Message box doesnt appear

[xatziaris]

this is my code:

main.cpp

#include <QApplication>
#include <QWidget>
#include "mybutton.h"

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);

    QWidget window1;
    window1.setFixedSize(640,480);

    QIcon openicon("/home/xatziaris/workspace/Qt/project8452/project8452/open.svg");
    QIcon saveicon("/home/xatziaris/workspace/Qt/project8452/project8452/save.png");
    QIcon closeicon("/home/xatziaris/workspace/Qt/project8452/project8452/close.png");
    QIcon abouticon("/home/xatziaris/workspace/Qt/project8452/project8452/about.png");

    mybutton openbutton;
    openbutton.setParent(&window1);
    openbutton.setText("Open");
    openbutton.setIcon(openicon);
    openbutton.setGeometry(1,1,80,45);

    mybutton savebutton;
    savebutton.setParent(&window1);
    savebutton.setText("Save");
    savebutton.setIcon(saveicon);
    savebutton.setGeometry(1,47,80,45);

    mybutton aboutbutton;
    aboutbutton.setParent(&window1);
    aboutbutton.parentwindow = &window1;
    aboutbutton.setText("About");
    aboutbutton.setIcon(abouticon);
    aboutbutton.setGeometry(1,93,80,45);

    mybutton closebutton;
    closebutton.setParent(&window1);
    closebutton.setText("Close");
    closebutton.setIcon(closeicon);
    closebutton.setGeometry(1,434,80,45);


    window1.show();

    QObject::connect(&closebutton,SIGNAL(clicked()),&a,SLOT(quit()));

    aboutbutton.setCheckable(true);
    QObject::connect(&aboutbutton,SIGNAL(clicked(bool)),&aboutbutton,SLOT(showinfo(bool)));

    return a.exec();
}

mybutton.cpp

#include "mybutton.h"

void mybutton::showinfo(bool a){

    if(a){

        this->setChecked(false);

        QMessageBox infobox;
        infobox.setText("\nA text editor by ME.\n\nCreated with open source Qt Creator.");
        infobox.show();

    }

}

mybutton.h

#ifndef MYBUTTON
#define MYBUTTON

#include <QPushButton>
#include <QWidget>
#include <QMessageBox>

class mybutton: public QPushButton{
    Q_OBJECT

public:
    QWidget *parentwindow;

public slots:
    void showinfo(bool a);
};

#endif // MYBUTTON

in main.cpp a create a window with 4 buttons. i want to click the "About" button to make a message box appear. but nothing happens.

the connection:

QObject::connect(&aboutbutton,SIGNAL(clicked(bool)),&aboutbutton,SLOT(showinfo(bool)));

the slot:

void mybutton::showinfo(bool a){

    if(a){

        this->setChecked(false);

        QMessageBox infobox;
        infobox.setText("\nA text editor by ME.\n\nCreated with open source Qt Creator.");
        infobox.show();

    }

}

ty :)

[SGaist]

Hi and welcome to devnet,

infobox is a local variable and currently will be destroyed at the end of showinfo if's true condition.

[mrjj]

Hi and welcome
Adding to @SGaist info,
You are using show() to show the dialog.
Show() will not block and the next lines will be executed;
So
dialog.show();
int a=10; << this is also executed while dialog is showing
// and if this is end of function, the dialog is deleted.

This is in contrast to
exec()
http://doc.qt.io/qt-5.5/qmessagebox.html#exec

dialog.exec(); << exec blocks and it stays "in" there.
int a=10; << this is NOT executed before dialog closes

[xatziaris]

thanks a lot guys :)

Any similar method for widgets?

[mrjj]

@xatziaris
Hi a QDialog is a widget. (also)
But the exec is in Dialog class so other Widgets don't have it.
You can how ever make a Widget "Dialog like"
QWidget *yourWidget = new QWidget(this, Qt::Popup | Qt:: Dialog);
yourWidget ->setWindowModality(Qt::WindowModal);
yourWidget->show();

But it can only use show(); not exec().

If you need a dialog, just use a dialog.
Nothing u cant do with Dialog that u can do with plain Widget.

If time permits, you should really read the
modal vs non modal (block vs non block)
http://doc.qt.io/qt-5.5/qdialog.html
as the use of exec() can give surprise if used inside
event handlers and the like.

The exec() is a message loop.
So when you are in such loop,
then normal mainwindow message loop is affected.

[xatziaris]

thanks a lot man :) happy new year

[mrjj]

@xatziaris
happy new year :)