Qt signal-slot not working

Question

I want to update text in a label in a first window from a second window where is a line edit to write some text. This text should be dispaly in first window. I spend a week for it. A famous connect doesn't work. Is somebody who correct below code and explain how connect should work? I use Qt in version 5.1.1

firstwindow.h

#ifndef FIRSTWINDOW_H
#define FIRSTWINDOW_H

#include <QMainWindow>
#include "secondwindow.h"

namespace Ui {
class Firstwindow;
}

class Firstwindow : public QMainWindow
{
    Q_OBJECT

public:
    explicit Firstwindow(QWidget *parent = 0);
    ~Firstwindow();
public slots:
    void addEntry();
private slots:
    void on_pushButton_clicked();

private:
    Ui::Firstwindow *ui;
    Secondwindow *asecondwindow;
    Secondwindow *absecondwindow;
    Secondwindow *abcsecondwindow;
};

#endif // FIRSTWINDOW_H

secondwindow.h

#ifndef SECONDWINDOW_H
#define SECONDWINDOW_H

#include <QDialog>
#include <QtWidgets>

namespace Ui {
class Secondwindow;
}

class Secondwindow : public QDialog
{
    Q_OBJECT

public:
    explicit Secondwindow(QWidget *parent = 0);
    ~Secondwindow();
QLineEdit *lineEdit;

private slots:
    void on_pushButton_clicked();

private:
    Ui::Secondwindow *ui;
    QPushButton *pushButton;
};

#endif // SECONDWINDOW_H

main.cpp

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

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

    return a.exec();
}

firstwindow.cpp

#include "firstwindow.h"
#include "ui_firstwindow.h"
#include <QtCore>
#include <QtGui>
#include <QtWidgets>

Firstwindow::Firstwindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::Firstwindow)
{
    ui->setupUi(this);
    asecondwindow = new Secondwindow();
    QObject::connect(asecondwindow->lineEdit,SIGNAL(textChanged()),this, SLOT(addEntry()));
}

Firstwindow::~Firstwindow()
{
    delete ui;
    delete asecondwindow;
    delete absecondwindow;
    delete abcsecondwindow;
}

void Firstwindow::on_pushButton_clicked()
{
    absecondwindow = new Secondwindow;
    absecondwindow->exec();

}

void Firstwindow::addEntry()
{
    abcsecondwindow = new Secondwindow;

    if (abcsecondwindow->exec()) {
        QString name = abcsecondwindow->lineEdit->text();
        ui->label->setText(name);

    }
}

secondwindow.cpp

#include "secondwindow.h"
#include "ui_secondwindow.h"
#include <QDialog>

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

Secondwindow::~Secondwindow()
{
    delete ui;
}
void Secondwindow::on_pushButton_clicked()
{
  //  emit ui->lineEdit->textChanged();
    QDialog::accept();
}

Answer 1

There is a fundamental design problem with what you're doing. There's no need to expose the second window's internal properties to the first window. Just listen for changes within the second window and emit a signal whenever it changes. Then the first window can just listen to the changes on the second window.

Here's a full example showing what I mean. main.cpp:

#include <QApplication>
#include <QDialog>
#include <QLabel>
#include <QMainWindow>
#include <QPushButton>
#include <QLineEdit>
#include <QVBoxLayout>

class SecondWindow : public QDialog {
    Q_OBJECT
public:
    SecondWindow(QMainWindow *parent = 0) : QDialog(parent) {
        QLineEdit *edit = new QLineEdit;
        QPushButton *close = new QPushButton(QStringLiteral("close"));
        QVBoxLayout *layout = new QVBoxLayout;
        layout->addWidget(edit);
        layout->addWidget(close);

        setLayout(layout);

        connect(edit, SIGNAL(textChanged(QString)), this, SIGNAL(textChanged(QString)));
        connect(close, SIGNAL(clicked()), this, SLOT(close()));
    }

signals:
    void textChanged(const QString &text);
};

class FirstWindow : public QMainWindow {
    Q_OBJECT
public:
    FirstWindow(QMainWindow *parent = 0) : QMainWindow(parent) {
        QWidget *central = new QWidget(this);

        QPushButton *button = new QPushButton(QStringLiteral("Open"));
        label = new QLabel(QStringLiteral("Output appears here"));

        QVBoxLayout *layout = new QVBoxLayout;
        layout->addWidget(button);
        layout->addWidget(label);
        central->setLayout(layout);

        setCentralWidget(central);

        connect(button, SIGNAL(clicked()), this, SLOT(createWindow()));
    }

private slots:
    void createWindow() {
        SecondWindow *window = new SecondWindow(this);
        connect(window, SIGNAL(textChanged(QString)), this, SLOT(setLabelText(QString)));
        window->resize(300, 300);
        window->exec();
    }

    void setLabelText(const QString &text) {
        label->setText(text);
    }

private:
    QLabel *label;
};



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

    FirstWindow w;
    w.resize(400, 400);
    w.show();

    return a.exec();
}

#include "main.moc"

Not that the SecondWindow listens for changes on the QLineEdit and emits its own signal when that value changes. Then the FirstWindow just connects to that signal and changes its own QLabel whenever it receives the signal.

Answer 2

I see the following issues:

• QLineEdit does not have a signal textChanged(). It should be textChanged(const QString &) instead. So you have to install your connection like:

  QObject::connect(asecondwindow->lineEdit, SIGNAL(textChanged(const QString &)), this, SLOT(addEntry(const QString &)));
  

  Please note that I changed the Firstwindow::addEntry() slot to Firstwindow::addEntry(const QString &) to match the signal's signature.

• I cannot find when and where your QLineEdit member variable of the Secondwindow class is created.