Vs2013编程友元函数无法访问类私有成员问题

今天学了重载运算符，然后我编写了如下代码：
#include
using namespace std;
class Complex
{
public:
Complex(double i = 0.0, double j = 0.0) :real(i), imag(j){}
friend Complex operator+ (const Complex &c1, const Complex &c2);
friend Complex operator- (const Complex &c1, const Complex &c2);
friend ostream & operator<< (ostream &cout, const Complex &c);
private:
double real;
double imag;
};

Complex operator+ (const Complex &c1, const Complex &c2)
{
return Complex(c1.real + c2.real, c1.imag + c2.imag);
}
Complex operator- (const Complex &c1, const Complex &c2)
{
return Complex(c1.real - c2.real, c1.imag - c2.imag);
}
ostream & operator<< (ostream &cout, Complex &c)
{
cout << c.real << "," << c.imag;
return cout;
}

int main()
{
Complex c1(5, 4), c2(2, 10), c3;
cout << "c1 = " << c1 << endl;
cout << "c2 = " << c2 << endl;
cout << "c3 = " << c3 << endl;
c3 = c1 + c2;
cout << "c3 = " << c3 << endl;
return 0;
}
然后调试一直通不过，说是不能访问私有成员，但是那不是友元函数么？怎么就不能了！！！
求大神解答……

函数头：
ostream & operator<< (ostream &cout, Complex &c)
应该改成：
ostream & operator<< (ostream &cout, const Complex &c)

定义与实现参数类型不一致。 实现里const string st2应该是const string& st2