Part 3. Why do we need both of them? The object-oriented programming paradigm (OOP) Two kinds of object. Important Special Kinds of Member Function

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Miles Crawford
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1 Part 3 The object-oriented programming paradigm (OOP) Two kinds of object Value objects The object contains the member data items Allocated automatically just like primitive (built-in) data items Suitable for elementary data types Language support: Not supported by Java C# declares with struct C++ declares with class Reference objects The object contains the location (address) of the member data items Allocated by invoking the new operator Suitable for composite data types Language support Only option in Java C# declares with class C++ declares with class Why do we need both of them? Choice between class & struct Largely an issue of efficiency when you copy or move an object: Small (value) objects can be moved quickly Large (reference) objects may not have to be moved; just copy the reference Important Special Kinds of Member Function Constructor: A function used to create new objects (instances) of the class, usually by initializing the member data items Accessor: A function that gives users access to an encapsulated data item Most of them are read accessors Write accessors (modifiers) are needed less often Overloaded operator: A function that extends the meaning of an existing operator (+ - * / == < > etc) to apply to one or two objects of the class COMP 170, Fall, copyright Conrad Weisert

2 C# Overloaded functions Two or more functions declared with the same name but different parameter types ("signatures") are said to be overloaded void sort (long [], short N); void sort (float [], short N); void sort (float [], short LB, short HB); short n = 100; float[n] x; What good is this capability? sort (x, n); Which function does this invoke? C# Overloaded member functions Similarly member functions ( methods ) having the same name in two or more classes Employee x, y; Customer z; if (xidentifier() == zidentifier()) The bad news: Except for the assignment (=) operator and the member selection () operator, C# knows nothing about applying operators to objects of a struct Complex x, y, z; illegal operation! z = x + y; The good news: In C++ and C# (but not Java) you can define ("overload") a class-specific meaning for most operators eg once you define overloaded + for Complex, the above is legal The usual operator syntax applies when an overloaded operator is used or invoked: x y for a binary operator y for a unary operator However, to declare or define an overloaded operator we use function notation C# understands that a function name of the form operator defines such an operator The compiler then considers x + y to be a shorthand for either of these invocations operator+(x,y) // a non-member function xoperator+(y) // a member function COMP 170, Fall, copyright Conrad Weisert

3 Example (inside the Complex class definition): public static // Non-member version Complex operator+ (Complex rs, Complex rs) {Complex result; resultrp = lsrp + rsrp; resultip = lsip + rsip; return result; } Complex x, y, z; z = x + y; Overloaded assignment Unlike the other operators, C# provides a default interpretation of assignment between objects of a class: member by member assignment (Why?) Thus: Complex x, y; y = x; gives the expected result even if we don't define operator= explicitly Might the default assignment operator ever give the wrong result? When? Compound assignment operators In C++ we still must define the compoundassignment operators (+=, etc), even though the appropriate interpretation is obvious But C# figures that out, and we only need define the simple operators There may be an efficiency problem, however, but we won't worry about that now Defining an operator: member or non-member? We can define most overloaded operators either as member functions or as non-member functions Thus we may declare: public Vector operator+(vector rs) inside the Vector class definition, or declare: public static Vector operator+(vector ls, Vector rs) anywhere COMP 170, Fall, copyright Conrad Weisert

4 Defining an operator: restrictions Operators are limited to those already defined in C++ We may not invent new syntax, such as a ** or operator We can't change the operator precedence rules Thus, in the unparenthesized expression a + b * c the * operation is always done before the +, no matter how we may define operator* and operator+ Constructor functions to initialize an object (C++, Java, and C#) Have the same name as the class Have no return value (not even void) Can assign values (usually derived from the function parameters) to the private data members Can allocate dynamic memory using new (and usually set private member to its address) or obtain other resources Constructor invocation: 4 cases (C++) 1 Declaring objects of the class: Complex x(10,10), y(10), z(x); (Presumably 3 constructors have been defined) 2 Declaring and initializing: Complex y = 10, z = x; (Just a notational variant of the second and third above) Constructor invocation 3 Explicit conversion (unnamed object): z = x + Complex (10, 10); 4 Implicit conversion (from a type used in a 1-parameter constructor) float w; Complex y; y = w; What happens here? COMP 170, Fall, copyright Conrad Weisert

5 Special kinds of C# constructor function Default constructor: no parameters: Complex (); invoked in declaration: Complex x; Copy constructor: takes (reference to) an object of the same class: Complex (Complex& x); The compiler will supply both by default (but they may not do what you want) The bad news We now know many of the C# facilities we need to define a new class But the three fundamental types demand very different approaches to class definition Textbooks, articles, and class libraries often fail to make the distinction Much emphasis on container classes and GUI classes (Why?) We'll start with elementary classes, and as time permits, then go on to the others More next week COMP 170, Fall, copyright Conrad Weisert

3.Constructors and Destructors. Develop cpp program to implement constructor and destructor.

3.Constructors and Destructors Develop cpp program to implement constructor and destructor. Constructors A constructor is a special member function whose task is to initialize the objects of its class.