Object Oriented programming in C Classes as units of encapsulation Information Hiding Inheritance polymorphism and dynamic dispatching Storage management multiple inheritance Classes Encapsulation of type and related operations class point double x y private data members public point int x0 int y0 public methods point x 0 y 0 a constructor void move int dx int dy void rotate double alpha int distance point p A class is a type objects are instances point p1 10 20 call constructor with given arguments point p2 call default constructor Methods are functions with an implicit argument p1 move 1 1 special syntax to indicate object in other languages might write move p1 1 1 special syntax inspired by message passing metaphor objects are autonomous entities that exchange messages Implementing methods No equivalent of a body each method can be defined separately void point rotate double alpha x x cos alpha y sin alpha y y cos alpha x cos alpha x and y are the data members of the object on which the method is being called if method is defined in class declaration it is inlined Constructors One of the best innovations of C special method s invoked automatically when an object of the class is declared point int x1 int x2 point point double alpha double r point p1 10 10 p2 p3 pi 4 2 5 Name of method is name of class Declaration has no return type The target of an operation The implicit parameter in a method call can be retrieved through this class Collection Collection insert thing x return reference modify data structure return this to modified object my collection insert x1 insert x2 Static members Need to have computable attributes for class itself independent of specific object e g number of objects created Static qualifier indicates that entity is unique for the class static int num objects 0 point num objects ditto for other constructors Can access static data using class name or object name if point num objects p1 num objects error Classes and private types If all data members are private class is identical to a private type visible methods including assignment A struct is a class with all public members How much to reveal is up to programmer define functions to retrieve not modify private data int xcoord return x int ycoord return y p2 x 15 error data member x is private Destructors If constructor allocates dynamic storage need to reclaim it class stack int contents int sz public stack int size contents new int sz size void push int pop int size return sz stack my stack 100 allocate storage dynamically when is my stack contents released If constructor uses resources class needs a destructor User cannot deallocate data because data member is private system must do it stack delete contents inventive syntax negation of constructor Called automatically when object goes out of scope Almost never called explicitly Copy and assignment point p3 10 20 point p5 p3 componentwise copy This can lead to unwanted sharing stack stack1 200 stack stack2 stack1 stack1 contents shared stack2 push 15 stack1 is modified Need to redefine assignment and copy Copy constructor stack const stack s reference to existing object contents new int sz s size for int I 0 I sz I contents I s contents I stack s1 100 stack s2 s1 invokes copy constructor Redefining assignment assignment can also be redefined to avoid unwanted sharing operator returns a reference so it can be used efficiently in chained assignments one two three stack operator const stack s if this s beware of self assignment delete contents discard old value contents new int sz s size for int I 0 I sz I contents I s contents I return this stack s1 100 s2 200 s1 s2 transfer contents Anomalies An array whose component type is a class can only be declared if there is a parameterless constructor for the class There is no way to pass parameters to the constructor polygon point 10 ok turing stack 2 illegal