初学c++，有一句代码看不明白，请大牛指点
class complex{
double re,im;
public:
complex(double r,double i): re(r),im(i) {}
complex operator + (complex);
complex operator * (complex);
};
class complex{
double re,im;
public:
complex(double r,double i): re(r), im(i) {}
//构造函数初始化列表 类似与
// complet(double r, double i) //这函数的功能和上面是一样的，
//但是在某些情况下必须要用初始化列表
{
re = r;
im = i;
}
complex operator + (complex);
complex operator * (complex);
