C++, Qt Signals & slots

Question

I'm trying to understand Qt 4.8 signals and slots so I wrote some code to test it out for myself. Eventually, I want to be able to use a common source file in my project so that serial ports can be accessed from any source file in the project.

I set up a Qt GUI application and added a C++ class header and source file, shown below.

When I try to build, I get the error message when I try to emit the signal.

/home/user/QTProjects/stest1/stest1/ser.cpp:25: error: invalid use of 'this' in non-member function

I haven't even gotten to the stage of setting up the connections yet!

My newbie status is obvious, I'd be grateful for any help.

Thanks, James

---

The following is the MainWindow.cpp:-

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include "ser.h"

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);

    ser *j = new ser;
    j->init();

    connect (this, SIGNAL(click()), ser, SLOT(testprint()));

}

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

void MainWindow::on_pushButton_clicked()
{
    QByteArray ba1;

    ba1.resize(6);
    ba1[0]='H'; ba1[1]='e'; ba1[2]='l'; ba1[3]='l'; ba1[4]='o'; ba1[5]='\n';

    this->printtext(ba1);
}

void MainWindow::printtext(const QByteArray &data)
{
    ui->textEdit->insertPlainText(QString(data));
}

The following is the MainWindow.h

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>

namespace Ui {
class MainWindow;
}

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();

private slots:
    void on_pushButton_clicked();
    void printtext(const QByteArray &data);

private:
    Ui::MainWindow *ui;

signals:
//    void click;
};

#endif // MAINWINDOW_H

The following is ser.cpp:-

#include "ser.h"

#include <QObject>

ser::ser(QObject *parent) :
    QObject(parent)
{
}

void ser::init()
{

 //   connect(this->, SIGNAL(testsignal), MainWindow, SLOT(printtext(const QByteArray &data)));

}

void ser::testprint()
{
    QByteArray ba1;

    ba1.resize(8);
    ba1[0]='S'; ba1[1]= '0'; ba1[2]= ' '; ba1[3]= 'l'; ba1[4]='o'; ba1[5]='n'; ba1[6]='g'; ba1[7]='\n';

    emit this->testsignal(ba1);
}

The following is ser.h

#ifndef SER_H
#define SER_H
#include "mainwindow.h"

#include <QObject>

class ser : public QObject
{
    Q_OBJECT
public:
    explicit ser(QObject *parent = 0);

    void init();

signals:
    void testsignal(const QByteArray &data);

private slots:
    void testprint();


public slots:


};

#endif // SER_H

Answer 1

Your method is implemented as void testprint() { ... } , but it should be void ser::testprint() { ... } . It's in your cpp file.

Also note that you don't need to use this-> to refer to class members. emit testsignal(ba1); will fork fine.

Answer 2

I think should be

connect (this, SIGNAL(click()), j, SLOT(testprint()));

instead of

connect (this, SIGNAL(click()), ser, SLOT(testprint()));

that apart, I can't spot where you connect testsignal

Answer 3

Great, that worked. connect (this, SIGNAL(click()), j, SLOT(testprint()));

My next problem is connecting the signal in ser to the slot in the MainWindow. I used

connect(j,
        SIGNAL(testsignal),
        this,
        SLOT(printtext(const QByteArray &data)));

It was inserted immediately after the other connect statement. This does not print out the expected message "Slong". It also does not give me any error! What is the problem? James