求助：友元函数和运算符重载

#include<iostream>
using namespace std;
class complex{
private:
     double real;
     double imag;
public:
     complex();
     complex(double x,double y);
     void operator~()const;
     void operator+(complex & num)const;
     void operator-(complex & num)const;
     void operator*(complex & num)const;
     friend ostream & operator<<(ostream & os,complex & num)const;
     friend istream & operator>>(istream & is,complex & num);
     friend void operator*(double n,complex & num);
};
int main()
{
     complex a(3.0,4.0);
     complex c;
     cout<<"Enter a cimplex number (q tu quit):\n";
     while(cin>>c)
     {
         cout<<"c is "<<c<<'\n';
         cout<<"complex conjugate is "<<~c<<'\n';
         cout<<"a is "<<a<<'\n';
         cout<<"a + c is "<<a+c<<'\n';
         cout<<"a - c is "<<a-c<<'\n';
         cout<<"a * c is "<<a*c<<'\n';
         cout<<"2 * c is "<<2*c<<'\n';
         cout<<"Enter a cimplex number (q tu quit):\n";
     }
     cout<<"Done\n";
     return 0;
}
complex::complex()
{
     real=0;
     imag=0;
}
complex::complex(double x,double y)
{
     real=x;
     imag=y;
}
void complex::operator~()const
{
     complex b(real,-imag);
     cout<<b;
}
void complex::operator+(complex & num)const
{
     cout<<complex(real+num.real,imag+num.imag);
}
void complex::operator-(complex & num)const
{
     cout<<complex(real-num.real,imag-num.imag);
}
void complex::operator*(complex & num)const
{
     cout<<complex(real*num.real-imag*num.imag,real*num.imag+imag*num.real);
}
ostream & operator<<(ostream & os,complex & num)const
{
     os<<'('<<num.real<<'，'<<num.imag<<"i)";
     return os;
}
istream & operator>>(istream & is,complex & num)
{
     cout<<"real:";
     is>>num.real;
     cout<<"imag:";
     is>>num.imag;
     return is;
}
void operator*(double n,complex & num)
{
     cout<<complex(n*num.real,n*num.imag);
} 基本上所有的错误都指向重载的<<符号
我这是哪里错了？