Qt Form Login. (Parent Window is Disabled After Child Window Login Success)

Hello, Now I want share how to use Qt, previously is using Wxwidget and now I want share using Qt (especially Qt5). Form login will look like this:

Form Login apps

Figure 1. Form Login Application

Okay to create the application like the picture above, follow this instructions:
First open your Qt creator and create new Project (based Qt widgets application) and give the name of the application with the name of the “FormLogin”.

Create new project

Figure 2. Create new project

After you create the project, the structure FormLogin application will look like this:

FormLogin Structure

Figure 3. FormLogin structure

After that, right click the project and select add new. Then choose C++ header file and give the name “formlogin”.

FormLogin Structure

Figure 4. Create formlogin.h

Repeat the above steps, but in popup window new file, select “C++ source file” and give the name “formlogin”. Then you will get a directory structure like this:

FormLogin Structure

Figure 5. Form Login stucture

And now we must coding😀. First open your formlogin.h and insert this code:


#include <QDialog>
#include <QVBoxLayout>
#include <QHBoxLayout>
#include <QLabel>
#include <QLineEdit>
#include <QPushButton>
#include <QMessageBox>

class FormLogin : public QDialog

    FormLogin(QWidget* parent = 0);

private slots:
    void OnQuit();
    void OnLogin();

    void reject();

    QLabel* userLabel;
    QLabel* passLabel;
    QLineEdit* userLineEdit;
    QLineEdit* passLineEdit;
    QPushButton* loginButton;
    QPushButton* quitButton;

#endif // FORMLOGIN_H

Then add this code on formlogin.cpp:

#include "formlogin.h"

FormLogin::FormLogin(QWidget* parent)
    : QDialog(parent)
    setFixedSize(300, 120);
    setWindowTitle("Form Login");

    userLabel = new QLabel("Username:");
    passLabel = new QLabel("Password:");
    userLineEdit = new QLineEdit();
    passLineEdit = new QLineEdit();
    loginButton = new QPushButton("Login");
    quitButton = new QPushButton("Quit");

    QVBoxLayout* vbox = new QVBoxLayout(this);
    QHBoxLayout* hbox1 = new QHBoxLayout();
    QHBoxLayout* hbox2 = new QHBoxLayout();
    QHBoxLayout* hbox3 = new QHBoxLayout();

    hbox1->addWidget(userLabel, 1);
    hbox1->addWidget(userLineEdit, 2);
    hbox2->addWidget(passLabel, 1);
    hbox2->addWidget(passLineEdit, 2);
    hbox3->addWidget(loginButton, 1, Qt::AlignRight);
    hbox3->addWidget(quitButton, 0, Qt::AlignRight);


    connect(quitButton, SIGNAL(clicked()), this, SLOT(OnQuit()));
    connect(loginButton, SIGNAL(clicked()), this, SLOT(OnLogin()));

void FormLogin::reject()

void FormLogin::OnQuit()

void FormLogin::OnLogin()
    QString username = userLineEdit->text();
    QString password = passLineEdit->text();

    // Checking if username or password is empty
    if (username.isEmpty() || password.isEmpty())
        QMessageBox::information(this, tr("Peringatan!"), "Username atau password tidak boleh kosong");

FormLogin::~FormLogin() {}


  1. Line 7. Setfixed is used to fixed the size of the window, so the window can’t resize and not have maximaze button.
  2. Line 8. This function is used to create the modal dialog, so if user click the main window, the form login will blink and sound an alert sound
  3. Line 9. Setting Qt::WA_DeleteOnClose means qt can delete anytime after you call close()
  4. Line 15. To display platform-dependent password mask characters instead of the characters actually entered.
  5. Line 28-29. Is used to make the button is positioned on the right, look login and quit button from above picture
  6. Line 31. This function is to add space between widget
  7. Line 40-43. This function will be called when the user clicks the close button and call FormLogin::OnQuit function
  8. Line 45-49. THis function will be called when the user clicks the quit button. Then this function will delete/close form login window and main window.
  9. Line 53-54. This function to get text from line edit
  10. Line 57-58. This code is if user click login button and the username and password is empty, user will get warning prompt
  11. Line 60 is used if user success login and form login will close

Now open mainwindow.h and write this code:


#include <QMainWindow>

class MainWindow : public QMainWindow


#endif // MAINWINDOW_H

And this is the mainwindow.cpp source:

#include "mainwindow.h"

    : QMainWindow()
    setMinimumSize(800, 600);

MainWindow::~MainWindow() {}

And the last is implement formlogin and mainwindow on main.cpp:

#include "mainwindow.h"
#include "formlogin.h"
#include <QApplication>

int main(int argc, char *argv[])
    QApplication a(argc, argv);
    MainWindow* mainWindow = new MainWindow();
    FormLogin* formLogin = new FormLogin(mainWindow);

    return a.exec();


  1. Line 8 is to create pointer of mainwindow
  2. Line 9 is to instance formlogin with the parent window is mainwidow. Look the formlogin.h line 17. This code is to add constructor with parent (default parent is null/0), and from formlogin.cpp line 3-4 is to call constructor QDialog with parent QDialog(parent).
    Now run the project, if no error you will get form login application like the first picture on this post.
    Source code: http://downloads.ziddu.com/download/24288305/FormLogin.7z.html


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