NOOB Signals and Slots

admiral142

I'm a beginner, and I've run into a question when it comes to Signals and Slots. What I've done so far is connect a QML onClick (signal) event to a slot in a C++ class, then have that class emit its own signal. Now I need another C++ class slot to listen to that signal. I'm having trouble connecting the signal to the slot though. I'm trying to connect C++ class A's signal to C++ class B's slot in C++ class B's constructor. I want to avoid connections in main, because I'm going to be creating more complex applications on the future. Here is some code to help visualize:

//DataObject HEADER
class DataObject : public QObject
{
    Q_OBJECT
public:
    DataObject(QObject *parent=0);
    DataObject(const QString &name, const double &max, const double &min, QObject *parent=0);
    QString name() const;
    void setName(const QString &name);
    double max() const;
    void setMax(const double &max);
    double min() const;
    void setMin(const double &min);
signals:
    void nameChanged();
    void maxChanged();
    void minChanged();
    void updateSelection(); //THIS IS THE SIGNAL I'M SENDING
public slots:
    void newSelection()  //THIS IS INVOKED BY THE QML
    {
        qDebug() << "Called the C++ slot";
        emit updateSelection();
    }
private:
    QString m_name;
    double m_max;
    double m_min;
  };

//UseData HEADER
class UseData : public QObject
{
    Q_OBJECT
public:
    UseData(QObject *parent=0);    
signals:
public slots:
    void dataRefresh();  //THE SLOT I WANT TO INVOKE
private:
};

//UseData DEFINITION
UseData::UseData(QObject *parent) : QObject(parent)
{
    connect(sender, SIGNAL(updateSelection()), this, SLOT(dataRefresh()));  //ERROR HERE
}
void UseData::dataRefresh()
{
    qDebug() << "Called UseData Slot";
}

The error I'm getting says, "'QObject::sender': function call missing argument list; use'&QObject::sender' to create a pointer to member

Any help would be great!

Joel Bodenmann

Hello & welcome,

The problem is that in the scope where you call QObject::connect() (namely in the constructor of your UseData class) you don't have any object named sender. The first parameter of the QObject::connect() method needs to be a valid QObject that emits the signal you are going to connect.
If the sender of the updateSelection() signal you are caring about isn't known/accessible in your UserData constructor you have to make the connect outside of the class (or anywhere else where the sender object is known).

Ni.Sumi

@admiral142

you need the pointer of the class DataObject in the class UseData. . The "sender" in the connect hast to be the pointe of the class DataObject.

connect(objectA, SIGNAL(xxx()), objectB, SLOT(xxxx()));
//this connect() is old. Please check the new syntax of the connecct() from 5.7(if your are using latest version)

class UseData : public QObject
{
    Q_OBJECT
public:
    UseData(QObject *parent=0);   
    DataObject*  dataobject;	//******** you need the object of the class where the signal is 
signals:
public slots:
    void dataRefresh();  
private:
};

//UseData DEFINITION
UseData::UseData(QObject *parent) : QObject(parent)
{
    connect(dataobject, SIGNAL(updateSelection()), this, SLOT(dataRefresh()));  //use that class object to connect
}
void UseData::dataRefresh()
{
    qDebug() << "Called UseData Slot";
}

admiral142

Ok, that makes sense. The problem then becomes that DataObject is being used in a QList. So I'm newing up instances of it to add to the QList. My question is, if I were to connect a general instance of DataObject in my main to my general instance of UseData in my main, would any signal from any DataObject then reach my UseData class? I believe that's a scope question. Here is what I'm thinking:

int main(int argc, char *argv[])
{
    QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
    QGuiApplication app(argc, argv);

    QQmlApplicationEngine engine;

    UseData useData;  //GENERAL INSTANCE OF UseData
    DataObject dataObject  //ADDED - GENERAL INSTANCE OF DataObject

    QList<QObject*> dataList;
    dataList.append(new DataObject("This", 100, 0, 80, 50));
    dataList.append(new DataObject("That", 100, 0, 75, 70));
    
    connect(dataObject, &dataObject::updateSelection, useData, &useData::dataRefresh );  //ADDED - Signal and Slot connection.  Correct Syntax?

    QQmlContext *ctxt = engine.rootContext();
    ctxt->setContextProperty("pidList", QVariant::fromValue(dataList));
    ctxt->setContextProperty("activeGauge", &gaugeData);

    engine.load(QUrl(QLatin1String("qrc:/main.qml")));

    return app.exec();

Would that do it? Thanks again!

admiral142

@Ni.Sumi Wouldn't including the DataObject class in my UseData class cause them to become tightly coupled?