How do I make my app calculate 1+1 in Function.cpp and then display the result using classes?

I would ultimately like to create a simple dialog-based app that does a simple calculation, like adding 1+1, and  displays the result in a text box.  The caveat is, that I would like the calculation performed in a separate .cpp file (Functions.cpp) so that I can isolate the calculation part of the program from everything else.  So, I did the following:

1.   Created an MFC dialog-based application in VS2008 called "FirstApp".  The files FirstApp.cpp and FirstAppDlg.cpp were created (along with their respective header files).  

2.  Using the Resource View tab, I inserted a button dialog called IDC_BUTTON1.  This is the button to be pushed.

3.  Then I Inserted an edit control dialog called IDC_EDIT1. This is where the results of 1+1 is to be displayed.  Added a variable (value) called m_strEdit1 of type String to the control.

4. I created Functions.cpp and Functions.h (no code in there yet)

5. Double clicked on the button dialog and inserted the code below

Question: How do make it so that I when I click on Button1 the program goes to Function.cpp to calculate 1+1..and then displays the result "2" in the Edit Control?  I plan on adding more complex calculation functions and using more variables I would like to use classes in the solution.  
// FirstAppDlg.cpp
void CFirstAppDlg::OnBnClickedButton1()
 UpdateData( TRUE ); // get text from edit control into 'm_strEdit1'
 m_strEdit1 = intToString( addInts( 1, 1 ) );
 UpdateData( FALSE ); // put the text from 'm_strEdit1' to the edit control

// function.h
#pragma once

int      addInts( int x, int y );
CString  intToString( int x );

// function.cpp
#include "stdafx.h"
#include "function.h"

int addInts( int x, int y )
 return x + y;

CString intToString( int x )
 CString strText;
 strText.Format( "%d", x );
 return strText;

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sarabandeConnect With a Mentor Commented:
if you want classes, you may add

class Calculator
     int add(int operand1, int operand2);


to functions.h.

implement Calculator::add in functions.cpp

then you would use it like

void OnBtnClicked()
    // get both values to add and convert to integer

   Calculator calc;
   int r = calc.add(operand1, operand2);

   // convert r to string and show it on screen

shaolinfunkAuthor Commented:
Many, many thanks!  I see what you did there....and I got my program to compile on the 1st try.  Thanks for helping me create my first working class!
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