Qt - Right Way to Implement Custom Signals and Slots

Question

I have one MainWindow and one Class; I want to send data between them using custom signal and slot. I can't seem to figure it out, I need help.

Here is my code: MainWindow.h

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>
#include <iostream>
#include "receive.h"
QT_BEGIN_NAMESPACE
namespace Ui { class MainWindow; }
QT_END_NAMESPACE

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    MainWindow(QWidget *parent = nullptr);
    ~MainWindow();
signals:
    void sendit(QString name);
private slots:
    void on_send_button_clicked();
    void display(QString e)
    {        
     std::cout<<"Here is where I am called this "<<e.toStdString()<<std::endl;
    }

private:
    Ui::MainWindow *ui;

};
#endif // MAINWINDOW_H

MainWindow.cpp

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include "receive.h"

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
    , ui(new Ui::MainWindow)
{
    ui->setupUi(this);    
   Receive *r = new Receive();
   connect(this, SIGNAL(sendit(QString)), r, SLOT(display(QString)));

}

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


void MainWindow::on_send_button_clicked()
{
    emit sendit(ui->lineEdit->text());

}

receive.h

#ifndef RECEIVE_H
#define RECEIVE_H

#include <iostream>
#include <QDialog>

class Receive : public QDialog
{
public:
    Receive();
private slots:
    void display(QString e);
};

#endif // RECEIVE_H

receive.cpp

#include "receive.h"
#include "mainwindow.h"

Receive::Receive()
{

}

void Receive::display(QString e)
{    
 std::cout<<"Here is where I am called this "<<e.toStdString()<<std::endl;
}

When I run this program, I get this message:

06:26:29: Starting C:\Users\Troy\Documents\build-tests-Desktop_Qt_5_14_1_MinGW_32_bit-Debug\tests.exe ... QObject::connect: No such slot QDialog::display(QString) in ..\tests\mainwindow.cpp:11 QObject::connect: (sender name: 'MainWindow')

How do I get this done, please?

Thank you for your help.

`Receive` is missing the Q_OBJECT macro – Frank Osterfeld Mar 18 '20 at 05:36 — Frank Osterfeld, Mar 18 '20 at 05:36

score 2 · Accepted Answer · answered Mar 18 '20 at 05:41

2

Your slot in Receive needs to be public not private. Much the same as other class members, private slots can only be used by the class itself.

If you use the modern connect syntax you'll get a better compile time error:

connect(this, &MainWindow::sendit, r, &Receive::display);

You also need to make sure you add Q_OBJECT to every Qt class, it is missing from Receive.

answered Mar 18 '20 at 05:41

Alan Birtles

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Thank you. I did and I got this error: Undefined reference to 'vtable for Receive' – delivite Mar 18 '20 at 05:49
You might need to re-run qmake, also make sure you have implemented all methods in the class that you've declared: https://stackoverflow.com/questions/2555816/qt-linker-error-undefined-reference-to-vtable – Alan Birtles Mar 18 '20 at 05:56
Quick question please, is it possible to have this connection without having a pointer to Receive class? – delivite Mar 18 '20 at 06:31
Nope, you need an object to call a class method whether that's through a signal or any other means (signals and slots are essentially just glorified function pointers) however using the new connect syntax you can attach whatever you like to a signal, could be a static method or even a lambda in which cases you don't need an object to target – Alan Birtles Mar 18 '20 at 06:38

Qt - Right Way to Implement Custom Signals and Slots

1 Answers1