How to add another Initialize function to base class but not edit base file?

Question

I am having trouble initializing the class, I want to extend it a bit. This is the baseclass:

basefile.cpp

Class Point3d
{
public:
    Point3d ();
    Point3d (double x, double y, double z);
    Point3d&   set (double x, double y, double z);
    double x, y, z;
}
Point3d :: Point3d (): x (0.0), y (0.0), z (0.0)
{
}
inline Point3d &
Point3d :: set (double xx, double yy, double zz)
{
     x = xx;
     y = yy;
     z = zz;
     return * this;
    }
.....

Edit

I want when declaring Point3D pt {1,1}, it means z = 0, so if I didn't edit the original file, how to add a default initialization function z = 0 on base class but from another file, it looks like this:

Point3d::Point3d(double xx, double yy, double zz=0) { x = xx; y = yy; z = zz; }
//or
Point3d :: Point3d (double xx, double yy)
{
     x = xx;
     y = yy;
     z = 0;
     return * this;
}

I thought of using a derived class, but what I wanted was to extend the base class because I wanted to use the base class directly, I searched the partial class as well as many different ways but still failed.

Answer 1

You can only add members to a class in its definition and there can be only one definition of a class.

Extending classes at a later point is done via inheritance as you note in your question.

If you want to have a function set in your class, you need to declare it in the class definition:

class Point3d
{
public:
    Point3d ();
    Point3d (double x, double y, double z);
    Point3d& set (double xx, double yy, double zz = 0);
    double x, y, z;
};

Also note that you don't need the second definition of set , since you are already using a default argument for z .

You also have a few syntax errors in your code. In particular set lacks return values in its function head.

Answer 2

You can't.

You must declare your member function, member method or constructor inside your class header to define it in another place.