Separating class code into a header and cpp file

The class declaration goes into the header file. It is important that you add the #ifndef  include guards. Most compilers now also support #pragma once. Also I have omitted the private, by default C++ class members are private.

// A2DD.h
#ifndef A2DD_H
#define A2DD_H

class A2DD
{
  int gx;
  int gy;

public:
  A2DD(int x,int y);
  int getSum();

};

#endif

and the implementation goes in the CPP file:

// A2DD.cpp
#include "A2DD.h"

A2DD::A2DD(int x,int y)
{
  gx = x;
  gy = y;
}

int A2DD::getSum()
{
  return gx + gy;
}