access variable by pressing address

Question

I'm a beginner and I want to use instance w of OlMainWindow in order to access _x by pressing address (OlMain->vref->_x=10;) through ui_MainWidow::setupUI() function, but when I compile this program then I get an error on line OlMain->vref->_x=10:

error: use of undefined type 'OlMainWindow'

Please help me fix this error.

ui_MainWidow.h

#ifndef UI_MAINWINDOW_H
#define UI_MAINWINDOW_H
#include <iostream>
#include "olvalue.h"

class OlValue;
class OlMainWindow;
class ui_MainWidow;

class ui_MainWidow {
public:

    void setupUI(OlMainWindow* OlMain) {
        OlMain->vref->_x=10; // _x = 10;
        OlMain->vref->_x = 10;
        std::cout << " ok";
    }
};
#endif //UI_MAINWINDOW_H

olvalue.h

#include <iostream>
#ifndef VALUE_H
#define VALUE_H

#include "ui_mainwindow.h"
class OlValue;
class OlMainWindow;
class ui_MainWidow;

class OlValue
{
public:
    int _x;
    int _y;
public:
    OlValue() {
        _x = 10;
        _y = 20;
    }
};
#endif //VALUE_H

olmainwindow.h

#ifndef OLMAINWINDOW_H
#define OLMAINWINDOW_H
#include <iostream>
#include <string>
#include "olvalue.h"

class OlValue;
class OlMainWindow;
class ui_MainWidow;

class OlMainWindow
{
public:


    OlValue* vref = new OlValue;

    OlMainWindow();
    ~OlMainWindow();

    void enterUsername();
private:    
    int val;
    std::string _Username;
};
#endif //OLMAINWINDOW_H

olmainwidow.cpp

#include "olmainwindow.h"
#include <iostream>
#include <string>


class OlValue;
class OlMainWindow;
class ui_MainWidow;

OlMainWindow::OlMainWindow() {
    ui_MainWidow* ui = new ui_MainWidow();
    ui->setupUI(this);

}
OlMainWindow::~OlMainWindow() {
    //delete ui;
}

void OlMainWindow::enterUsername() {
    std::cout << "Please, enter Username: ";
    std::getline(std::cin, _Username);
    std::cout << "You'v accessed Username: " << _Username << std::endl;
}

main.cpp

#include "olmainwindow.h"
#include <iostream>

int main(int argc, char* argv[])
{
    OlMainWindow* w = new OlMainWindow();
    return 0;
}

Answer 1

In ui_MainWidow.h, the implementation of setupUI() is inlined, but when it tries to access the members of OlMainWindow it fails because the OlMainWindow class hasn't actually been defined yet, merely forward declared.

Your files are not setup correctly. You are creating dependencies where they don't belong, and then you are trying to use forward declarations to solve circular dependencies issues that shouldn't exist in the first place.

Try this instead:

OlValue.h

#ifndef OLVALUE_H
#define OLVALUE_H

class OlValue
{
public:
    int _x = 10;
    int _y = 20;
};

#endif //OLVALUE_H

ui_MainWindow.h

#ifndef UI_MAINWINDOW_H
#define UI_MAINWINDOW_H

class OlMainWindow;

class ui_MainWindow {
public:
    void setupUI(OlMainWindow* OlMain);
};

#endif //UI_MAINWINDOW_H

ui_MainWindow.cpp

#include "ui_MainWindow.h"
#include "OlMainWindow.h"
#include <iostream>

void ui_MainWindow::setupUI(OlMainWindow* OlMain) {
    OlMain->vref->_x = 10;
    OlMain->vref->_x = 10;
    std::cout << " ok";
}

olMainWindow.h

#ifndef OLMAINWINDOW_H
#define OLMAINWINDOW_H

#include "OlValue.h"
#include <string>

class ui_MainWidow;

class OlMainWindow
{
public:
    OlValue* vref = new OlValue;

    OlMainWindow();
    ~OlMainWindow();

    void enterUsername();
private:    
    ui_MainWidow* ui;
    int val;
    std::string _Username;
};

#endif //OLMAINWINDOW_H

olMainWindow.cpp

#include "olMainWindow.h"
#include <iostream>
#include <string>

OlMainWindow::OlMainWindow() {
    ui = new ui_MainWindow();
    ui->setupUI(this);
}

OlMainWindow::~OlMainWindow() {
    delete ui;
}

void OlMainWindow::enterUsername() {
    std::cout << "Please, enter Username: ";
    std::getline(std::cin, _Username);
    std::cout << "You'v accessed Username: " << _Username << std::endl;
}

main.cpp

#include "olMainWindow.h"
#include <iostream>

int main(int argc, char* argv[])
{
    OlMainWindow w;
    w.enterUsername();
    return 0;
}