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.

Files:

Contents

Multiple Inheritance Example

CalculatorForm Definition

CalculatorForm Implementation

main() Function

`main()` Function