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

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.

WhatIf

I want to keep the logic of how the program progresses from page to page in MainWindow. But I want other classes to generate the pages to be displayed. Basically I want to be able to manage the program without having a single .cpp file that is a thousand lines or so.

SGaist

Then why not make them full QWidget derivative ?

That's what is usually done, cut your application in logical pieces and then build as many QWidget based classes as needed to keep the code clean, simple and tidy.

WhatIf

Can you give an example please? Or direct me to any tutorial on the topic?

SGaist

The scribble example is a simple example to start with.

WhatIf

I went over the example but I still have some difficulty converting it to my use. I created a MainWindowContent that is a subclass of QWidget and created a

QGridLayout* MainWindowContent::getPageWidgetsLayout()

the exact same function I provided in the code above.

I don't think that this is the intended goal of this example because in MainWindow an instance of the QWidget subclass (ScribbleArea) is passed to setCentralWidget not a call to a function using the instance. Am I supposed to move all the code from getPageWidgetsLayout() to the constructor and declare the label, lineedit, and pushbutton as private variables.

SGaist

No the example shows what I suggested: create a separated widget that encapsulates the functionality of interest and use that one in the MainWindow.