Question

I have been trying to do forward declaration to allow access between classes. I have read here that:

• I can’t include the “a.h” file when forward declaring A in b.h

I haven’t been able to find very much about namespaces during forward declaration. And I’m fairly certain I am messing this up (I just don’t know where to put them). The errors I get are after the relevant code snippets.

This is what I did:

• I have split my class definitions from all being in .h to .cpp and .h

• I have #ifndef guards in my .h files

These are my files:

Form1.cpp

#include "stdafx.h"
#include "OpenGL.h"
#include "Form1.h"
#include "serialcom.h"
#include "calculations.h"

using namespace GUI_1;

GUI_1::Form1::Form1(void)
    {....
    }
void GUI_1::Form1::chlabel2(float num)
    {....
    }
int GUI_1::Form1::updateHand(int source){....}
void GUI_1::Form1::resetHand(){....}

Errors for Form1.cpp It’s the same thing for every definition

error C2872: 'GUI_1' : ambiguous symbol
could be 'GUI_1'
or       OpenGLForm::GUI_1'

Form1.h

#ifndef form1
#define form1

using namespace OpenGLForm; 
//error C2871: 'OpenGLForm' : a namespace with this name does not exist
ref class COpenGL;

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;

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

    public ref class Form1 : public System::Windows::Forms::Form
    {
    public:
        OpenGLForm::COpenGL^ o_gl;
            // error C2653: 'OpenGLForm' : is not a class or namespace name
        Form1(void);
        void chlabel2(float num);

    protected:
        ...
...};}

OpenGL.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

....

namespace OpenGLForm 
{
    using namespace System::Windows::Forms;
    using namespace GUI_1;
    // error C2871: 'GUI_1' : a namespace with this name does not exist

    ref class GUI_1::Form1;
    // error C2653: 'GUI_1' : is not a class or namespace name
    // 'Form1' uses undefined class 'OpenGLForm::GUI_1'

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

        Form1^ form1;
            // error C2059: syntax error : ';'
            // error C2238: unexpected token(s) preceding ';'
            // error C2143: syntax error : missing ';' before '^'

        ...
};
}
#endif

OpenGL.cpp – no errors here

#include "stdafx.h"
#include "OpenGL.h"
#include "Form1.h"

OpenGLForm::COpenGL::COpenGL(){};
... other functions that go the same way

GUI_1.cpp – the main function

#include <vcclr.h>
#include <stdio.h>
#include <stdlib.h>
#include "Form1.h"
#include "calculations.h"
#include "serialcom.h"
#include "OpenGL.h"

using namespace GUI_1;
using namespace OpenGLForm;

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

    GUI_1::Form1 ^form1 = gcnew GUI_1::Form1();
    // error C2059: syntax error : '='
    OpenGLForm::COpenGL ^open_gl = gcnew OpenGLForm::COpenGL();

    form1->o_gl = open_gl;
    // error C2143: syntax error : missing ';' before '->'

    open_gl->form1 = form1;
    // error C2059: syntax error : '='

    return 0;
}

I am going to keep trying to decypher these messages, but I’d appreciate any help meanwhile.

Answer 1

• In OpenGL.h, you need to forward-declare Form1 in the correct namespace:

  namespace GUI_1 {
  ref class Form1;
  }

• And forward-declare COpenGL in Form1.h the same way:

  namespace OpenGLForm {
  ref class COpenGL;
  }

Important: Make sure these declareations are outside of other namespace blocks, and remove your existing forward-declarations from inside the classes.

• In Form1.cpp, it would be clearer to define the member functions within a namespace block:

  namespace GUI_1 {
  Form1::Form1(void)
  …
  }

• The two .cpp files include Form1.h and OpenGL.h in different orders. It would be better to only include Form1.h, and have Form1.h include OpenGL.h.