Invalid use of void expression, when trying to connect signal/slot

[Christianvs]

Hi

I got this QPushButton in my GUI which i want to connect to a public slot in an object called db of class database.

The class database has following source:

#include "database.h"

database::database(QObject *parent) : QObject(parent)
{

}


void database::storeData(rtl_msgs::current_set_point *rtlcsp, rtl_msgs::state *rtls)
{
    //Storring rlt states and control params in a 2 dimensional vector

    QVector<float> rtlRowOfData;

    rtlRowOfData.push_back(rtls->position);
    rtlRowOfData.push_back(rtls->velocity);
    rtlRowOfData.push_back(rtls->force);
    rtlRowOfData.push_back(rtlcsp->target_position);
    rtlRowOfData.push_back(rtlcsp->target_velocity);
    rtlRowOfData.push_back(rtlcsp->target_force);


    rtlData.push_back(rtlRowOfData);
}


void database::saveData()
{
    std::ofstream rtlDataStream;
    rtlDataStream.open("rtlData.csv");

    if(rtlDataStream.is_open())
    {
        rtlDataStream << "Position,Velocity,Force,Target position,Target velocity, Target force" << std::endl;

        // For each loop
        for(const auto& singleRow : rtlData){
        for(const auto& singleVal : singleRow){
        rtlDataStream << singleVal << ",";
        }
        rtlDataStream << std::endl;
        }

        ROS_INFO_STREAM("Data saved");
    }
    else
    {
        ROS_INFO_STREAM("Stream not open");
    }

    rtlDataStream.close();
}

And the following header:

#ifndef DATABASE_H
#define DATABASE_H

#include <QObject>
#include <QVector>
#include <rtl_msgs/state.h>
#include <rtl_msgs/current_set_point.h>
#include "ros_comm.h"
#include "mainwindow.h"
#include <iostream>
#include <fstream>

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

    void storeData(rtl_msgs::current_set_point *rtlcsp, rtl_msgs::state *rtls);


private:
    QVector< QVector<float> > rtlData;

signals:

public slots:
    void saveData();
};

#endif // DATABASE_H

Now when i initialize my MainWindow i create an object db of class database and connect the QPushButton->clicked with the slot database::saveData as follows:

MainWindow::MainWindow(int argc, char **argv, QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow),
    ros_comm_(argc, argv)
{
    ui->setupUi(this);

    database *db = new database(this);
//    connect(ui, SIGNAL(clicked()), db, SLOT(saveData()));
    connect(ui->saveRTLData->clicked(), &QPushButton::clicked, db, &database::saveData);
    
    QTimer *timer = new QTimer(this);
    connect(timer, SIGNAL(timeout()), this, SLOT(gui_update()));
    timer->start(10);
}

I tried both the new and old connect and cannot seem to get any of them working.

I get following error in my console when i try to compile:

Can anyone see what i did wrong?

[Pablo J. Rogina]

@Christianvs said in Invalid use of void expression, when trying to connect signal/slot:

connect(ui->saveRTLData->clicked(), &QPushButton::clicked, db, &database::saveData);

I guess you are providing the signal but not the sender object, ui->saveRTLData->clicked() instead of ui->saveRTLData as required

connect(
    sender, &Sender::signal,
    receiver, &Receiver::slot
);

There's more information about new syntax of signals & slots

[mranger90]

I think the problem is the parentheses on ui->saveRTLData->clicked().

[aha_1980]

@mranger90

No, you have to omit clicked completely:

connect(ui->saveRTLData, &QPushButton::clicked, db, &database::saveData);

[Christianvs]

@Pablo-J.-Rogina

That was it! Thanks a lot :)