Question

Edit: new files. I’m having troubles accessing a public function in the Form1 class. I get identifier not found when I try to use it.
Form1:

#pragma once

#include "OpenGL.h"
#include "serialcom.h"
#include "calculations.h"

namespace GUI_1 {

    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 OpenGLForm;


    /// <summary>
    /// Summary for Form1
    /// </summary>

    public ref class Form1 : public System::Windows::Forms::Form
    {
    public:
        Form1(void)
        {
            InitializeComponent();
            OpenGL = gcnew COpenGL(this->panel4, this->label16, 785, 530);
        }
        void changelabel2(float num)
        {
            label2 -> Text = " " + num;
        }
    protected: ...

OpenGL.h:

#include "stdafx.h"

#ifndef opengl
#define opengl

#pragma comment(lib, "opengl32.lib")
#pragma comment(lib, "glu32.lib")

#include <windows.h>
#include <GL/gl.h>
#include <GL/glu.h>
#include <GL/glut.h>
#include <math.h>

// Declare globals
...

using namespace System::Windows::Forms;

namespace OpenGLForm 
{
    public ref class COpenGL: public System::Windows::Forms::NativeWindow
    {
    public:

        COpenGL(System::Windows::Forms::Panel ^ parentForm, System::Windows::Forms::Label ^ lbl, GLsizei iWidth, GLsizei iHeight)
        {
            CreateParams^ cp = gcnew CreateParams;

            c_p_v v1, v2;
            changelabel2(189); 
...

So that doesn’t work (above, in “changelabel2”). Perhaps because I’m not using class name?

This is my main:

#include "stdafx.h"
#include <string.h>
#include <iostream>
#include <stdio.h>
#include < vcclr.h >
#include < stdio.h >
#include < stdlib.h >
#include < vcclr.h >    
#include "Form1.h"
#include "calculations.h"
#include "serialcom.h"

using namespace GUI_1;

[STAThreadAttribute]
int main(array<System::String ^> ^args)
{
    // Enabling Windows XP visual effects before any controls are created
    Application::EnableVisualStyles();
    Application::SetCompatibleTextRenderingDefault(false); 


    // Create the main window and run it
    Form1^ form = gcnew Form1();
    Application::Run(form);

    return 0;
}

Calling form.changelabel2 doesn’t work either.

Answer 1

It looks like you have a circular dependency between OpenGL.h and Form1.h

Try removing the #include "Form1.h" if you can, or converting it to a forward declaration like class Form1;

Also, be careful when using using namespace in headers as it will pollute the namespace of any file it is subsequently included in.