//MainWindow.h
#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>
#include <DbManager.h>

namespace Ui {
class MainWindow;
}

class MainWindow : public QMainWindow
{
    Q_OBJECT

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

private:
    Ui::MainWindow *ui;

    DbManager db_test;
};

#endif // MAINWINDOW_H

// MainWindow.cpp
#include "Mainwindow.h"
#include "ui_Mainwindow.h"

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
}

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


// Bar.h
#ifndef BAR_H
#define BAR_H

#include <QWidget>

namespace Ui {
class Bar;
}

class Bar : public QWidget
{
    Q_OBJECT

public:
    explicit Bar(QWidget *parent = 0);
    ~Bar();

private:
    Ui::Bar *ui;

};

#endif // BAR_H

// Bar.cpp
#include "Bar.h"
#include "ui_Bar.h"

Bar::Bar(QWidget *parent) :
    QWidget(parent),
    ui(new Ui::Bar)
{
    ui->setupUi(this);
}

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


//Foo.h
#ifndef FOO_H
#define FOO_H

#include <QWidget>

namespace Ui {
class Foo;
}

class Foo : public QWidget
{
    Q_OBJECT

public:
    explicit Foo(QWidget *parent = 0);
    ~Foo();

private:
    Ui::Foo *ui;
};

#endif // FOO_H

//Foo.cpp
#include "Foo.h"
#include "ui_Foo.h"

Foo::Foo(QWidget *parent) :
    QWidget(parent),
    ui(new Ui::Foo)
{
    ui->setupUi(this);
}

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

//DbManager.h
#ifndef DBMANAGER_H
#define DBMANAGER_H

#include <QDebug>

class DbManager
{
public:
    DbManager();

    void do_something();
};

#endif // DBMANAGER_H

//DbManager.cpp
#include "DbManager.h"

DbManager::DbManager()
{

}

void DbManager::do_something()
{
    qDebug() << "do something";
}

//main.cpp
#include "Mainwindow.h"
#include <QApplication>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    w.show();

    return a.exec();
}