【运算符重载】类型转换运算符

class Complex {
     double real, image;
public:
    Complex(...) {}
    operator double () { return real; }
}
main() {
     Complex c(1.2, 3.4);
    cout << (double)c << endl; // 输出 1.2
    double n = 2 + c; // 等价于 double n = 2 + c.operator double()
    cout << n; // 输出 3.2
}