CSC 143F 1 CSC 143 Constructors Revisited

CSC 143F 2 Constructors In C++, the constructor is a special function automatically called when a class instance is declared  Constructor’s name is class name  No explicit return type, not even void...

CSC 143F 3 Multiple Constructors  May be several reasonable ways to initialize a class instance  Multiple constructors  All have same name (name of class)  Distinguished by number and types of arguments  Example of “overloading” (more later)

CSC 143F 4 Multiple Constructors (review) //complex.h--simple complex number class class Complex { public: // constructors: Complex( );// = 0+0i Complex(double a, double b); // = a+bi // double -> Complex constructor Complex(double r);// = r+0i... };

CSC 143F 5 Default parameters //complex.h--simple complex number class class Complex { public: // combine all previous constructors // in one single constructor // Complex( );// = 0+0i // Complex(double r);// = r+0i Complex(double a=0, double b=0); // Complex() means Complex(0,0) // Complex(3.2) means Complex(3.2,0) };

CSC 143F 6 Default Constructor  If no explicit constructor is given, a default is supplied by compiler  Takes no arguments, does nothing  Not guaranteed to perform any initialization  Invisible  If a class has one or more “non-default” constructor:  then NO “default” constructor will be supplied; variable declaration without initialization fails.

CSC 143F 7 Guidelines for Constructors  A constructor cannot return a value  so it must be declared without a return type  A class may provide multiple constructors  Compiler will choose appropriate one, depending on context.  Syntax for invoking a constructor Complex a1; Complex a3 = Complex(123.45); Complex a2(10.0, -1); But not this: Complex a4();

CSC 143F 8 CSC 143 Overloading [Chapter 8, pp ]

CSC 143F 9 Name Scope and Visibility  Variable and function names may be repeated in different scopes int myVar; void Snork(int myVar) {... while ( a > b){ int myVar = 0;... } }  Style issue: using same name in multiple scopes can be confusing

CSC 143F 10 Overloading  Reusing names  Function names can be reused  Differentiate based on scope and context Global vs. class vs. local, etc.  Different functions in same scope can have same name if argument number and/or types different  Name is said to be overloaded  Operators (+, =, etc.) can also be overloaded (ex. int+int, double+double, Complex+Complex)

CSC 143F 11 Overloaded Function Names  Useful if one name has multiple meanings or multiple interfaces  Constructors are common example of overloading since all must have same name  Compiler determines which function to call at compile-time

CSC 143F 12 Resolving Overloaded Functions To "resolve" mean to decide which version of the overloaded function is being called  Determined by matching actual arguments against possible formal arguments  Compiler gives error if not exactly one matches  Complete matching algorithm rather complex  If match is not exact, automatic type conversions are used

CSC 143F 13 Matching Algorithm  Function declarations void Snark(int); void Snark(double); void Snipe(char []); void Snipe(double); void Sneep(char); void Sneep(double); Snark(1);// Integer Snark Snipe(1);// Double Snipe Sneep(1);// Ambiguous

CSC 143F 14 Example of Resolving  Function declarations void PrintData(int data) { cout << "int = " << data << endl; } void PrintData(char data) { cout << "char = '" << data << "'\n"; } void PrintData(double data) { cout << "double = " << data << endl; }  Which calls are valid? PrintData(3); PrintData("Hello"); PrintData( ); PrintData('m');

CSC 143F 15 Overloaded Operators  For convenience, can define functions named +, -, *, =, /, ==, etc. on classes  Gives natural expression to some operations  Very confusing if abused  Operator functions may be members or friends  Function “names” are operator+, operator-,...  Several ways of calling them