well defined e.g. sphere, cube, plane  'fuzzy' e.g. a river, a chemical process, deformation

Objects have states and behaviour State : Static properties - inherit features or characteristics.  e.g. height, volume, width, State: Dynamic values - assigned to a feature  e.g. colour, level of detail, position Behavior is how an object acts and reacts An object may support 'services' that are invoked when a message is sent an object may invoke 'services' in other objects by passing a message to the object

What is an Object color draw() erase() move() getColor() setColor() Identity state behaviour Object Shape

Key OO Concepts There are 4 'pillars' or key concepts to OO these are  Abstraction  Encapsulation  Modularity  Hierarchy

Abstraction Two abstract conceptualisations of a PC  An Intel based processor with peripherals  A box that runs programs “An abstraction denotes the essential characteristics of an object that distinguishes it from all other kinds of objects and thus provides crisply defined conceptual boundaries, relative to the perspective of the viewer”

Encapsulation An Object supports  'Visible' services defined in an interface.  'Hidden' state defined inside the object The state of an object may only be accessed through the services supported in the object interface. However as we shall see when we look at the code we can ignore this (and usually do in graphics programming as we need speed) 'Encapsulation serves to separate the conceptual interface of an abstraction and its implementation '

Modularity An object is a data centred module, with encapsulated state and behavior that operates on that state. Objects communicate through message passing.  This raises the following issues : Deriving objects from the problem domain building 'well-engineered' objects Connecting objects in a meaningful way

Hierarchy Hierarchy is a ranking and ordering of abstractions Two hierarchical schemes in OO :  'Is part of' - the object is a part of a larger object e.g. Engine is part of a car  'Is a' is also known as 'inheritance' we can see this when looking at the hyper- graph in Maya showing how all the different objects in a scene are connected together (especially when using groups).