Invalid use of incomplete type 'class Ui::dialog (QT error )

Question

I would like to make a simple QT mainwindow with the button to open a second window or dialog. I followed literally the step from the QT link "Using a Designer UI File in Your Application" and following the single inheritance example.

But QT gives 4 errors , which you will see a snapshot of below.

Now, what I did is I created a mainwindow in Qt designer, then I added a second form to the project , which will be the second dialog window when a button clicked. Because I created the form manually "mydialog.ui", I added class "mydialog.h and mydialog.cpp" and put the header of "ui-mydialog" in the source file "mydialog.cpp".

I' not sure what am I missing ?

Below is the code :

- mydialog.h

#ifndef MYDIALOG_H 
#define MYDIALOG_H
#include<QWidget>

class mydialog ;

namespace Ui {
class mydialog;
}

class mydialog : public QWidget
{
    Q_OBJECT

public:

    explicit mydialog(QWidget *parent = 0);
    virtual ~mydialog();
private :

    Ui::mydialog *ui;

};

#endif // MYDIALOG_H

- mainwindow.h

#ifndef MAINWINDOW_H
#define MAINWINDOW_H
#include <QtCore/QtGlobal>
#include <QMainWindow>

QT_USE_NAMESPACE
QT_BEGIN_NAMESPACE
namespace Ui {
class MainWindow;
}
QT_END_NAMESPACE

class mydialog;

class MainWindow : public QMainWindow
{
    Q_OBJECT

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

private slots:
    void on_Start_clicked();

private:
    Ui::MainWindow *ui;
    mydialog *dialog1;
};

#endif // MAINWINDOW_H

- mydialog.cpp

#include"mydialog.h"
#include "ui_mydialog.h"


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


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

- mainwindow.cpp

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include"mydialog.h"


MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    dialog1 = new mydialog ;
}

MainWindow::~MainWindow()
{
    delete ui;
    delete dialog1;
}

void MainWindow::on_Start_clicked()
{

}

- main.cpp

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

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

    return a.exec();
}

- The .pro file

#-------------------------------------------------
#
# Project created by QtCreator 2015-12-17T00:10:58
#
#-------------------------------------------------

QT       += core gui

greaterThan(QT_MAJOR_VERSION, 4): QT += widgets

TARGET = TestTool
TEMPLATE = app


SOURCES += main.cpp\
        mainwindow.cpp \
    mydialog.cpp

HEADERS  += mainwindow.h \
    mydialog.h

FORMS    += mainwindow.ui \
    mydialog.ui

RESOURCES += \
    misc.qrc

- Qt compilation output error

Compilation error

The generated file Ui_mydialog.h is :

#ifndef UI_MYDIALOG_H
#define UI_MYDIALOG_H

#include <QtCore/QVariant>
#include <QtWidgets/QAction>
#include <QtWidgets/QApplication>
#include <QtWidgets/QButtonGroup>
#include <QtWidgets/QDialog>
#include <QtWidgets/QDialogButtonBox>
#include <QtWidgets/QHeaderView>

QT_BEGIN_NAMESPACE

class Ui_Dialog
{
public:
    QDialogButtonBox *buttonBox;

    void setupUi(QDialog *Dialog)
    {
        if (Dialog->objectName().isEmpty())
            Dialog->setObjectName(QStringLiteral("Dialog"));
        Dialog->resize(400, 300);
        buttonBox = new QDialogButtonBox(Dialog);
        buttonBox->setObjectName(QStringLiteral("buttonBox"));
        buttonBox->setGeometry(QRect(30, 240, 341, 32));
        buttonBox->setOrientation(Qt::Horizontal);
        buttonBox->setStandardButtons(QDialogButtonBox::Cancel|QDialogButtonBox::Ok);

        retranslateUi(Dialog);
        QObject::connect(buttonBox, SIGNAL(accepted()), Dialog, SLOT(accept()));
        QObject::connect(buttonBox, SIGNAL(rejected()), Dialog, SLOT(reject()));

        QMetaObject::connectSlotsByName(Dialog);
    } // setupUi

    void retranslateUi(QDialog *Dialog)
    {
        Dialog->setWindowTitle(QApplication::translate("Dialog", "Dialog", 0));
    } // retranslateUi

};

namespace Ui {
    class Dialog: public Ui_Dialog {};
} // namespace Ui

QT_END_NAMESPACE

#endif // UI_MYDIALOG_H

Answer 1

This is because of the diferences between the names in your UI- und C++sourcecode files. For example if in your UI-sourcecode file you have a name like "StatusBarPart" but the name of your class in the C++ file is "StatusBar"

<class>StatusBarPart</class>
<widget class="QWidget" name="StatusBarPart">

---

StatusBar::StatusBar(QWidget *parent)
    : PartBase(parent),
      ui(new Ui::StatusBar)

then you you get these error message you see.

Solution: You can edit the UI file in some external editor and make the names equal. Save changes. Compile your app. Be happy ;-)