Question

what am I doing wrong?

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>
#include "fileoperations.h"

namespace Ui {
class MainWindow;
}
class FileOperations;
class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();
    FileOperations FileController;

private slots:
    void on_OpenButton_clicked();
    void on_SaveButton_clicked();
    void on_EncodeButton_clicked();
    void on_DecodeButton_clicked();
private:
    Ui::MainWindow *ui;
};

#endif // MAINWINDOW_H

When i try to compile and run the program, it says:

g:\ke\c++ projects\projects\qt\shitlencoder\mainwindow.h:18: error: C2079: 'MainWindow::FileController' uses undefined class 'FileOperations'

Here’s the strange thing, if I change ‘FileOperations FileController;’ to ‘FileOperations *FileController;'(Obviously this compiles wrongly, because the rest of my codes that you can’t see havn’t been adapted to ‘->’ instead of ‘.’)
Then if I change it back to ‘FileOperations FileController;’ it lets me compile the program once (And it works fine), then it has the error the next time I try to compile it.

I’m using Qt 5.0.

fileoperations.h:

#ifndef FILEOPERATIONS_H
#define FILEOPERATIONS_H
#include "ui_mainwindow.h"
#include "mainwindow.h"
#include <QFileDialog>
#include <string>
#include <time.h>
#include <iostream>
#include <conio.h>
#include <windows.h>
#include <fstream>
using namespace std;
class FileOperations
{
public:
    FileOperations();
    void SetInputFile(QString x);
    void SetOutputFile(QString x);
    void EncryptAndSave(Ui::MainWindow *NUI);
    void DecryptAndSave(Ui::MainWindow *NUI);
    void createid(int id, int id2);
    int GetCFuncion();
    void SetCFuncion(int x);
    long long Get_Size(string filename);
    bool Get_Toobig(string path);
    //DWORD WINAPI Thread_no_1();
private:
    string InputFilename;
    string OutputFilename;
    int CFuncion;//CurrentFunction;
    vector<int> conbyte1;
    vector<int> conbyte2;
    vector<int> opbyte1;
    vector<int> opbyte2;
    vector<int> passwordbytes;
};

#endif // FILEOPERATIONS_H

Answer 1

Since you are holding a FileOperations object in your class, you need the full class declaration. This means you have to include the header, you cannot simply forward declare the class like you are doing now. If you hold only a pointer, and do not have any code in your header that attempts to dereference the pointer, then the forward declaration is enough.

EDIT You have a cyclical include. You are including mainwindow.h in fileoperations.h. You can fix if by removing that include completely.