In Qt, each interaction between a user and a widget is considered an event, such as clicking a button or closing a window. These events emit signals, like a button-click signal or a window-close signal. Widgets in Qt can receive multiple signals and respond with specific actions, known as slots, which are essentially callback functions. The signals and slots mechanism enables communication between independent widgets, allowing acsions like closing a window when a button is clicked.

Signals involve three components: the source widget emitting the signal, the signal type based on user action, and the slot function handling the signal. Qt uses the connect function to link signals to slots, automating slot execution upon signal emission.

The connect function, a static member of QObject, has a traditional prototype:

connect(const QObject *sender, const char *signal, const QObject *receiver, const char *method, Qt::ConnectionType type = Qt::AutoConnection);

Example usage:

QPushButton *closeBtn = new QPushButton(this);
closeBtn->setText("Close");
closeBtn->move(200, 200);
connect(closeBtn, &QPushButton::clicked, this, &MainWindow::closeWindow);

Custom slots are defined similarly to regular member functions. Historically, they were placed in sections like public slots, but modern Qt allows automatic connections via naming conventions, such as on_objectName_signal(). For instance, a slot named on_closeButton_clicked() can be auto-connected to a button's click signal without explicit connect calls.

Custom signals are less common and require declaration with the signals keyword, returning void, with optional parameters and support for overloading. Qt automatically genertaes their implementations during compilation. For example:

signals:
    void dataUpdated(int value);

Parameters in signals and slots must match in type, with signals having at least as many parameter as slots to allow argument passing. Classes using signals and slots must include the Q_OBJECT macro at the start.

To disconnect signals and slots, use disconnect with similar syntax to connect:

disconnect(ui->actionBtn, &QPushButton::clicked, this, &Widget::handleAction);

Lambda expressions offer a concise way to define slot functions for one-time use, supporting variable capture. Capture all local variables by value with [=] or by reference with [&], though pointer captures are typical in Qt. Ensure captured variables remain valid during callback execution. Example:

connect(ui->calcBtn, &QPushButton::clicked, this, [=]() {
    int result = computeValue(input->text());
    output->setText(QString::number(result));
});

Lambda syntax requires C++11, enabled by default in Qt5+ or via CONFIG += c++11 in older versions.