Passing custom arguments to a slot in Qt/C++

Question

I'm having some troubles trying to pass a custom widget to a custom slot inside my Qt App.

Here an example of what I need: (Note the slots)

void MainWindow::HttpRequest(const QString & URL, QCustomWidget *Feed) {
    manager = new QNetworkAccessManager(this);
    reply = manager->get(QNetworkRequest(QUrl(URL)));
    connect(reply, SIGNAL(error(QNetworkReply::NetworkError)), this, SLOT(HttpImageError(Feed)));
    connect(reply, SIGNAL(finished()), this, SLOT(HttpImageFinished(Feed)));
}

I've already searched on Google and I've found that QSignalMapper is used to pass arguments to slots, but QSignalMapper is usable only to pass integers, QStrings , QObjects and QWidgets . I need to pass a custom widget. I've also read that there are tricks to wrap the custom widget inside a struct or something like that, but I'm very confused on how to do that.

Does anybody can help me?

Answer 1

With Qt 5 and C++11 you can use a new lambda syntax:

connect(reply, &QNetworkReply::NetworkError, [this, Feed]() {
    this->HttpImageError(Feed);
});

Here an additional Feed parameter to the slot function is added to lambda capture block.

Answer 2

Just for further information... I found another solution to the problem, in fact to pass a custom widget I've subclassed the QSignalMapper class (I haven't studied OOP at the University yet, so be patient with me! :P), here some code:

QCustomMapper.h

#ifndef QCUSTOMMAPPER_H
#define QCUSTOMMAPPER_H

#include <QSignalMapper>
#include <QHash>
#include "customwidget.h"

class QCustomMapper : public QSignalMapper
{
    Q_OBJECT
public:
    explicit QCustomMapper(QObject *parent = 0);
    void setMapping(QObject *sender, CustomWidget *customWidget);
    void removeMappings(QObject *sender);
    QHash<QObject *, CustomWidget *> customHash;

Q_SIGNALS:
    void mapped(CustomWidget *);

public slots:
    void senderDestroyed() {
        removeMappings(sender());
    }
    void map(QObject *sender);
    void map();

};

#endif // QCUSTOMMAPPER_H

QCustomMapper.cpp

#include "qcustommapper.h"

QCustomMapper::QCustomMapper(QObject *parent) : QSignalMapper(parent) {

}

void QCustomMapper::setMapping(QObject *sender, CustomWidget *customWidget) {
    customHash.insert(sender, customWidget);
    connect(sender, SIGNAL(destroyed()), this, SLOT(senderDestroyed()));

}

void QCustomMapper::removeMappings(QObject *sender) {
    customHash.remove(sender);
}

void QCustomMapper::map(QObject *sender) {
    if (customHash.contains(sender)) {
        emit mapped(customHash.value(sender));
    }
}

void QCustomMapper::map() {
    map(sender());
}

NOTE: I don't know if this is an "elegant" method or if the subclassing is done correctly.. (I've done it without the right knoledgments) anyway it's working fine for me! (miracles happen :D) I hope that this post will help someone that has the same problem!

Bye, Matteo.