有如下程序： #include ＜iostream＞ using namespace std; class Complex { double re, im, public: Complex(double r, double i): re(r), im(i) {} double real() const {return re;} double image() const {return im,} Complex& operator +=(Complex a) { re +=a.re; im +=a.im; return *this; } }; ostream& operator ＜＜ (ostream& s, const Complex& z) { return s＜＜'('＜＜z.real()＜＜','＜＜z.image()＜＜')'; } int main() { Complex x(1,-2), y(2,3); cout ＜＜ (x+=y) ＜＜ endl; return 0; } 执行这个程序的输出结果是( )。