## How to define and implement a new GObject

This chapter focuses on the implementation of a subtype of GObject, for example to create a custom class hierarchy, or to subclass a GTK+ widget.

Throughout the chapter, a running example of a file viewer program is used, which has a ViewerFile class to represent a single file being viewed, and various derived classes for different types of files with special functionality, such as audio files. The example application also supports editing files (for example, to tweak a photo being viewed), using a ViewerEditable interface.

## Boilerplate header code

The first step before writing the code for your GObject is to write the type's header which contains the needed type, function and macro definitions. Each of these elements is nothing but a convention which is followed by almost all users of GObject, and has been refined over multiple years of experience developing GObject-based code. If you are writing a library, it is particularly important for you to adhere closely to these conventions; users of your library will assume that you have. Even if not writing a library, it will help other people who want to work on your project.

Pick a name convention for your headers and source code and stick to it:

- use a dash to separate the prefix from the typename: `viewer-file.h` and `viewer-file.c` (this is the convention used by Nautilus and most GNOME libraries).

- use an underscore to separate the prefix from the typename: `viewer_file.h` and `viewer_file.c`.

- Do not separate the prefix from the typename: `viewerfile.h` and `viewerfile.c`. (this is the convention used by GTK+)

Some people like the first two solutions better: it makes reading file names easier for those with poor eyesight.

The basic conventions for any header which exposes a GType are described in the section called “Conventions”.

If you want to declare a type named ‘file’ in namespace ‘viewer’, name the type instance `ViewerFile` and its class `ViewerFileClass` (names are case sensitive). The recommended method of declaring a type differs based on whether the type is final or derivable.

Final types cannot be subclassed further, and should be the default choice for new types — changing a final type to be derivable is always a change that will be compatible with existing uses of the code, but the converse will often cause problems. Final types are declared using `G_DECLARE_FINAL_TYPE`, and require a structure to hold the instance data to be declared in the source code (not the header file).

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    /*
     * Copyright/Licensing information.
     */

    /* inclusion guard */
    #ifndef __VIEWER_FILE_H__
    #define __VIEWER_FILE_H__

    #include <glib-object.h>
    /*
     * Potentially, include other headers on which this header depends.
     */

    G_BEGIN_DECLS

    /*
     * Type declaration.
     */
    #define VIEWER_TYPE_FILE viewer_file_get_type ()
    G_DECLARE_FINAL_TYPE (ViewerFile, viewer_file, VIEWER, FILE, GObject)

    /*
     * Method definitions.
     */
    ViewerFile *viewer_file_new (void);

    G_END_DECLS

    #endif /* __VIEWER_FILE_H__ */

Derivable types can be subclassed further, and their class and instance structures form part of the public API which must not be changed if API stability is cared about. They are declared using `G_DECLARE_DERIVABLE_TYPE`:

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    /*
     * Copyright/Licensing information.
     */

    /* inclusion guard */
    #ifndef __VIEWER_FILE_H__
    #define __VIEWER_FILE_H__

    #include <glib-object.h>
    /*
     * Potentially, include other headers on which this header depends.
     */

    G_BEGIN_DECLS

    /*
     * Type declaration.
     */
    #define VIEWER_TYPE_FILE viewer_file_get_type ()
    G_DECLARE_DERIVABLE_TYPE (ViewerFile, viewer_file, VIEWER, FILE, GObject)

    struct _ViewerFileClass
    {
      GObjectClass parent_class;

      /* Class virtual function fields. */
      void (* open) (ViewerFile  *file,
                     GError     **error);

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

    /*
     * Method definitions.
     */
    ViewerFile *viewer_file_new (void);

    G_END_DECLS

    #endif /* __VIEWER_FILE_H__ */

The convention for header includes is to add the minimum number of `#include` directives to the top of your headers needed to compile that header. This allows client code to simply `#include "viewer-file.h"`, without needing to know the prerequisites for `viewer-file.h`.