Differences between String-Based and Functor-Based Connections

String-based connections type-check by comparing strings at run-time. There are three limitations with this approach:

1. Connection errors can only be detected after the program has started running.

2. Implicit conversions cannot be done between signals and slots.

3. Typedefs and namespaces cannot be resolved.

Limitations 2 and 3 exist because the string comparator does not have access to C++ type information, so it relies on exact string matching.

In contrast, functor-based connections are checked by the compiler. The compiler catches errors at compile-time, enables implicit conversions between compatible types, and recognizes different names of the same type.

For example, only the functor-based syntax can be used to connect a signal that carries an int to a slot that accepts a double. A QSlider holds an int value while a QDoubleSpinBox holds a double value. The following snippet shows how to keep them in sync:

    auto slider = new QSlider(this);
    auto doubleSpinBox = new QDoubleSpinBox(this);

    // OK: The compiler can convert an int into a double
    connect(slider, &QSlider::valueChanged,
            doubleSpinBox, &QDoubleSpinBox::setValue);

    // ERROR: The string table doesn't contain conversion information
    connect(slider, SIGNAL(valueChanged(int)),
            doubleSpinBox, SLOT(setValue(double)));

The following example illustrates the lack of name resolution. QAudioInput::stateChanged() is declared with an argument of type "QAudio::State". Thus, string-based connections must also specify "QAudio::State", even if "State" is already visible. This issue does not apply to functor-based connections because argument types are not part of the connection.

    auto audioInput = new QAudioInput(QAudioFormat(), this);
    auto widget = new QWidget(this);

    // OK
    connect(audioInput, SIGNAL(stateChanged(QAudio::State)),
            widget, SLOT(show()));

    // ERROR: The strings "State" and "QAudio::State" don't match
    using namespace QAudio;
    connect(audioInput, SIGNAL(stateChanged(State)),
            widget, SLOT(show()));

    // ...

The functor-based connection syntax can connect signals to C++11 lambda expressions, which are effectively inline slots. This feature is not available with the string-based syntax.

In the following example, the TextSender class emits a textCompleted() signal which carries a QString parameter. Here is the class declaration:

class TextSender : public QWidget {
    Q_OBJECT

    QLineEdit *lineEdit;
    QPushButton *button;

signals:
    void textCompleted(const QString& text) const;

public:
    TextSender(QWidget *parent = nullptr);
};

Here is the connection which emits TextSender::textCompleted() when the user clicks the button:

TextSender::TextSender(QWidget *parent) : QWidget(parent) {
    lineEdit = new QLineEdit(this);
    button = new QPushButton("Send", this);

    connect(button, &QPushButton::clicked, [=] {
        emit textCompleted(lineEdit->text());
    });

    // ...
}

In this example, the lambda function made the connection simple even though QPushButton::clicked() and TextSender::textCompleted() have incompatible parameters. In contrast, a string-based implementation would require extra boilerplate code.

The string-based syntax can connect C++ objects to QML objects, but the functor-based syntax cannot. This is because QML types are resolved at run-time, so they are not available to the C++ compiler.

In the following example, clicking on the QML object makes the C++ object print a message, and vice-versa. Here is the QML type (in QmlGui.qml):

Rectangle {
    width: 100; height: 100

    signal qmlSignal(string sentMsg)
    function qmlSlot(receivedMsg) {
        console.log("QML received: " + receivedMsg)
    }

    MouseArea {
        anchors.fill: parent
        onClicked: qmlSignal("Hello from QML!")
    }
}

Here is the C++ class:

class CppGui : public QWidget {
    Q_OBJECT

    QPushButton *button;

signals:
    void cppSignal(const QVariant& sentMsg) const;

public slots:
    void cppSlot(const QString& receivedMsg) const {
        qDebug() << "C++ received:" << receivedMsg;
    }

public:
    CppGui(QWidget *parent = nullptr) : QWidget(parent) {
        button = new QPushButton("Click Me!", this);
        connect(button, &QPushButton::clicked, [=] {
            emit cppSignal("Hello from C++!");
        });
    }
};

Here is the code that makes the signal-slot connections:

    auto cppObj = new CppGui(this);
    auto quickWidget = new QQuickWidget(QUrl("QmlGui.qml"), this);
    auto qmlObj = quickWidget->rootObject();