invalid use of incomplete type 'class Point'

为什么会出现这种情况

#include<iostream>
using namespace std;
class Point;
class Complex
{
    int real;
    int img;
    public:
        friend void Point::print(const Complex &m);
        Complex(int a,int b)
        {
            real=a;
            img=b;
        }
        void show()
        {
            cout<<real<<" "<<img; 
        }
};
class Point
{
    int x;
    int y;
    public:
        Point(int j,int k)
        {
            x=j;
            y=k;
        }
        void print(const Complex &m);
};
void Point::print(const Complex &m)
        {
            cout<<x<<" "<<y<<endl;
            m.show();
        }
int main()
{
    Complex c1(4,5);
    Point c2(2,4);
    c2.print(c1);
}

#include<iostream>
using namespace std;
class Complex;
class Point
{
    int x;
    int y;
    public:
        Point(int j,int k)
        {
            x=j;
            y=k;
        }
        void print(const Complex &m);
};

class Complex
{
    int real;
    int img;
    public:
        friend void Point::print(const Complex &m);
        Complex(int a,int b)
        {
            real=a;
            img=b;
        }
        void show() const
        {
            cout<<real<<" "<<img;
        }
};

void Point::print(const Complex &m)
{
    cout<<x<<" "<<y<<endl;
    m.show();
}

int main()
{
    Complex c1(4,5);
    Point c2(2,4);
    c2.print(c1);
}