Error: no matching function for call to 'QObject::connect

[WhatIf]

Hi,

I'm building a simple program to learn how to use widgets, layouts, etc without using the designer but ran through a problem. Basically, the program is designed to have a typical main and MainWindow but instead of having all gui code in MainWindow, I have a class called MainWindowContent which builds the gui and returns the layout containing all the widgets which is added to MainWindow's central widget.

#include "mainwindowcontent.h"

MainWindowContent::MainWindowContent()
{

}

QGridLayout* MainWindowContent::getPageWidgetsLayout()
{
    QGridLayout *gridLayout  =new QGridLayout();

    QLabel *label = new QLabel("");

    QLineEdit *lineEdit = new QLineEdit("Enter Text!");

    QPushButton *button = new QPushButton("OK!");

    QObject::connect(button, SIGNAL (clicked()),this, SLOT (handleButton(label, lineEdit)));

    gridLayout->addWidget(label,0,0);
    gridLayout->addWidget(lineEdit,1,1);
    gridLayout->addWidget(button,2,2);
   

    return gridLayout;
}

//slot to handle button clicks
void MainWindowContent::handleButton(QLabel* label,QLineEdit *lineEdit)
{
    label->setText(lineEdit->text());
}

I keep getting

error: no matching function for call to 'QObject::connect(QPushButton*&, const char*, MainWindowContent*, const char*)'
QObject::connect(button, SIGNAL (clicked()),this, SLOT (handleButton(label, lineEdit)));

[mrjj]

@WhatIf said in error: no matching function for call to 'QObject::connect:

QObject::connect(button, SIGNAL (clicked()),this, SLOT (handleButton(label, lineEdit)));

Hi
That is not correct.
The clicked() signal do not have a label and lineEdit as parameters so you cannot connect like that.
the signal must have those parameters too.

Can i ask what the logic should be ?

[WhatIf]

Even if I delete the parameters label and lineEdit from the slot, its function implementation and in the .h file, I still get the same error.

[Paul Colby]

@WhatIf,

Show us the updated header declaration, updated connect call, and the resulting error message.

Cheers.

[VRonin]

QObject::connect(button, &QPushButton::clicked,this, std::bind(&MainWindowContent::handleButton,this,label,lineEdit));

https://wiki.qt.io/New_Signal_Slot_Syntax

[WhatIf]

private slots:
    void handleButton();

QObject::connect(button, SIGNAL (clicked()),this, SLOT (handleButton()));

void MainWindowContent::handleButton()
{
    //label->setText(lineEdit->text());
}

error: no matching function for call to 'QObject::connect(QPushButton*&, const char*, MainWindowContent*, const char*)'
QObject::connect(button, SIGNAL (clicked()),this, SLOT (handleButton()));

I also tried

QObject::connect(button, &QPushButton::clicked,this, std::bind(&MainWindowContent::handleButton,this,label,lineEdit));

and got

error: no matching function for call to 'QObject::connect(QPushButton*&, void (QAbstractButton::)(bool), MainWindowContent, std::_Bind_helper<false, void (MainWindowContent::)(QLabel, QLineEdit*), MainWindowContent*, QLabel*&, QLineEdit*&>::type)'
QObject::connect(button, &QPushButton::clicked,this, std::bind(&MainWindowContent::handleButton,this,label,lineEdit));

[hskoglund]

Hi, maybe you forgot to inherit from QObject or QWidget, i.e. your mainwindowcontent.h should look something like this:

#pragma once
#include "qobject.h"

class MainWindowContent : QObject
{
     Q_OBJECT

public:
    MainWindowContent();

private slots:
    void handleButton(QLabel* label,QLineEdit *lineEdit);
}

[VRonin]

@WhatIf said in error: no matching function for call to 'QObject::connect:

I also tried
QObject::connect(button, &QPushButton::clicked,this, std::bind(&MainWindowContent::handleButton,this,label,lineEdit));

and got

error: no matching function for call to 'QObject::connect(QPushButton*&, void (QAbstractButton::)(bool), MainWindowContent, std::_Bind_helper<false, void (MainWindowContent::)(QLabel, QLineEdit*), MainWindowContent*, QLabel*&, QLineEdit*&>::type)'
QObject::connect(button, &QPushButton::clicked,this, std::bind(&MainWindowContent::handleButton,this,label,lineEdit));

read the error. (MainWindowContent::)(QLabel, QLineEdit*) did you forget a * next to QLabel in handleButton?

[WhatIf]

I forgot to subclass QObject!

Now the program compiles but the slot is never called. here is how the code looks at this point.

signals:

private slots:
    void handleButton(); //handles button clicks

QObject::connect(button, SIGNAL (clicked()),this, SLOT (handleButton()));

void MainWindowContent::handleButton()
{
    qDebug() << "INSIDE SLOT!";
}

[mrjj]

Hi
try with
qDebug () << "conn:" << QObject::connect(button, SIGNAL (clicked()),this, SLOT (handleButton()));
and see it says
conn: true

[hskoglund]

Hi, also to add to @mrjj, perhaps the slot is never called because the button isn't visible, you could try insert a show() just after you've created the button, say like this:

QPushButton* button = new QPushButton("OK!");
button->show();

QObject::connect(button, SIGNAL (clicked()),this, SLOT (handleButton()));

[WhatIf]

qDebug () << "conn:" << QObject::connect(button, SIGNAL (clicked()),this, SLOT (handleButton()));

shows conn: true

using button->show(); doesn't change anything.

Basically, main.cpp calls mainwindow.cpp to get the central widget. mainwindow.cpp in turn calls mainwindowcontent.cpp to get the layout to set as centralwidget's.

I'm passing "this" (connect 3rd argument) of mainwindowcontent.cpp to the connect statement.

Will this make the connect work when mainwindowcontent.cpp returns the layout to mainwindow.cpp?

[SGaist]

Hi,

Did you check whether you have any warning at run time ?

How do you know it's not getting called ? Did you put a breakpoint in your slot or a qDebug statement ?

[WhatIf]

No warnings at run time.

I used both breakpoints and a qDebug statement.

[SGaist]

Then a silly question: how are you ensuring that you are clicking on the correct button ?

[WhatIf]

There is only one button

[hskoglund]

Hi, just guessing, but how do you assign the grid layout into the centralwidget in your mainwindowp.cpp, is it something similar to this:

...
setCentralWidget(new QWidget());
centralWidget()->setLayout(mainWindowContent->getPageWidgetsLayout());
...

[Paul Colby]

What's the lifetime of your MainWindowContent instance?

It looks to me that MainWindowContent::getPageWidgetsLayout() could be entirely static (if you wanted it to be) except for the this in the connect call. So if, for example, your MainWindowContent instance is destroyed, and re-created later, the slot will have been automatically disconnected, and not reconnected.

Just something else to check...

Cheers.

[WhatIf]

main.cpp

#include "mainwindow.h"
#include <QApplication>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;

    w.buildPage(); 

    w.show();

    return a.exec();
}

mainwindow.cpp

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

#include <QLabel>
#include <QHBoxLayout>

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

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

void MainWindow::buildPage()
{
    MainWindowContent winContent;

    ui->centralWidget->setLayout(winContent.getPageWidgetsLayout());
}

mainwindowcontent.cpp

#include "mainwindowcontent.h"

#include <QDebug>

MainWindowContent::MainWindowContent(QObject *parent) : QObject(parent)
{

}

QGridLayout* MainWindowContent::getPageWidgetsLayout()
{
    QGridLayout *gridLayout  =new QGridLayout();

    QLabel *label = new QLabel("");

    QLineEdit *lineEdit = new QLineEdit("Enter Text!");

    QPushButton *button = new QPushButton("OK!");

    QObject::connect(button, SIGNAL (clicked()),this, SLOT (handleButton()));

    gridLayout->addWidget(label,0,0);
    gridLayout->addWidget(lineEdit,1,1);
    gridLayout->addWidget(button,2,2);
   

    return gridLayout;
}

void MainWindowContent::handleButton()
{
    qDebug() << "INSIDE SLOT!";
}

[Paul Colby]

The problem is with the winContent lifetime.

void MainWindow::buildPage()
{
    MainWindowContent winContent;

    ui->centralWidget->setLayout(winContent.getPageWidgetsLayout());
    // at this point, winContent is destroyed, and automatically disconnected.
}

You probably want to do something more like:

void MainWindow::buildPage()
{
    MainWindowContent * winContent = new MainWindowContent(this);

    ui->centralWidget->setLayout(winContent->getPageWidgetsLayout());
}

Or move the slot to the MainWindow class.

Cheers.