SIGNAL is not emitted - c++

Question

I have issue with SIGNAL not being emitted. Here is minimal example of this issue:
main.cpp - standard main
#include "mainwindow.h"
#include <QApplication>
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
MainWindow w;
w.show();
return a.exec();
}
mainwindow.h
#ifndef MAINWINDOW_H
#define MAINWINDOW_H
#include <QMainWindow>
#include <QVBoxLayout>
#include "combobox.h"
class MainWindow : public QMainWindow
{
Q_OBJECT
public:
explicit MainWindow(QWidget *parent = 0);
~MainWindow();
};
#endif // MAINWINDOW_H
mainwindow.cpp
#include "mainwindow.h"
MainWindow::MainWindow(QWidget *parent) : QMainWindow(parent)
{
QWidget *mainWidget = new QWidget;
setCentralWidget(mainWidget);
QVBoxLayout *mainLayout = new QVBoxLayout;
combobox combo_box;
mainLayout->addWidget(combo_box.box);
mainWidget->setLayout(mainLayout);
}
MainWindow::~MainWindow()
{
}
combobox.h
#ifndef COMBOBOX_H
#define COMBOBOX_H
#include <QComboBox>
#include <QDebug>
class combobox : public QWidget
{
Q_OBJECT
public:
combobox();
~combobox();
QComboBox *box = new QComboBox;
public slots:
void selection_changed();
};
#endif // COMBOBOX_H
combobox.cpp
#include "combobox.h"
combobox::combobox()
{
QString string = "test 1";
box->addItem(string);
string = "test 2";
box->addItem(string);
box->setCurrentIndex(0);
connect(box, SIGNAL(currentIndexChanged(int)), this, SLOT(selection_changed()));
}
combobox::~combobox()
{
}
void combobox::selection_changed()
{
qDebug() << "Selection changed";
box->setCurrentIndex(-1);
}
You need to run qmake before compiling.
When I run this program and change combobox index selection_changed is not executed. Why?
SIGNAL and SLOT connection definitely works, because if I add box->setCurrentIndex(1); at the end of combobox constructor, selection_changed will be executed once.
I am using QT 5.4 with QT Creator
Thanks in advance!

Answer 1

This problem because of you create your ComboBox class object at stack. So if you write some debug information at destructor of this class you can see object destroyed immediately after MainWindow constructor out of scope:
ComboBox::~ComboBox()
{
qDebug() << Q_FUNC_INFO;
}
MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent)
{
setupUi(this);
QWidget *mainWidget = new QWidget;
setCentralWidget(mainWidget);
QVBoxLayout *mainLayout = new QVBoxLayout;
ComboBox combo_box;
mainLayout->addWidget(combo_box.box);
mainWidget->setLayout(mainLayout);
qDebug() << Q_FUNC_INFO;
}
And qDebug() got the following result:
MainWindow::MainWindow(QWidget*)
virtual ComboBox::~ComboBox()
Thats why signal don't emitted. So you need create object on the heap or make something tricks.
Also, you have dynamically allocated object *box at your ComboBox class and don't specify parent of this object, so it seems memory leak is possible. Specify parent or delete object at destructor of ComboBox class.