Question

I am not sure if I am doing the overload correctly.

…\point.h(42) : error C2061: syntax error : identifier ‘Vec3’
Point operator +(Vec3 a) const;

Here is my .h file:

    #include <fstream>
    #include <iostream>
    #include "vec3.h"

    using std::ofstream;
    using std::ifstream;
    using std::cout;
    using std::cin;
    using std::endl;
    using namespace std;

    #ifndef POINT_H
    #define POINT_H

    //Define ourselves a basic 2D point class
    class Point
    {

        friend ofstream& operator <<(ofstream& output, const Point& p);
        friend ifstream& operator >>(ifstream& input, Point& p);
\    
        public:

        Point();
        Point(double _x, double _y);
        Point(double _x, double _y, double _z);

        double x,y,z;

        //Operators
        Point operator -(Point a) const;
        Point operator /(double s) const;
        Point operator *(double s) const;

        // Used to do vector point addition
        Point operator +(Vec3 a) const;

    };

    #endif

Here is my .cpp file

#include "point.h"

Point::Point(double _x, double _y, double _z)
{
    x= _x;
    y= _y;
    z= _z;
}

Point :: Point()
{
    x = 0.0;
    y = 0.0;
    z = 0.0;
}

Point::Point(double _x, double _y)
{
    x= _x;
    y= _y;
    z= 0;
}

    Point Point::operator -(Point a) const
    {
        return Point(x-a.x, y-a.y, z-a.z);
    }

    Point Point::operator /(double s) const
    {
        return Point(x/s, y/s, z/s);
    }

    Point Point::operator *(double s) const
    {
        return Point(x*s, y*s, z*s);
    }


// Vector Point Addition
    Point Point::operator +(Vec3 a) const
    {
        return Point(x+a.x, y+a.y, z+a.z);
    }

    ofstream& operator <<(ofstream& output, const Point& p)
    {
        output << p.x << " " << p.y << " " << p.z << "\n";
        return output;
    }

    ifstream& operator >>(ifstream& input, Point& p)
    {
        input >> p.x >> p.y >> p.z;
        return input;
    }

Here is the Vec3.h

    #ifndef VEC3_H
#define VEC3_H

#include "point.h"

class Vec3
{
    friend ofstream& operator <<(ofstream& output, const Vec3& p);
    friend ifstream& operator >>(ifstream& input, Vec3& p);

    public: 
    Vec3();
    Vec3(double _x, double _y);
    Vec3(double _x, double _y, double _z);

    double x,y,z;

    //Operators
    Vec3 operator -(Vec3 a) const;
    Vec3 operator /(double s) const;
    Vec3 operator *(double s) const;

    // Used to do vector Vec3 addition
    Vec3 operator +(Vec3 a) const;
    Point operator +(Point a) const;

};

#endif

Answer 1

vec3.h includes point.h and point.h includes vec3.h. You need to remove the circular dependency, by forward declaring one of the classes.