12.5 MEMBER-FUNCTION OVERLOAD DEFINITIONS FOR UNARY OPERATORS

Paralleling our discussion for binary operators in Section 12.3, the following example shows a member-function overload definition for ‘-’ as a unary operator.

 
 //OverloadUnaryMemb.cc #include <iostream> using namespace std; class MyComplex {     double re, im; public:     MyComplex( double r, double i ) : re(r), im(i) {}     MyComplex operator-() const;     friend ostream&operator<< ( ostream&, MyComplex&); }; //Member-function overload definition for "-""-" MyComplex MyComplex::operator-() const {                       //(A)     return MyComplex( -re, -im ); } //This overload definition has to stay global ostream&operator<< ( ostream&os, const MyComplex&c ) {     os << "(" << c.re << "," << c.im << ")" << endl;     return os; } int main() {     MyComplex c(3, 4);     MyComplex z = -c;                                          //(B)     cout << z << endl;                 // (-3, -4)     return 0; }

Note that the following statement in line (B) of the program

      MyComplex z = -c;

is translated by the compiler into

      MyComplex z = c.operator-();

which makes c the invoking object. Thus the values of re and im available inside the definition of the operator- function in line (A) would then correspond to the MyComplex number c.