Conventions

There are a number of conventions users are expected to follow when creating new types which are to be exported in a header file:

Use the object_method pattern for function names: to invoke the method named foo on an instance of object type bar, call bar_foo.
Use prefixing to avoid namespace conflicts with other projects. If your library (or application) is named Maman, ^[3] prefix all your function names with maman_. For example: maman_object_method.
Create a macro named PREFIX_TYPE_OBJECT which always returns the GType for the associated object type. For an object of type Bar in a libray prefixed by maman, use: MAMAN_TYPE_BAR. It is common although not a convention to implement this macro using either a global static variable or a function named prefix_object_get_type. We will follow the function pattern wherever possible in this document.
Create a macro named PREFIX_OBJECT (obj) which returns a pointer of type PrefixObject. This macro is used to enforce static type safety by doing explicit casts wherever needed. It also enforces dynamic type safety by doing runtime checks. It is possible to disable the dynamic type checks in production builds (see building glib). For example, we would create MAMAN_BAR (obj) to keep the previous example.
If the type is classed, create a macro named PREFIX_OBJECT_CLASS (klass). This macro is strictly equivalent to the previous casting macro: it does static casting with dynamic type checking of class structures. It is expected to return a pointer to a class structure of type PrefixObjectClass. Again, an example is: MAMAN_BAR_CLASS.
Create a macro named PREFIX_IS_BAR (obj): this macro is expected to return a gboolean which indicates whether or not the input object instance pointer of type BAR.
If the type is classed, create a macro named PREFIX_IS_OBJECT_CLASS (klass) which, as above, returns a boolean if the input class pointer is a pointer to a class of type OBJECT.
If the type is classed, create a macro named PREFIX_OBJECT_GET_CLASS (obj) which returns the class pointer associated to an instance of a given type. This macro is used for static and dynamic type safety purposes (just like the previous casting macros).

The implementation of these macros is pretty straightforward: a number of simple-to-use macros are provided in gtype.h. For the example we used above, we would write the following trivial code to declare the macros:

#define MAMAN_TYPE_BAR                  (maman_bar_get_type ())
#define MAMAN_BAR(obj)                  (G_TYPE_CHECK_INSTANCE_CAST ((obj), MAMAN_TYPE_BAR, MamanBar))
#define MAMAN_BAR_CLASS(klass)          (G_TYPE_CHECK_CLASS_CAST ((klass), MAMAN_TYPE_BAR, MamanBarClass))
#define MAMAN_IS_BAR(obj)          (G_TYPE_CHECK_INSTANCE_TYPE ((obj), MAMAN_TYPE_BAR))
#define MAMAN_IS_BAR_CLASS(klass) (G_TYPE_CHECK_CLASS_TYPE ((klass), MAMAN_TYPE_BAR))
#define MAMAN_BAR_GET_CLASS(obj)  (G_TYPE_INSTANCE_GET_CLASS ((obj), MAMAN_TYPE_BAR, MamanBarClass))

Note

Stick to the naming klass as class is a registered c++ keyword.

The following code shows how to implement the maman_bar_get_type function:

GType maman_bar_get_type (void)
{
  static GType type = 0;
  if (type == 0) {
    static const GTypeInfo info = {
      /* You fill this structure. */
    };
    type = g_type_register_static (G_TYPE_OBJECT,
                                   "MamanBarType",
                                   &info, 0);
  }
  return type;
}

When having no special requirements you also can use the G_DEFINE_TYPE macro:

G_DEFINE_TYPE (MamanBar, maman_bar, G_TYPE_OBJECT)

^[3] Maman is the French word for mum or mother - nothing more and nothing less.