Question

I’ve been looking for ways of solving this problem for quite some time now. What I have is a standard Form1.h with some global variables declared in it. I want to access the properties of this form from a function in a separate .cpp file. So here is how I try to accomplish this:

//Form1.h

#pragma once

#include "stdafx.h"
#include "test.h"


unsigned char vMAC1;
unsigned char vMAC2;
unsigned char vMAC3;
unsigned char vMAC4;
unsigned char vMAC5;
unsigned char vMAC6;


extern long pNum;

//ARP Variables

unsigned char gMAC1;
unsigned char gMAC2;
unsigned char gMAC3;
unsigned char gMAC4;
unsigned char gMAC5;
unsigned char gMAC6;

extern unsigned char mMAC1;
extern unsigned char mMAC2;
extern unsigned char mMAC3;
extern unsigned char mMAC4;
extern unsigned char mMAC5;
extern unsigned char mMAC6;

namespace Artemis_v {
    using namespace System;
    using namespace System::ComponentModel;
    using namespace System::Collections;
    using namespace System::Windows::Forms;
    using namespace System::Data;
    using namespace System::Drawing;
    using namespace System::Runtime::InteropServices;
    using namespace System::Threading;
    using System::IntPtr;
    /// <summary>
    /// Summary for Form1
    /// </summary>

    public ref class Form1 : public System::Windows::Forms::Form
    {
    public:
        Form1(void)
        {
            InitializeComponent();
            //
            //TODO: Add the constructor code here
            //
        }

    protected:
        /// <summary>
        /// Clean up any resources being used.
        /// </summary>
        ~Form1()
        {
            if (components)
            {
                delete components;
            }
        }

    protected: 

    ///// And so on just standard compiler-created statements..

    private: System::Void button1_Click(System::Object^  sender, System::EventArgs^  e) {
        ChangeText(this);
    }


// test.h

#ifndef TEST_H
#define TEST_H

namespace Artemis_v
{
    ref class Form1;
    void ChangeText(Form1 ^frm);
}

#endif


// test.cpp

#include "StdAfx.h"
#include "test.h"
#include "Form1.h"

namespace Artemis_v
{
    void ChangeFormText(Form1 ^frm)
    {
        frm->Text="Hello!";
    }
}

This code gives me LNK2005 Already defined errors and I know it’s because of my variables being redeclared when I include Form1.h in test.cpp. Can I find any workarounds to this problem or should I remove the variables?

Answer 1

One of your .cpp files should declare the variables without “extern” and your .h file should declare them with “extern”