单项选择题

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) { remm 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

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