Qt 5.13 | Qt UI Tools | Multiple Inheritance Example

Qt 5.13.2 Reference Documentation

# Multiple Inheritance Example

Using a form created with Qt Designer in an application.

The Multiple Inheritance Example shows how to use a form created with Qt Designer in an application by subclassing both QWidget and the user interface class, which is `Ui::CalculatorForm`.

To subclass the `calculatorform.ui` file and ensure that `qmake` processes it with the `uic`, we have to include `calculatorform.ui` in the `.pro` file, as shown below:

      QT += widgets

      HEADERS = calculatorform.h
      SOURCES = calculatorform.cpp main.cpp
      FORMS = calculatorform.ui

When the project is compiled, the `uic` will generate a corresponding `ui_calculatorform.h`.

#### CalculatorForm Definition

In the `CalculatorForm` definition, we include the `ui_calculatorform.h` that was generated earlier.

      #include "ui_calculatorform.h"

As mentioned earlier, the class is a subclass of both QWidget and `Ui::CalculatorForm`.

      class CalculatorForm : public QWidget, private Ui::CalculatorForm
      {
          Q_OBJECT

      public:
          explicit CalculatorForm(QWidget *parent = nullptr);

      private slots:
          void on_inputSpinBox1_valueChanged(int value);
          void on_inputSpinBox2_valueChanged(int value);
      };

Two slots are defined according to the automatic connection naming convention required by `uic`. This is to ensure that QMetaObject's auto-connection facilities connect all the signals and slots involved automatically.

#### CalculatorForm Implementation

In the constructor, we call `setupUi()` to load the user interface file. Note that setupUi is a method of `Ui::CalculatorForm`.

      CalculatorForm::CalculatorForm(QWidget *parent)
          : QWidget(parent)
      {
          setupUi(this);
      }

We include two slots, `on_inputSpinBox1_valueChanged()` and `on_inputSpinBox2_valueChanged()`. These slots respond to the valueChanged() signal that both spin boxes emit. Whenever there is a change in one spin box's value, we take that value and add it to whatever value the other spin box has.

      void CalculatorForm::on_inputSpinBox1_valueChanged(int value)
      {
          outputWidget->setText(QString::number(value + inputSpinBox2->value()));
      }

      void CalculatorForm::on_inputSpinBox2_valueChanged(int value)
      {
          outputWidget->setText(QString::number(value + inputSpinBox1->value()));
      }

#### `main()` Function

The `main()` function instantiates QApplication and `CalculatorForm`. The `calculator` object is displayed by invoking the show() function.

      int main(int argc, char *argv[])
      {
          QApplication app(argc, argv);
          CalculatorForm calculator;
          calculator.show();
          return app.exec();
      }

There are various approaches to include forms into applications. The Multiple Inheritance approach is just one of them. See Using a Designer UI File in Your Application for more information on the other approaches available.

Example project @ code.qt.io