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Object methods

Just as with C++, there are many different ways to define object methods and extend them: the following list and sections draw on C++ vocabulary. (Readers are expected to know basic C++ concepts. Those who have not had to write C++ code recently can refer to e.g. http://www.cplusplus.com/doc/tutorial/ to refresh their memories.)

  • non-virtual public methods,

  • virtual public methods and

  • virtual private methods

Non-virtual public methods

These are the simplest, providing a simple method which acts on the object. Provide a function prototype in the header and an implementation of that prototype in the source file. 

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/* declaration in the header. */
void viewer_file_open (ViewerFile  *self,
                       GError     **error);

/* implementation in the source file */
void
viewer_file_open (ViewerFile  *self,
                  GError     **error)
{
  g_return_if_fail (VIEWER_IS_FILE (self));
  g_return_if_fail (error == NULL || *error == NULL);

  /* do stuff here. */
}

Virtual public methods

This is the preferred way to create GObjects with overridable methods:

  • Define the common method and its virtual function in the class structure in the public header

  • Define the common method in the header file and implement it in the source file

  • Implement a base version of the virtual function in the source file and initialize the virtual function pointer to this implementation in the object’s class_init function; or leave it as NULL for a ‘pure virtual’ method which must be overridden by derived classes

  • Re-implement the virtual function in each derived class which needs to override it

Note that virtual functions can only be defined if the class is derivable, declared using G_DECLARE_DERIVABLE_TYPE so the class structure can be defined. 

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/* declaration in viewer-file.h. */
#define VIEWER_TYPE_FILE viewer_file_get_type ()
G_DECLARE_DERIVABLE_TYPE (ViewerFile, viewer_file, VIEWER, FILE, GObject)

struct _ViewerFileClass
{
  GObjectClass parent_class;

  /* stuff */
  void (*open) (ViewerFile  *self,
                GError     **error);

  /* Padding to allow adding up to 12 new virtual functions without
   * breaking ABI. */
  gpointer padding[12];
};

void viewer_file_open (ViewerFile  *self,
                       GError     **error);

/* implementation in viewer-file.c */
void
viewer_file_open (ViewerFile  *self,
                  GError     **error)
{
  ViewerFileClass *klass;

  g_return_if_fail (VIEWER_IS_FILE (self));
  g_return_if_fail (error == NULL || *error == NULL);

  klass = VIEWER_FILE_GET_CLASS (self);
  g_return_if_fail (klass->open != NULL);

  klass->open (self, error);
}

The code above simply redirects the open call to the relevant virtual function.

It is possible to provide a default implementation for this class method in the object's class_init function: initialize the klass->open field to a pointer to the actual implementation. By default, class methods that are not inherited are initialized to NULL, and thus are to be considered "pure virtual". 

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static void
viewer_file_real_close (ViewerFile  *self,
                        GError     **error)
{
  /* Default implementation for the virtual method. */
}

static void
viewer_file_class_init (ViewerFileClass *klass)
{
  /* this is not necessary, except for demonstration purposes.
   *
   * pure virtual method: mandates implementation in children.
   */
  klass->open = NULL;

  /* merely virtual method. */
  klass->close = viewer_file_real_close;
}

void
viewer_file_open (ViewerFile  *self,
                  GError     **error)
{
  ViewerFileClass *klass;

  g_return_if_fail (VIEWER_IS_FILE (self));
  g_return_if_fail (error == NULL || *error == NULL);

  klass = VIEWER_FILE_GET_CLASS (self);

  /* if the method is purely virtual, then it is a good idea to
   * check that it has been overridden before calling it, and,
   * depending on the intent of the class, either ignore it silently
   * or warn the user.
   */
  g_return_if_fail (klass->open != NULL);
  klass->open (self, error);
}

void
viewer_file_close (ViewerFile  *self,
                   GError     **error)
{
  ViewerFileClass *klass;

  g_return_if_fail (VIEWER_IS_FILE (self));
  g_return_if_fail (error == NULL || *error == NULL);

  klass = VIEWER_FILE_GET_CLASS (self);
  if (klass->close != NULL)
    klass->close (self, error);
}

Virtual private Methods

These are very similar to virtual public methods. They just don't have a public function to call directly. The header file contains only a declaration of the virtual function: 

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/* declaration in viewer-file.h. */
struct _ViewerFileClass
{
  GObjectClass parent;

  /* Public virtual method as before. */
  void     (*open)           (ViewerFile  *self,
                              GError     **error);

  /* Private helper function to work out whether the file can be loaded via
   * memory mapped I/O, or whether it has to be read as a stream. */
  gboolean (*can_memory_map) (ViewerFile *self);

  /* Padding to allow adding up to 12 new virtual functions without
   * breaking ABI. */
  gpointer padding[12];
};

void viewer_file_open (ViewerFile *self, GError **error);

These virtual functions are often used to delegate part of the job to child classes: 

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/* this accessor function is static: it is not exported outside of this file. */
static gboolean 
viewer_file_can_memory_map (ViewerFile *self)
{
  return VIEWER_FILE_GET_CLASS (self)->can_memory_map (self);
}

void
viewer_file_open (ViewerFile  *self,
                  GError     **error)
{
  g_return_if_fail (VIEWER_IS_FILE (self));
  g_return_if_fail (error == NULL || *error == NULL);

  /*
   * Try to load the file using memory mapped I/O, if the implementation of the
   * class determines that is possible using its private virtual method.
   */
  if (viewer_file_can_memory_map (self))
    {
      /* Load the file using memory mapped I/O. */
    }
  else
    {
      /* Fall back to trying to load the file using streaming I/O… */
    }
}

Again, it is possible to provide a default implementation for this private virtual function: 

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static gboolean
viewer_file_real_can_memory_map (ViewerFile *self)
{
  /* As an example, always return false. Or, potentially return true if the
   * file is local. */
  return FALSE;
}

static void
viewer_file_class_init (ViewerFileClass *klass)
{
  /* non-pure virtual method; does not have to be implemented in children. */
  klass->can_memory_map = viewer_file_real_can_memory_map;
}

Derived classes can then override the method with code such as: 

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static void
viewer_audio_file_class_init (ViewerAudioFileClass *klass)
{
  ViewerFileClass *file_class = VIEWER_FILE_CLASS (klass);

  /* implement pure virtual function. */
  file_class->can_memory_map = viewer_audio_file_can_memory_map;
}

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