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

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.

WhatIf

Thank you very much for all the help!

The last problem I have is how to pass the label and lineedit to the slot. I don't want to make them global variables. if I create a custom signal with the label and lineedit to match the slot, I'm confused about where to emit the signal.

SGaist

Before that, why do you need the MainWindowContent. It looks like a convoluted mean to create the central widget content.