Question

Im trying to display 3 forms(calender, history, statistics) which has 3 classes into a MainWindow Class which has three tabs created using the designer. When the application runs for the first time, it displays the history form into the tab. But when it is being run the second time, the form is displayed over the tabs that they are not visible.

MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{

ui->setupUi(this);
CalenderForm *pCal = new CalenderForm();
lay = new QVBoxLayout(ui->tab);
lay->addWidget(pCal);
connect(ui->tabWidget,SIGNAL(currentChanged(int)),this,SLOT(a(int)));


}


void MainWindow::a(int index)
{
switch(index)
{
case 0:
    callCal();
    break;

case 1:
    callHist();
    break;

default:
    break;
}


}

void MainWindow::callHist()
{

    HistoryForm *pHis = new HistoryForm();
    pHis->DisplayHistory();
    pHis->show();
    lay2 = new QVBoxLayout(ui->tab_2);
    lay2->addWidget(pHis);
}

Answer 1

Everytime you switch the tab index, you create a new form. I am not sure if this is the cause of your problem, but it definitely is a problem.

You should ensure the forms are only created once. For example you may create them in the constructor of MainWindow and store pointers to them in member variables. You should also already assign any required layouts there.

When you switch the index, you just call the DisplayHistory() or equivalent method.

PS: If you still want to understand your code next year, you should probably find a more speaking name than “a” for that slot 😉

[Edit]
Here is a sample header and cpp file. Be adviced that it might not compile as is and you might have to do some adjustments, but I wanna show you the general idea.

Header file

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QtGui/QMainWindow>

// forward declarations
class CalendarForm;
class HistoryForm;
class StatisticsForm; // or whatever your 3rd form is called

class MainWindow : QMainWindow
{
public:
    MainWindow(QWidget *parent);
    ~MainWindow();

private slots:

    void currentTabChanged( int tabIndex );

private:

    // these are called "member variables" as they are a member of a class
    // Sample naming convention: m_ for members, p for Pointer, you can use 
    // whatever you prefer, as long as you use it consistently.

    CalendarForm*   m_pCalendar;
    HistoryForm*    m_pHistory;
    StatisticsForm* m_pStatistics;


    // not sure about this part but it is generated by Qt anyway
    Ui::MainWindowClass* ui;


};

// not sure about this part but it is generated by Qt anyway
namespace Ui {
    class MainWindowClass;
}
#endif // MAINWINDOW_H

CPP File:

// these includes should be present already anyway
#include "MainWindow.h"
#include "ui_mainwindow.h"

#include "CalendarForm.h"
#include "HistoryForm.h"
#include "StatisticsForm.h"

MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
    m_pCalendar   = new CalendarForm();
    m_pHistory    = new HistoryForm();
    m_pStatistics = new StatisticsForm();

    ui->tabWidget->addTab( m_pCalendar );
    ui->tabWidget->addTab( m_pHistory );
    ui->tabWidget->addTab( m_pStatistics );

    connect( ui->tabWidget, SIGNAL( currentChanged   ( int ) ),
             this         , SLOT  ( currentTabChanged( int ) );
}

MainWindow::~MainWindow()
{
    delete m_pCalendar;
    delete m_pHistory;
    delete m_pStatistics;
}

void MainWindow::currentTabChanged( int tabIndex )
{
    switch( tabIndex )
    {
    case 0:
        // update calculator if necessary
        break;
    case 1:
        // Assumption: You need to update the view every time you activate 
        // the tab. If this assumption is wrong, you don't need this slot
        // at all, just call the method once in the constructor.
        m_pHistory->DisplayHistory();
        break;
    case 2:
        // update statistics if necessary
        break;
    default:
        break;
    }
}