How to Use the Qt PIMPL Idiom in C ?

Using the Qt's PIMPL Idiom

Introduction

The PIMPL (Pointer to Implementation) idiom allows for separating the public interface from the private implementation details of a class, preventing users of the class from being concerned with those details. Qt employs its own implementation of the PIMPL idiom, which is documented below.

The Interface

The PIMPL-based interface for Qt looks as follows:

<code class="cpp">class CoordinateDialog : public QDialog
{
  Q_OBJECT
  Q_DECLARE_PRIVATE(CoordinateDialog)
#if QT_VERSION  const d_ptr;
public:
  CoordinateDialog(QWidget * parent = 0, Qt::WindowFlags flags = 0);
  ~CoordinateDialog();
  QVector3D coordinates() const;
  Q_SIGNAL void acceptedCoordinates(const QVector3D &);
};</code>

Here, the Q_DECLARE_PRIVATE macro declares the PIMPL class and provides the necessary mechanisms to access it.

The Implementation

The PIMPL class, CoordinateDialogPrivate, is defined in the implementation file:

<code class="cpp">class CoordinateDialogPrivate {
  Q_DISABLE_COPY(CoordinateDialogPrivate)
  Q_DECLARE_PUBLIC(CoordinateDialog)
  CoordinateDialog * const q_ptr;
  QFormLayout layout;
  QDoubleSpinBox x, y, z;
  QDialogButtonBox buttons;
  QVector3D coordinates;
  void onAccepted();
  CoordinateDialogPrivate(CoordinateDialog*);
};</code>

Q_DECLARE_PRIVATE

The Q_DECLARE_PRIVATE macro simplifies the declaration of the PIMPL class and associates it with the interface class. It generates inline implementations of the d_func() helper method, which provides access to the PIMPL with appropriate constness.

Q_PRIVATE_SLOT

This macro is used for Qt 4 compatibility or when targeting non-C 11 compilers. It declares a private slot for internal use.

Q_DECLARE_PUBLIC

The Q_DECLARE_PUBLIC macro provides access to the interface from the PIMPL. It generates inline implementations of the q_func() helper method, similar to d_func().

Common Gotchas

• The PIMPL class should not be a private class within the interface class itself.
• The PIMPL pointer should be const for non-copyable/non-assignable classes like QObject.
• Avoid using placement new and the Fast Pimpl idiom for performance reasons.

Non-QObject Copyable Classes

The PIMPL idiom can also be used for copyable, non-QObject classes. However, the PIMPL pointer must be non-const. The Rule of Four (copy constructor, move constructor, assignment operator, destructor) and a free-standing swap function should be implemented.

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