求教高手,下面的程序哪错了

#include <iostream.h>
class complex{
int re,im;
public:
complex (int r=0,int i=0){ //写上默认参数才可这样定义complex c;
re=r;im=i;}
complex operator *=(complex &a);
void show(){
cout<<re<<"+"<<im<<"i"<<endl;}
};

complex complex::operator *=(complex &a){ //compelx complex
complex c;
c.re=re*a.re-im*a.im;
c.im=re*a.im+im*a.re;
return c;
}

void main(){
complex a(1,2);
complex b(2,3);
b*=a;
b.show();
}