Operator Overloading What is operator overloading Most predefined

Operator Overloading What is operator overloading? • Most predefined operators (arithmetic, logic, etc. ) in C++ can be overloaded when applied to objects of a class by providing an operator function. Examples of those that can not be overloaded are : : , . , and ? : • Operators keep their predefined precedence and associatively class X{public: X operator -(); // unary -ve operator invoked by X a; -a; X operator ++(); // prefix increment invoked by X a; ++a; X operator ++(int); // postfix increment, X a; a++; X operator -(X&); // binary -, X b; a - b ; X operator +(X&); // add operator, a + b; // The return type X is needed to cascade operators // global functions can also be used to overload operators on objects

Operator Overloading A simple example: class Complex{ double real, imag; public: Complex(double r=0. 0, double I=0. 0){real = r; imag = I; } Complex operator ++(){real += 1. 0; imag += 1. 0; return *this; } Complex operator +(Complex& c){ Complex temp; temp. real = real + c. real; temp. imag = imag + c. imag; return temp; // this invokes the default copy constructor // can also be implemented in one line as return Complex(real + c. real , imag + c. imag); } }; main(){ Complex c 1(1. 5, 2. 0), c 2(1. 2, 2. 2), c 3; c 3 = ++c 1 + ++c 2; //why the ++ functions must return a Complex data type

Operator Overloading What happens when the assignment exists as in operators +=, -=, *= assume we will add the following member functions to class Complex& operator +=(Complex&); Complex& operator -=(Complex&); Complex& operator *=(Complex&); Complex& Complex: : operator +=(Complex& a){ real += a. real; imag += a. imag return *this; } Complex& Complex: : operator *=(Complex& a){ real = real*a. real - imag * a. imag; // there is an error here imag = real * a. imag + imag * a. real; return *this; }

Operator Overloading The indexing operator [] for an array class Vector{ float *p; int size; public: Vector(int n){ p = new float[n]; size=n; } ~Vector() {delete p; } float& operator [](int I){return p[I]; } Vector operator +(const Vector & a){ Vector temp(size); for(int I=0; I<size; I++) temp. p[I] = p[I] + a. p[I]; return temp; // this will destroy the resulting Vector why? // a default copy constructor will be used to copy the attributes p and size // only // To remedy this, a special copy constructor must be defined as follows, Vector(Vector& c){p = new float[c. size], size = c. size; for(int I=0; I<size; I++) p[I] = c. p[I]; }// this will form a new dynamic object for the result

Operator Overloading The assignment operator Although the assignment operator is supported by a default copy operator, the following example will produce a Null pointer assignment fun(){ Vector x 1(20), x 2(10); x 2 = x 1; // now x 2. p and x 1. p have the same address, size = 20 }// the destructor function will be called once for x 1 and once for x 2 // upon function exit, the same array will be deleted twice The following assignment operator function is needed to remedy this Vector& operator = (const Vector& a){ if(this != &a){// check that this is not x=x delete p; // clean up p = new float [size = a. size]; for(int I =0; I<size; I++) p[I] = a. p[I]; } return *this; } // every class should contain a copy constructor and an assignment operator

Operator Overloading Non-member operator functions used to define operations between different types of objects e. g. Vector operator * (const Matrix& a, const Vector& b){ Vector temp; for(int I = 0; I < a. no_rows; I++) for(int j = 0; j < a. no_cols; j++) temp[I] = a[I][j] * b[I]; return temp; } The above function should be declared as friend function in both class matrix and class vector as follows, class Matrix{…friend Vector operator * (const Matrix&, const Vector&); } class Vector{…friend Vector operator * (const Matrix&, const Vector&); }

Operator Overloading the stream input/output operators >> and << this is needed to apply these operators to the instances of objects e. g. , Complex x(0. 0, 0. 0); cin >> x; cout << x; // this needs the following functions which input or outputs complex numbers as (flt. pt , flt. pt. ) istream& operator >> (istream& input, const Complex& a){ double re=0, im=0; char c; input >> c; // input a character in c if(c == ‘(‘ ){ input >> re >> c; // input real part followed by , if ( c == ‘, ’ ) {input >> im >> c; // input imag part followed by ) if ( c == ‘)’ ){ a = Complex(re, im); return input; }}} cout << “error in input format”; } ostream& operator << (ostream& output, const Complex& a){ return output << ‘n’ << ‘(‘ << a. real << ‘, ’ << a. imag << ‘)’ << ‘n’; }// the above functions must be declared as friends in class Complex