I am not able to use a friend function properly - c++

Question

I am writing a code to declare a member function of a class as a friend, but I am getting errors.
The code is
#include<iostream>
class Vect;
class Coordinate {
float x;
float y;
public:
Coordinate(float a,float b):x(a),y(b){}// constructor
Coordinate():x(0),y(0){};//constructor
display()
{
std::cout<<"\nx:"<<x<<" "<<"y:"<<y;
}
friend Vect::add(Coordinate B);
};
class Vect {
public:
add(Coordinate A)
{
std::cout<<A.x;
}
};
The exact errors are
invalid use of incomplete type 'class Vect'| and forward declaration
of 'class Vect'|

Answer 1

class Y; //forward declaration of class Y
class X
{
public:
int getmark(Y);
};
class Y
{
int mark;
public:
Y() {}
friend int X::getmark(Y);
};
int X::getmark(Y obj) {
cin >> obj.mark;
return obj.mark;
}
int main()
{
X a;
Y b;
a.getmark(b);
}
At first, when the object a (class X) is created, a forward declaration of class Y is necessary in order to declare the Y argument to X::getmark().
Creating object b (class Y) wont be a problem as the compiler knows class X exists (for the friend function).
Then, simply call the function getmark() through the object a.
Note: It is necessary to declare the function getmark() after the declaration of class Y, or else compiler will consider the class Y as an incomplete type.

Answer 2

you can use this :
class Vect;
class Coordinate
{
public:
Coordinate(float a = 0f, float b = 0f) :x(a), y(b) {}// constructor
float x;
float y;
void display()
{
std::cout << "\nx:" << x << " " << "y:" << y;
}
friend int add(Coordinate B);
};
class Vect
{
friend int add(Coordinate B);
public:
};
int add(Coordinate B)
{
std::cout << B.x << std::endl;
return B.x;
}