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connect with SLOT/SIGNAL: QPushButton clicked signal not received by main window

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I added a QPushButton to my ui file named pushButton, and manually connected it using connect.

  • There are no errors or warnings emitted at any point in the compilation stage
  • The button does show up on the UI.
  • When I click on the button, nothing happens.

MainWindow.cpp:

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include <QMessageBox>

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

    connect(ui->pushButton, SIGNAL(clicked()), this, SLOT(pushButton_Clicked));
}

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

void MainWindow::pushButton_Clicked()
{
    QMessageBox::information(nullptr, tr("Status"), tr("You clicked the button!"));
}

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 = 0);
    ~MainWindow();

private:
    Ui::MainWindow *ui;

private slots:
    void pushButton_Clicked();
};

#endif // MAINWINDOW_H

What do I need to do to make MainWindow::pushButton_Clicked() get called when the button is clicked?

Why should this post be closed?

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1 answer

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Qt's SLOT macro is very specific in what it accepts.

When specifying a slot in the connect function, it's required to put parenthesis, so the correct code would be:

connect(ui->pushButton, SIGNAL(clicked()), this, SLOT(pushButton_Clicked()));

With this syntax, the button clicked() signal will be received by MainWindow.

Some background information

When you use the SLOT() macro, you aren't passing in a function pointer, it actually stringizes what you pass in behind the scenes.

So you could actually do something like this, and it would still be accepted.:

connect(ui->pushButton, SIGNAL(clicked()), this, SLOT(gibberish));

Unfortunately there are no errors or warnings to help find typos in usages of the SLOT macro, be very careful with this macro.

Alternatives

It's possible to use function pointers instead of using the SIGNAL/SLOT macros.

connect(ui->pushButton, &QPushButton::clicked, this, &MainWindow::pushButton_Clicked);

For overloaded functions, it's necessary to explicitly specify the type of function pointer, (auto is no help).

void (QComboBox::*comboBox_IndexChanged)(int) = &QComboBox::currentIndexChanged;

QObject::connect(itemComboBox, comboBox_IndexChanged, this, &MainWindow::OnCellComboBoxChanged, Qt::AutoConnection);

// note: 
// QObject::connect(itemComboBox, &QComboBox::currentIndexChanged(int), this, &MainWindow::OnCellComboBoxChanged(int));
// does NOT work. Unfortunately you cannot specify an overloaded method like this.

I think there are some other methods too, but these are the best options I was able to find in Qt 5.7.0

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