Question

I have the code:

// class declaration
class Vector3D;

// class declaration and definition
class Point3D { 
    // ...

    // function declaration (only needs class declarations)
    Point3D operator+(const Vector3D &);
};

// class definition
class Vector3D {
    // ...
};

// function definition (needs class definitions)
inline Point3D Point3D::operator+(const Vector3D &vector) {
    // ...
}

But I get errror:
‘Graphic::Point3D::operator +’ : redefinition; different type modifiers

Answer 1

The code in your question is well-formed. The Visual C++ 2012 Release Candidate accepts the code without error (I mention this because the text of your error is identical to that of Visual C++ error C2373).

Either your compiler has a bug, or the code you present in your question is not the same as the code you are compiling.

---

In any case: an operator+ does not need to be a member function. It would be simpler to use a nonmember function (or two, to handle different operand orderings):

Point3D operator+(Point3D  const& lhs, Vector3D const& rhs);
Point3D operator+(Vector3D const& lhs, Point3D  const& rhs);

If you do keep your operator+ member function(s), they should be const-qualified so that they can be called with a const-qualified left-hand argument:

Point3D operator+(const Vector3D &) const;
                                    ^ const required