How to pass and invoke lambda function at slot ?

[huseyinkozan]

Hi,

I want to take lambda as function parameter and invoke it, but couldn't realize how to do that. For ex:

MyObject::ask(const QString & text, QObject * receiver, / * param type ? */ func) {
    // setup m_msgBox to ask question
    connect(m_msgBox,  &QMessageBox::finished, this, [&](int result) {
        func(result); // how to call lambda function ?
    });
    m_msgBox->open();
}

MyObject::someFunction() {
    ask(tr("How?"), this, [&](int result) {
        // process result
    });
}

[fcarney]

std::function

MyObject::ask(const QString & text, QObject * receiver, std::function<void(int)> func)

or

template<class T>
MyObject::ask(const QString & text, QObject * receiver, T func)

The template one can get messy for class members, but is still doable.

[huseyinkozan]

@fcarney Thanks.

I tried to did first way, and call like:

MyObject::ask(const QString & text, QObject * receiver, std::function<void(int)> func) {
...
func(result);

But failed at functional header:

__throw_bad_function_call();

Also, should I use receiver ?

[fcarney]

Why are you passing this here?:

connect(m_msgBox,  &QMessageBox::finished, this, [&](int result) {

Can you create a minimal project that shows the error? Something we can compile?

[huseyinkozan]

@fcarney Right, no need to pass this.
Found the problem, changing the capture from & to = solved the exception.

#include <QApplication>
#include <QMessageBox>
#include <QPushButton>
#include <QDebug>
#include <QScreen>
#include <QStyle>

class Button : public QPushButton
{
    Q_OBJECT
public:
    Button(QWidget *parent = nullptr) : QPushButton(parent) {
        setText(tr("Click"));
        m_msgBox = new QMessageBox(this);
        connect(this, &QPushButton::clicked, [&](){
            askProceed();
        });
    }
    virtual ~Button() {}

    void ask(
            const QString & text,
            const QStringList & buttons,
            int acceptButton,
            int rejectButton,
            std::function<void(int)> func)
    {
        m_msgBox->setText(text);
        m_msgBox->setIcon(QMessageBox::Question);
        m_msgBox->setStandardButtons(QMessageBox::NoButton);
        QList<QAbstractButton*> buttonList;
        for (int i = 0; i < buttons.count(); ++i) {
            if (i == acceptButton) {
                QPushButton * btn = m_msgBox->addButton(buttons.at(i), QMessageBox::AcceptRole);
                m_msgBox->setDefaultButton(btn);
                buttonList.append((QAbstractButton*)btn);
            }
            else if (i == rejectButton) {
                QPushButton * btn = m_msgBox->addButton(buttons.at(i), QMessageBox::RejectRole);
                m_msgBox->setEscapeButton((QAbstractButton*)btn);
                buttonList.append((QAbstractButton*)btn);
            }
            else {
                Q_ASSERT(false);
            }
        }
        connect(m_msgBox,  &QMessageBox::finished, [=](int) {
            int res = rejectButton;
            for (int i = 0; i < buttonList.count(); ++i) {
                if (m_msgBox->clickedButton() == buttonList.at(i)) {
                    res = i;
                    break;
                }
            }
            QList<QAbstractButton*> listToDelete = m_msgBox->buttons();
            foreach (QAbstractButton * btn, listToDelete) {
                m_msgBox->removeButton(btn);
                btn->deleteLater();
            }
            m_msgBox->disconnect();
            func(res);
        });
        m_msgBox->open();
    }
    void askProceed() {
        ask(
                    tr("Which do you prefer?"),
                    QStringList() << tr("Apple") << tr("Pineapple"),
                    0, 1, [&](int result) {
            if (result == 0)
                qDebug() << "Selected Apple";
            else if (result == 1)
                qDebug() << "Selected Pineapple";
            else
                qDebug() << "Unexpected result = " << result;

        });
    }
private:
    QMessageBox * m_msgBox = nullptr;
};

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    Button btn;
    btn.resize(100, 30);
    btn.setGeometry(
        QStyle::alignedRect(
            Qt::LeftToRight,
            Qt::AlignCenter,
            btn.size(),
            qApp->screens().first()->availableGeometry()
        )
    );
    btn.show();
    return a.exec();
}

#include "main.moc"

Also added to my snippets:
https://gitlab.com/snippets/1888176

[huseyinkozan]

@fcarney said in How to pass and invoke lambda function at slot ?:

Why are you passing this here?:
connect(m_msgBox, &QMessageBox::finished, this, [&](int result) {

Should we check if the object and function exists before calling the callback ?

[KroMignon]

@huseyinkozan said in How to pass and invoke lambda function at slot ?:

Should we check if the object and function exists before calling the callback ?

When a QObject is deleted, all connection on this object are removed ==> Take a look at QObject::~QObject() for more details

[huseyinkozan]

@kromignon Thanks.
Then we can be sure that all childs of deleted QObject will disconnect.

At my example, m_msgBox will be deleted after btn delete, and lambda for &QMessageBox::finished will not call. I prefer checking func is not a dangling pointer. But seems not possible with std::function::operator==.

I tried to find at QObject::connect() code at woboq, but couldn't find how to handle lambdas.

[KroMignon]

@huseyinkozan said in How to pass and invoke lambda function at slot ?:

but couldn't find how to handle lambdas.

Qt documentation is big, so find the right information is not that easy, but Google is very performant for "data mining".
A simple search with "connect functor" and you will find Making Connections to Lambda Expressions

[huseyinkozan]

@kromignon Thanks.
I had found the page that you gave, but couldn't realise how QObject::connect() accepts lambdas.

I am using lambdas newly. Just read this article, and helped alot:
https://blog.feabhas.com/2014/03/demystifying-c-lambdas/

[KroMignon]

@huseyinkozan Thanks for the link, I've found this one https://medium.com/genymobile/how-c-lambda-expressions-can-improve-your-qt-code-8cd524f4ed9f

[huseyinkozan]

@kromignon Thanks.

I had also read that article, but that also not defines how Qt handle this stuation:

In this version, we pass monitor as a context to connect(). It won't affect the execution of our lambda, but when monitor is deleted, Qt will notice and will disconnect Worker::progress() from our lambda.

I tried with below, but could not find how to detect I have already disconnect:

connect(receiver, &QObject::destroyed, [this](){
    qDebug() << "receiver destroyed";
    m_msgBox->disconnect();
});

Full code with receiver (deletes btn after 10 sec!):

#include <QApplication>
#include <QMessageBox>
#include <QPushButton>
#include <QDebug>
#include <QScreen>
#include <QStyle>
#include <QTimer>

class Button : public QPushButton
{
    Q_OBJECT
public:
    Button(QWidget *parent = nullptr) : QPushButton(parent) {
        setText(tr("Click"));
        m_msgBox = new QMessageBox(this);
        connect(this, &QPushButton::clicked, [this](){
            ask(tr("Which do you prefer?"),
                QStringList() << tr("Apple") << tr("Pineapple"),
                0, 1, this,
                [](int result) {
                  if (result == 0)
                      qDebug() << "Selected Apple";
                  else if (result == 1)
                      qDebug() << "Selected Pineapple";
                  else
                      qDebug() << "Unexpected result = " << result;
            });
        });
    }
    virtual ~Button() {}

    void ask(
            const QString & text,
            const QStringList & buttons,
            int acceptButton,
            int rejectButton,
            const QObject * receiver,
            std::function<void(int)> func)
    {
        m_msgBox->setText(text);
        m_msgBox->setIcon(QMessageBox::Question);
        m_msgBox->setStandardButtons(QMessageBox::NoButton);
        QList<QAbstractButton*> buttonList;
        for (int i = 0; i < buttons.count(); ++i) {
            if (i == acceptButton) {
                QPushButton * btn = m_msgBox->addButton(buttons.at(i), QMessageBox::AcceptRole);
                m_msgBox->setDefaultButton(btn);
                buttonList.append((QAbstractButton*)btn);
            }
            else if (i == rejectButton) {
                QPushButton * btn = m_msgBox->addButton(buttons.at(i), QMessageBox::RejectRole);
                m_msgBox->setEscapeButton((QAbstractButton*)btn);
                buttonList.append((QAbstractButton*)btn);
            }
            else {
                Q_ASSERT(false);
            }
        }
        connect(m_msgBox,  &QMessageBox::finished, [this, rejectButton, buttonList, func](int) {
            m_msgBox->disconnect();
            int res = rejectButton;
            for (int i = 0; i < buttonList.count(); ++i) {
                if (m_msgBox->clickedButton() == buttonList.at(i)) {
                    res = i;
                    break;
                }
            }
            QList<QAbstractButton*> listToDelete = m_msgBox->buttons();
            foreach (QAbstractButton * btn, listToDelete) {
                m_msgBox->removeButton(btn);
                btn->deleteLater();
            }
            func(res);
        });
        connect(receiver, &QObject::destroyed, [this](){
            qDebug() << "receiver destroyed";
            m_msgBox->disconnect();
        });
        m_msgBox->open();
    }
private:
    QMessageBox * m_msgBox = nullptr;
};

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    Button * btn = new Button;
    btn->resize(100, 30);
    btn->setGeometry(
        QStyle::alignedRect(Qt::LeftToRight, Qt::AlignCenter, btn->size(),
                            qApp->screens().first()->availableGeometry()));
    btn->show();
    QTimer::singleShot(10000, [&](){
        delete btn;
        btn = nullptr;
    });
    int res = a.exec();
    if (btn)
        delete btn;
    return res;
}

#include "main.moc"