undefined reference to QMessageBox

[Ahti]

Hi
I am trying to show simple error message with QMessageBox but I keep getting this error:
undefined reference to `_imp___ZN11QMessageBoxD1Ev'

My Code:

#include <QtWidgets/QMessageBox>

QMessageBox msg;
msg.setWindowTitle("Error");
msg.setIcon(QMessageBox::Critical);
msg.setText("Error: Something went wrong!");
msg.exec();

Error:

Please help

[Christian Ehrlicher]

"QT +=widgets" in the pro file should help.

[Ahti]

@Christian-Ehrlicher Okay I added widgets in pro file and this is how it looks now:

pro file:

TEMPLATE = app

QT += qml quick
QT += widgets

CONFIG += c++11
...

But when I run my project I get this error now:

[SGaist]

Hi,

Run it through the debugger. That will show you where the failure happens.

[Ahti]

@SGaist Its says Cannot create a QWidget without QApplication so I changed this:

#include <QGuiApplication>
int main(int argc, char *argv[])
{
    QGuiApplication app(argc, argv);
    ...
}

to this:

#include <QApplication>
int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    ...
}

It works but now I get a message on console saying: ReferenceError: adjust is not defined ( adjust is the object of some class which I use in qml to access its method )

[Pradeep P N]

Hi @Ahti

Your intention is to show the QMessageBox on the QML UI ? If not correct me.

If you have the QML UI why dont you use the MessageDialog.

As per my understanding,

• QCoreApplication is the base class, QGuiApplication extends the base class with functionality related to handling windows and GUI (non widget, e.g. OpenGL or QtQuick, QML),
• QApplication extends QGuiApplication with functionality related to handling widgets.

All the best.

[Ahti]

@Pradeep-P-N Well the error message should be shown when the c++ method I am calling from qml fails.

[Pradeep P N]

@Ahti Fine, The UI you have is of QML or QWidget ?

[Ahti]

@Pradeep-P-N I use qml for front-end and c++ as back-end

[Pradeep P N]

@Ahti Cool.

Just wrote a small example. Which brings the QMessageBox on launch.

As you use QML front-end the Best solution would be prefer you to use the MessageDialog which react for your C++ function.

    MessageDialog {
        id: messageDialog

        title: "May I have your attention please"
        text: "It's so cool that you are using Qt Quick."

// Option 1
        visible: adjust.functionCall() // functionCall() returns bool.

        onAccepted: {
            console.log("And of course you could only agree.")
            Qt.quit()
        }
    }

// Option 2

you can even set messageDialog.visible = true/false from other components or signals on functionCall from C++

Please check this and let me know if its helpful.

// .pro file

QT += quick core gui widgets

// main.cpp

#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include <QApplication>
#include <QQmlContext>

#include "MyClass.h"

int main(int argc, char *argv[])
{
    QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);

    QApplication app(argc, argv);
    QQmlApplicationEngine engine;

    // Registers the C++ type in the QML
    // qmlRegisterType<MyClass>("MyClass", 1, 0, "Adjust");

    MyClass myclass;
    engine.rootContext()->setContextProperty("adjust", &myclass);

    engine.load(QUrl(QStringLiteral("qrc:/main.qml")));
    if (engine.rootObjects().isEmpty())
        return -1;

    return app.exec();
}

// main.qml

import QtQuick 2.9
import QtQuick.Window 2.2

// If qmlRegisterType is used
// import MyClass 1.0

Window {
    visible: true
    width: 640
    height: 480
    title: qsTr("Hello World")

    // If qmlRegisterType is used
    // Adjust { }

    Component.onCompleted: {
        adjust.name = "My Name";

        console.log(adjust.name)
    }
}

// MyClass. cpp 

#include "MyClass.h"
#include <QMessageBox>

MyClass::MyClass(QObject *parent) : QObject(parent)
{    
    QMessageBox msg;
    msg.setWindowTitle("Error");
    msg.setIcon(QMessageBox::Critical);
    msg.setText("Error: Something went wrong!");
    msg.exec();
}

QString MyClass::name() const
{
    return m_name;
}

void MyClass::setName(const QString &name)
{
    m_name = name;

    emit nameChanged();
}

// MyClass.h

#ifndef MYCLASS_H
#define MYCLASS_H

#include <QObject>

class MyClass : public QObject
{
    Q_PROPERTY(QString name READ name WRITE setName NOTIFY nameChanged)
    Q_OBJECT
public:
    explicit MyClass(QObject *parent = nullptr);

    QString name() const;
    void setName(const QString &name);

signals:
    void nameChanged();

public slots:

private:
    QString m_name;

};

#endif // MYCLASS_H

[Pradeep P N]

Hi @Ahti
Below is sample code for MessageDialog

QML side:

import QtQuick 2.9
import QtQuick.Window 2.2
import QtQuick.Dialogs 1.3

Window {
    visible: true
    width: 640
    height: 480
    title: qsTr("Hello World")

    MessageDialog {
        id: messageDialog
        title: "May I have your attention please"
        text: "It's so cool that you are using Qt Quick."

        visible: adjust.Adjust

        onAccepted: {
            console.log("And of course you could only agree.")
        }
    }
}

on the C++ side :

 MyClass myclass;
    engine.rootContext()->setContextProperty("adjust", &myclass);


// Here i am just exposing the Q_PROPERTY

    Q_PROPERTY(bool Adjust READ Adjust WRITE setAdjust NOTIFY AdjustChanged)


// You can change it as per your requirement.

MyClass::MyClass(QObject *parent)
    : QObject(parent)
    , m_Adjust(false)
{    
    QTimer *_timer = new QTimer(this);

    _timer->start(3000);

    connect(_timer, &QTimer::timeout, [this] {
        setAdjust(!m_Adjust);
    });

}


bool MyClass::Adjust() const
{
    return m_Adjust;
}

void MyClass::setAdjust(bool Adjust)
{
    if (Adjust == m_Adjust)
        return;
    m_Adjust = Adjust;

    emit AdjustChanged();
}