no matching member function for call to 'connect' - simple example why doesn't this work??

pmh4514

Hi,

I made a very simple test project, just a QMainWindow. I instantiated a QSpinBox in the constructor and tried to connect it to a public slot of MainWindow. Yet I get the error "no matching member function for call to 'connect'. This using both the old and new style connect syntax. My header includes Q_OBJECT. Both QMainWindow and QSpinBox derive from QObject.

I must be missing/forgetting something very simple here. Why on earth doesn't this work?

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 = nullptr);
    ~MainWindow();

public slots:
    void doTest(int val);

private:
    Ui::MainWindow *ui;
};

#endif // MAINWINDOW_H

mainwindow.cpp

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

#include <QSpinBox>

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

    QSpinBox* oSpinBox = new QSpinBox(this);
    connect(oSpinBox, &QSpinBox::valueChanged, this, &MainWindow::doTest);

}

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

void MainWindow::doTest(int val)
{
    // do something
}

J.Hilk

@pmh4514 said in no matching member function for call to 'connect' - simple example why doesn't this work??:

valueChanged

value changed has 2 overloads int and string, the new connect does not know how to handle that,

but the spinbox documentation actually offers you the solution:

https://doc.qt.io/qt-5/qspinbox.html#valueChanged

aha_1980

@pmh4514

Please post the exact error message with the corresponding code.

Your problem is most likely, that the spinbox is a local variable which does not outlive the ctor.

Regards

Bondrusiek

@pmh4514 said in no matching member function for call to 'connect' - simple example why doesn't this work??:

onnect(oSpinBox, &QSpinBox::valueChanged, this, &MainWindow::doTest);

Problem is that valuChanged is overloaded so you must use other connect's construction.
Try this:

connect(oSpinBox, QOverload<int>::of(&QSpinBox::valueChanged), this, &MainWindow::doTest);

https://doc.qt.io/qt-5/qspinbox.html#valueChanged

pmh4514

I posted all the code and the error message exactly.

Then as suggested I tried making the QSpinBox a member variable of MainWindow so that it would outlive the constructor.

I still got the "no matching member function for call to 'connect' error.

However, if instead I use the old style connect syntax that explicitly specifies the valueChanged(int) interface, then it works:

connect(oSpinBox, SIGNAL(valueChanged(int)), this, SLOT(doTest(int)));

The documentation linked above and its use of QOverload answers the rest.

Thanks everyone!

JKSH

@pmh4514 said in no matching member function for call to 'connect' - simple example why doesn't this work??:

However, if instead I use the old style connect syntax that explicitly specifies the valueChanged(int) interface, then it works:

Yes: https://doc.qt.io/qt-5/signalsandslots-syntaxes.html#selecting-overloaded-signals-and-slots