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GOB2 - The GObject Builder


gob2 [ option ] ... file


GObject Builder is a simple preprocessor for easily creating GObject objects. It does not
parse any C code and ignores any C errors. It is in spirit similar to things like lex or
yacc. In some ways it also resembles java. But it is really just a simple preprocessor
for creating GObjects for use in C or C++ and it is not a programming language.


-? -h --help
Display a simple help screen.

Display version information

-w --exit-on-warn
Exit with an error code even when you encounter a warning.

Exit with an error only on errors, not on warnings, this is the default.

Generate C++ code.

Never add the extern "C" to the header.

Never generate any code with GNU C extensions. However all the GNU C extensions
are always wrapped in #ifdef __GNUC__, so code using them compiles correctly even
on non-GNU compilers. This option is for purists only. (using GNU extensions some
warnings are eliminated, some ugly hacks and there is better argument type safety,
so it´s good to use them)

Don´t touch output files unless they really changed (implies --no-touch-headers).
Be careful with automake, see section PREVENTING SPURIOUS BUILDS.

Don´t touch the generated header file unless it really changed, this avoids
spurious rebuilds, but can confuse some make systems (automake in particular), so
it is not enabled by default. Private header is still touched even if unchanged

Always create a <basename>-private.h file, even if it would be empty.

Create the private header only if it would have something in it, that is, if there
are some private data members or protected methods. This is the default.

Never create a private header file. If we use any private data members, define the
private data structure at the point in the .c source where the class definition

--m4 Preprocess source with m4. Following args will be passed to m4.

Print directory that will be searched for m4 files.

-n --no-write
Do not write any output files, just check syntax of the input file.

Do not print out the ´#line´ statements into the output. Useful for debugging the
auto-generated generated code.

Do not create the Self and SelfClass type aliases and the SELF, IS_SELF and
SELF_CLASS macros.

Do not remove the initial underscore from method names.

Always include the private pointer in the public header file. This is useful for
files which are part of a library and you want to reserve the right to add some
private data members without breaking binary compatibility.

-o --output-dir
The directory into which output should be placed.

Replace default ´-´ file name separator. If no separator character is given then
none is used. Only one character can be used.

--gtk3 Use gtk3.


Because we need to parse out different parts of the typename, sometimes you need to
specify the typename with some special syntax. Types are specified in capitalized form
and words are separated by ´:´. The first word of the type (which can be empty) is the
"namespace". This fact is for example used for the type checking macro and the type
macro. For "Gtk:New:Button", the macros will be GTK_IS_NEW_BUTTON and
GTK_TYPE_NEW_BUTTON. This colon separated format of typenames is used in the class
declaration header and for method argument types.


The filenames are created from the typename. The words are separated by ´-´ (this can be
changed with --file-sep option) and all in lower case. For example for an object named
"Gtk:New:Button", the files are gtk-new-button.c and gtk-new-button.h. If you are using
C++ mode, the output .c file will in fact be a .cc file. If you have any private data
members, a private header file will also be created, called <basename>-private.h (for the
example above it would be gtk-new-button-private.h). The public header file is created to
be human readable and to be used as a reference to the object. The .c source file is not
created as a human readable source and is littered with #line statements, which make the
compiler attempt to point you to the right line in your .gob file in case of parsing
errors. The output should not be edited by hand, and you should only edit the .gob file.


To include some code directly in the output C file begin with ´%{´ on an empty line and
end the code with a ´%}´ on an empty line. These sections will appear in the output files
in the order they are given. There are several other sections to which you can put code.
You can put it in the ´header´ section (which can be abbreviated ´h´) and it will go into
the public header file. You can also put it in the ´privateheader´ section (abbreviated
´ph´) which will make the code go into the private header file. Sometimes you want some
code (other includes) to appear before the extern "C" and the protecting define. To do
this you can put them into the ´headertop´ (or ´ht´) section. You may wish to include
code or comments in all the files, which you can do by putting them into the ´all´ (or
´a´) section. Similarly, code you wish to appear at the top of all files go in the
´alltop´ (or ´at´) section. When you want code to appear as in alltop but only in the
cfile you use the ´ctop´ (or ´ct´) section. Note that ctop requires 2.0.18. Finally,
´afterdecls´ includes code between the declarations and the method implementations, but
note that ´afterdecls´ requires version 2.0.16. For example:

/* this will be at the very top of all output files */

/* this will be at the very top of the C file */
/* Requires 2.0.18 */

/* this will be on top of the public header */

/* this will go into the private header file */

/* will be included in the header */
void somefunc(int i);

/* will be included in all files */

/* between the declarations and the method implementations */
/* Requires gob version 2.0.16 */

/* will be included in the C file */
void somefunc(int i)
/* some code */


Gob will automatically include the class header file at the top of the .c source file. If
you wish to include it somewhere else, put the include into some %{ %} section above the
class definition, and gob will not include it automatically. This way you can avoid
circular includes and control where in the file do you want to include the header.

If you made any data members private, gob will also create a source file that will be
called <basename>-private.h. Same rule as above applies for this just as it does for the
regular header file. If you do explicitly include the regular header file, you should
always include this private header file below it. That is, if you use any private data
members. If you don´t, the private header file automatically includes the public header
file, and thus the public header file will be indirectly included at the very top of the


There can be only one class per input file. Defining a class is sort of like in Java, you
define the class and write inline code directly into the class definition. To define a
class you need to specify the new object name and the name of the object from which it is
derived from, such as this "class <new type> from <parent type> { <class code> }". For

class Gtk:New:Button from Gtk:Button {
<class code>

To make an abstract class (to pass G_TYPE_FLAG_ABSTRACT) add ´(abstract)´ before the curly
braces above. This works since version 2.0.13.


There are five types of data members. Three of them are normal data members, one is class
wide (global) in scope and one is a virtual one, usually linked to a normal data member or
a class wide data member. The three normal data members are public, protected and
private. Public and protected are basically just entries in the object structure, while
private has it´s own dynamically allocated private structure. Protected members are
always put after the public one in the structure and are marked protected in the header
file. There is only one identifier allowed per typename unlike in normal C. Example:

public int i;
private GtkWidget *h;
protected long k;

Public and protected data members are accessed normally as members of the object struct.
Example where ´i´ is as above a public data member:

object->i = 1;

The private data members are defined in a structure which is only available inside the .c
file, or by including a private header file. You must access them using the structure
_priv. Example where ´h´ is the private data member (as in the above example):

object->_priv->h = NULL;

The _priv structure is defined in the <basename>-private.h. This file is automatically
included if you don´t include it yourself. You should always explicitly include it in
your .gob file if you explicitly also include the main header file. The reason it is a
separate header file is that you can also include it in other places that need to access
this objects private data, such as if you have the majority of functionality of an object
in a separate .c file. Or if a derived object needs to access the protected methods.

In case you use the --no-private-header option, no private header file is created and you
can only access the _priv pointer below the class definition in the .gob file.

Also note that this structure is dynamically allocated, and is freed in the finalize
handler. If you override the finalized handler, your code will be run first and only then
will the _priv structure be freed.

Classwide data members:

Sometimes you want a datamember to be shared by all objects. You then need the
"classwide" scope keyword. So for example the following adds a global member foo:

classwide int foo;

To access the member you can use the SELF_GET_CLASS macro (or YOUR_OBJECT_NAME_GET_CLASS)
to get at the class. Thus the following would work:

SELF_GET_CLASS(object)->foo = 20;

Automatic Initialization:

You can automatically initialize the public private and protected data members without
having to add an init method. The advantage here is that initialization is kept close to
the definition of the data member and thus it´s easier to check. To do this, just add a
´=´ followed by a number or a token. It is also possible to include arbitrary C code for
more elaborate initializations by putting it all in curly braces. Note that the curly
braces will not be printed into the output, but since gob does not C parsing it needs them
to figure out where the C code ends. The code will be inserted into the init method,
above the user defined body. So for example the following will initialize an integer to
-1 and a string with a newly allocated string of "hello".

public int foo = -1;
private char *bar = {g_strdup("hello")};

Automatic Destruction:

Most data stored as pointers needs to have a function called when the object is finalized
to either free the data. Gob will let you define a function to be called on the data the
object is finalized. This is achieved by putting ´destroywith´ followed by a function
name after the variable definition. It is only called if the data you defined this on is
not NULL, so you cans specify functions which do not handle NULL. It is very much like
the GDestroyNotify function used in GTK+ and glib in many places. Unlike many other
places, gob will not enforce any kind of type safety here so be a little bit more careful.
Any function you give it will be called as a "void function(void *)". It will in fact be
cast into such a form before called. This is to avoid spurious warnings for gtk calls to
subclass methods. The function needs not be of that form exactly, it just has to take one
argument which is the pointer to the data. You should also not define this on any non-
pointer data as the results may be undefined. Example:

public char *foo = {g_strdup("bar")}
destroywith g_free;

Note that the function name you give must be a real function and not macro. Also note
that this is always called in the "finalize" method of GObject. It is always called after
any user defined body of the finalize handler.

Sometimes you may want to run arbitrary code on destruction. While this can be perfectly
well done in the finalize handler. Depending on the style you may want to include all
destruction/initialization code together with the definition of the data member. Thus you
may want to put arbitrary code which will then be inserted into the "finalize" method of
GObject. This can be done with the "destroy" keyword followed by arbitrary code in curly
braces. Inside this code a macro called VAR will be define which refers to your variable.
So for example destroying a GString can be either done with a helper routine or the
following code:

public GString *string = {g_string_new(NULL)}
destroy {
if(VAR) g_string_free(VAR, TRUE);

The thing to remember with these is that there are many ways to do this and you´d better
be consistent in your code in how you use the above things. Also defining a helper
routine that will do the destruction will be a nicer thing to do if that´s a possibility.
The "destroy" keyword with code does take up more space in the file and it may become more

The data is zeroed out after being destroyed. This is to make debugging easier in case
your code might try to access an already finalized object. In case you have overridden
the finalize method, your code will be run first and only then will the destructors be
called. You should not however make any assumptions about the order at which the
destructors are called. If you have interdependencies between destructors for different
data members, you will have to do this in your own finalize override function.

Automatic Unreffing:

This is very much like the automatic destruction, but is instead run in the dispose method
(it is among other places called from the "destroy" method of GtkObject). All data and
other objects that you need to unref should be done here, and not at finalize time. The
semantics are otherwise the same as for the "destroywith" and "destroy" keywords, except
that you use "unrefwith" and "unref".

public G:Object *foo = NULL
unrefwith g_object_unref;
public G:Object *bar = NULL
unref {
g_object_unref (VAR);


The fourth type of a data member a property type. It is a named data member which is one
of the features of the GObject system. It just defines a way to get and set some data,
but you have to take care of storing that data somewhere. So it is normal to also have a
normal private (or public) data member where you store the real data. You normally need
to define a get and a set handler. They are fragments of C code that will be used to get
the value or set the value of the argument. Inside them you can use the define VAL to
which you assign the data or get the data. You should treat this VAL as a GValue which
stores the data of the correct type. You can also use the identifier "self" as pointer to
the object instance. The type is defined as one of the GObject type enums, but without
the G_TYPE_ prefix. There are also some attributes of a property which you can set. For
example the following is a definition of an integer property ´height´ which will be
synchronized with a private integer data member also of the name ´height´.

private int height;
property INT height
name = "height",
nick = _("Short nickname"),
blurb = _("Long description"),
minimum = 10,
maximum = 200,
default_value = 100)
set { self->_priv->height = g_value_get_int (VAL); }
get { g_value_set_int (VAL, self->_priv->height); };

The attributes are really optional though you should at least set some of them. All
property types have a ´nick´ and a ´blurb´ attribute and you should set those accordingly.
This will make runtime querying the object nicer as things such as gui editors and class
browsers can be more verbose about the class itself.

The ´name´ property is canonical name of property. It is useful when you try to implement
properties with no C names like ´vertical-scroll´. The ´name´ property can be omitted.

You can use the ´_("string")´ notation instead of just "string", and that will mark the
string for translation.

Almost all types also have a ´default_value´ attribute which sets the initial value of
this property (on object initialization, the set handler will be run automatically with
this value). This value will be overridden if the user sets a value of this property on
the call to g_object_new.

All the numeric types (including CHAR) have ´minimum´ and ´maximum´ attributes which can
restrict the range. If you do not specify these the range will be the full range that the
data type can handle.

Types such as UNICHAR and BOOLEAN only have the ´nick´, ´blurb´ and ´default_value´

The ENUM type has an ´enum_type´ attribute which is the exact type of the enum. This is
so that the property knows which exact type you can set, rather then just knowing it is an
enum. You should always create an enum type specific for the enum itself (see section on
the enum types)

Similarly FLAGS type has a ´flags_type´ which again you should set to the specific type of
this flags data member.

There is a STRING type which has only the extra ´default_value´ attribute.

The OBJECT type is one of the types that doesn´t have a ´default_value´ and it only has an
´object_type´ attribute (in addition to nick and blurb of course) that is the exact object
type that this property accepts. The object_type should be as a type, that is for example

There is a BOXED type which is a pointer which has a boxed type defined (such that GObject
knows how to copy and destroy this pointer). Here you will need to specify the
´boxed_type´ attribute with the specific type of the boxed pointer.

There is also a POINTER type, which has only the ´nick´ and ´blurb´ attributes. This is
for storing arbitrary pointers. You should be careful with this one, as GObject knows
nothing about the data stored at this pointer. It is somewhat like a ´void *´ type.

There is also the PARAM type for storing parameters with a ´param_type´ attribute.

You should notice that this list is pretty much like the list of g_param_spec_* functions
from gobject/gparamspecs.h, and the attributes are like the arguments of those functions.
Note however that value array is NOT supported yet.

You can also specify extra flags, such as CONSTRUCT or CONSTRUCT_ONLY using the ´flags´
attribute. You can specify multiple flags by oring them together with ´|´. These flags
correspond to the GParamFlags enumeration except do not include the G_PARAM_ prefix. So
for example to define an enumeration property, which is a CONSTRUCT_ONLY property, we
could do the following:

private SomeEnumerationType foo;
property ENUM foo
(nick = _("Short nickname"),
blurb = _("Long description"),
enum_type = Some:Enumeration:Type

The above example also gives an example of automatic linking to a standard data memember.
By including the attribute ´link´ a get and set handlers will be automatically added
without having to type them by hand. This is useful for a vast majority data types that
are just linked to some standard data member and do not need to do anything extra on get
or set.

Another extra feature of properties is the possibility of automatically exporing methods
to get and set the property. That is without having to use g_object_set and g_object_get.
This is achieved by adding an ´export´ attribute to the list of property attributes.

If you do not define a set or get handler, the property will automatically be only
readable or writable as appropriate.

Gob2 also creates macros which can be used for type safe access to properties through
g_object_set and g_object_get. The macros are called <type>_PROP_<argument name>(x) and
<type>_GET_PROP_<argument name>(x). They define both the string and the value part of the
argument. So for setting an argument of height, one would use (for object type

g_object_set (G_OBJECT (object),

And for getting, you would use:

int height;
g_object_get (G_OBJECT (object),

Note however that the type safety only works completely on GNU C compilers. The code will
compile on other compilers but with minimal type safety. For complete type safety it is
useful to use the get/set methods that are defined by using the ´export´ attribute.

To get bettery type safety on some of the property types, you can specify the ´type´
attribute which will add casts where appropriate in code dealing with this property. This
is especially useful for POINTER and OBJECT types. But even for others.

You can also override properties from parent objects (that is override their
implementation, not their attributes). Do this by adding the special ´override´
attribute. For example if the parent object had a ´height´ property then you could
override it by

private int height;
property INT height
set { self->_priv->height = g_value_get_int (VAL); }
get { g_value_set_int (VAL, self->_priv->height); };

Overriding is supported since gob 2.0.10.


There is a whole array of possible methods. The three normal, "familiar" method types are
private, protected and public. Public are defined as normal functions with a prototype in
the header file. Protected methods are defined as normal methods (which you can call from
other files), but their prototype is placed in the private header file. Private methods
are defined as static functions with prototypes at the top of the .c file. Then there are
signal, virtual and override methods. More on those later. You can also define init and
class_init methods with a special definition if you want to add code to the constructors
or you can just leave them out. You can also not define a body for a method, by just
using ´;´ instead of a body. This will define an empty function. You can´t do this for
non-void regular public, private or protected methods, however it is acceptable for non-
void virtual, signal and override methods.

Function argument lists:

For all but the init and class_init methods, you use the following syntax for arguments.
The first argument can be just "self", which gob will translate into a pointer to the
object instance. The rest of the arguments are very similar to normal C arguments. If
the typename is an object pointer you should use the syntax defined above with the words
separated by ´:´
<type> <argument id>
<type> <argument id> (check <list of checks>)

The checks are glib type preconditions, and can be the following: "null", which tests
pointers for being NULL, "type" which checks GTK+ object pointers for being the right
type, "<test> <number>" which tests numeric arguments for being a certain value. The test
can be a <,>,<=,>= != or ==. Example:

public int
foo (self,
int h (check > 0 < 11),
Gtk:Widget *w (check null type))

This will be the prototype of a function which has a self pointer as the first argument,
an integer argument which will be checked and has to be more then 0 and less then 11, and
a pointer to a GtkWidget object instance and it is checked for being null and the type
will also be checked.

Function attributes:

For method that aren't virtual, signal or override methods, and aren't init or class_init,
GLib function attribute macros G_GNUC_PRINTF, G_GNUC_SCANF, and G_GNUC_FORMAT can
optionally be included after the argument list. Simply include an ´attr´ keyword and the
C code to include in the file. You have to include braces and anything inside the braces
will be printed into the header file after the function declaration and before the
trailing semicolon. The braces themselves are not printed. For example:

public void
print (self, const char *format (check null), ...)
attr {G_GNUC_PRINTF(2, 3)}

This will produce a prototype which will generate a warning at compile time if the
contents of the format argument (argument number 2) aren't consistent with the types and
number of the subsequent variadic arguments (the first of which is argument number 3).
Only one ´attr´ keyword per method is allowed. If you have more than one attribute to
include, you should put them all within the braces. Note that function attributes were
aded in version 2.0.16.

Error return:

Methods which have a return value, there also has to be something returned if there is an
error, such as if a precondition is not met. The default is 0, casted to the type of the
method. If you need to return something else then you can specify an ´onerror´ keyword
after the prototype and any optional function attribute macros, and after that a number, a
token (an identifier) or a bit of C code enclosed in braces {}. The braces will not be
printed into the output, they just delimit the string. For example:

public void * get_something (self, int i (check >= 0)) onerror NULL {

The onerror value is also used in overrides that have a return value, in case there isn´t
a parent method, PARENT_HANDLER will return it. More about this later.

Default return:

Some signal and virtual methods have a return type. But what happens if there is no
default handler and no one connects to a signal. GOB2 will normally have the wrappers
return whatever you specify with onerror or ´0´ if you haven´t specified anything. You
can also specify a default return value with the keyword ´defreturn´. It´s use is
identical to the use of onerror, and you can in fact use both at the same time. Example

virtual int get_some_int (self) onerror -1 defreturn 10 ;

That is an empty virtual method (in C++ terms a pure virtual). If you never specify any
handler for it in the derived children it will just return 10.

Constructor methods:

There are two methods that handle the construction of an object, init and class_init. You
define them by just using the init or class_init keyword with an untyped argument in the
argument list. The argument will be usable in your function as a pointer to your object
or class depending if it´s init or class_init. For example:

init (self) {
/* initialize the object here */
self->a = 9;
self->b = 9;

class_init (class) {
/* initialize the class, this is rarely needed */
class->blah = NULL;

The class_init function is very rarely needed as all standard class initialization is
taken care of for you by gob itself. The init function should on the other hand be used
whenever you need to construct or initialize anything in the object to put it into a sane

Constructor, dispose, finalize methods:

Since 2.0.16, you can also easily add code to the object's constructor, dispose, and
finalize methods. See GObject documentation on how these are run. The code you add will
be run before calling the parents function for dispose and finalize, and after the parent
function for constructor. The syntax is just like init and class_init. For example:

constructor (self) {
/* constructor method */

dispose (self) {
/* dispose method */

finalize (self) {
/* finalize method */

You can also just override those methods as usual, but the above is much easier and nearly
as flexible.

Virtual methods:

Virtual methods are basically pointers in the class structure, so that one can override
the method in derived methods. That is to implement the method in a derived class, you
must then use an override method (more on those later). They can be empty (if you put ´;´
instead of the C code). A wrapper will also be defined which makes calling the methods he
same as public methods. This type of method is just a little bit "slower" then normal
functions, but not as slow as signals. You define them by using "virtual" keyword before
the prototype. If you put the keyword "private" right after the "virtual" keyword, the
wrapper will not be a public method, but a private one. You can do the same with
"protected" to make a protected wrapper.


Signals are methods to which the user can bind other handlers and override the default
handler. The default handler is basically the method body. This is the most versatile
and flexible type of a method and also the slowest. You need to specify a whole bunch of
things when you define a signal. One thing is when the default handler will be run, first
or last. You specify that by "first" or "last" right after the "signal" keyword. Then
you need to define the GObject enum types (again without the G_TYPE_ prefix). For that
you define the return types and the types of arguments after the "self" pointer (not
including the "self" pointer). You put it in the following syntax "<return type> (<list
of arguments>)". If the return type is void, the type should be "NONE", the same should
be for the argument list. The rest of the prototype is the same as for other method
types. The body can also be empty, and also there is a public method wrapper which you
can use for calling the signal just like a public method. Example:

signal first INT (POINTER, INT)
int do_something (self, Gtk:Widget *w (check null type), int length)


signal last NONE (NONE) void foo (self);

You can include name of signal, if this name is not a C variable name. Example:

signal first INT "do-something" (POINTER, INT)
int do_something (self, Gtk:Widget *w (check null type), int length)

If you don´t want the wrapper that emits the signal to be public, you can include the
keyword "private" after the "signal" keyword. This will make the wrapper a normal private
method. You can also make a protected wrapper by using "protected" instead of "private".

If you don´t define a "first" or a "last", the default will be taken as "last".

You can also add additional flags. You do this just like with the argument flags,
although this is probably very rare. These are the G_SIGNAL_* flags, and you can add them
without the G_SIGNAL_ prefix into a parenthesis, just after the "signal" keyword. By
default all public signals are G_SIGNAL_ACTION.

Also gob2 creates a wrapper macros for typesafe signal connection. That is you will be
warned by the compiler if you pass a callback that is not the correct prototype. This
will again only warn you on gcc, but it will compile without warning on another compiler.
So as with all the typesafety hacks in gob, it is better to test your objects under gcc to
get any warnings even if you are using a different compiler in the end.

The methods that are created for you are:

<class_name>_connect__<signal_name> (<object>, <callback>, <data>)
<class_name>_connect_after__<signal_name> (<object>, <callback>, <data>)
<class_name>_connect_data__<signal_name> (<object>, <callback>, <data>,
<destroy_notify>, <flags>)

These three functions correspond to the g_signal_connect, g_signal_connect_after and
g_signal_connect_data functions that you would normally use, except they are for a
specific signal. Also do note the two underscores between the method name and the signal
name. For example to connect the signal "foo" on the object "Test:Object" you would do:

test_object_connect__foo (object, callback, data);

To use BOXED in the signal arguments you need to tell gob which type of boxed argument you
want to use. For this you can just add BOXED_GTK_TYPE_STRING instead of BOXED. For
example BOXED_GTK_TYPE_TREE_ITER for GtkTreeIter. This works since version 2.0.13.

Override methods:

If you need to override some method (a signal or a virtual method of some class in the
parent tree of the new object), you can define and override method. After the "override"
keyword, you should put the typename of the class you are overriding a method from. Other
then that it is the same as for other methods. The "self" pointer in this case should be
the type of the method you are overriding so that you don´t get warnings during
compilation. Also to call the method of the parent class, you can use the PARENT_HANDLER
macro with your arguments. Example:

override (Gtk:Container) void
add (Gtk:Container *self (check null type), Gtk:Widget *wid (check null type))
/* some code here */
PARENT_HANDLER(self, wid);

If the function has a return value, then PARENT_HANDLER is an expression that you can use.
It will return whatever the parent handler returned, or the "onerror" expression if there
was no parent handler.

Method names:

Inside the code, aliases are set for the methods, so that you don´t have to type the class
name before each call, just type self_ instead of the name of the class. So to call a
method called blah, you would use the name self_blah. Example:

private int
foo (self)
return self->len;

private int
bar (self, int i)
return self_foo (self) + i;


You should define a new method which should be a normal public method. Inside this
method, you can use the GET_NEW macro that is defined for you and that will fetch a new
object, so a fairly standard new method would look like:

public GObject *
new (void) {
GObject *ret = GET_NEW;
return G_OBJECT (ret);

You should not a subtle peculiarity of the GObject system here. If there is any code
inside the G_OBJECT macro argument, it will get executed multiple times. This means that
things such as G_OBJECT(GET_NEW) would actually create 4 objects, leaking 3 of them. A
good rule (as with anywhere in C) is to be careful with all macros.


Self alias casts:

There are some standard casts defined for you. Instead of using the full macros inside
the .c file, you can use SELF, IS_SELF and SELF_CLASS. Using these makes it easier to for
example change class names around.

Self alias types:

There are also the Self and SelfClass types inside your .c file. These serve the same
function as the above, they make it easier to type and easier to change typenames around
which can help a lot during prototyping stage. However you should note that the Self type
should not be used in function prototypes as one of the arguments or as a return value
type. This is because this is a simple C typedef which is only available inside your .c
file and not in the header files. You can disable both the self casting macros and the
self type aliases by passing --no-self-alias to gob.



In your generated C file, you can use the defines GOB_VERSION_MAJOR GOB_VERSION_MINOR and
GOB_VERSION_PATCHLEVEL if you wish to for example use a feature that is only available in
some newer gob version. Note however that you can only use these defines in the C code
portions of your .gob file, and #ifdef´s cannot span multiple functions. Check the BUGS
section for more on using the C preprocessor and gob.

Minimum version requires:

You can also make your .gob file require at least certain version of gob. You do this by
putting ´requires x.y.z´ (where x.y.z is the version number) outside of any C block,
comment or class, usually you should make this the first line in the file or close to the
top. If gob finds this and the version of gob used to compile the code is lower then that
listed in the require, gob will generate an error and exit. For example to require that
gob2 version 2.0.0 or higher be used to compile a file, put this at the top of that file:

requires 2.0.0


You can create new GObject ENUM, FLAGS and GError types for use in your classes easily.
Glib includes some utilities for handling these, however it may be cleaner to use the
below specified way in your classes. It also then doesn´t require any Makefile setup.
Make sure this is defined in the same section as the class, that is not in any of the
´%?{´ ´%}´ sections.

You use the keywords ´enum´ ´flags´ and ´error´ as you would use the ´class´ keyword.
Then you give a prefix for the values in the enumeration. Then you define a list of
values just like in C. For ´enum´ types you can also specify the values assigned to each
string. Then you specify the type in the standard gob style of specifying types. Here
are a few examples of all of these:

NONE = 9,
} Test:Enum;

flags BUGA_BUGA {
} Some:Flags;

} Test:Object:Error;

This will for example define an enum that is equivalent to the following C code:

typedef enum {
} TestEnum;


There is a C++ mode so that gob creates C++ compiler friendly files. You need to use the
--for-cpp argument to gob. This will make the generated file have a .cc instead of a .c
extension, and several things will be adjusted to make it all work for a C++ compiler.
One thing that will be missing is an alias to the new method, as that clashes with C++, so
instead you´ll have to use the full name of the method inside your code. Also note that
gob does not use any C++ features, this option will just make the generated code compile
with a C++ compiler.


The get_type is not really a method, but a function which initializes your object.
Recently objects appeared which require you to make a custom get_type function. So it is
possible to override this function. To do so, just define a new public method called
get_type, with no arguments. Example:

public GType
get_type (void)
/* code goes here */
return some_type;


Currently gob will only allow you to implement interfaces (that is, define new classes
which implement an interface) and doesn´t yet have support for making new interfaces, but
this will be coming at some point in the future.

To define a class that implements an interface add a class flag ´interface´ with the type
name of the interface as an argument. Then to implement a specific method of the
interface, just add ´interface <typename>´ before the method definition. The method can,
and probably should be, private.

The following example implements a new object, that implements the Gtk:Tree:Model
interface and implements the get_flags method of that interface. Do note that except for
standard (GTK+ and glib) specific interfaces which seem to have a non-standard name for
the interface structure, the structure should end with and Iface, if you are implementing
an interface. That is for example for the Gtk:Tree:Model, the structure containing the
table of methods should be named GtkTreeModelIface.
class Some:Object from G:Object
(interface Gtk:Tree:Model)
/* function implemented for the Gtk:Tree:Model interface */
interface Gtk:Tree:Model
private GtkTreeModelFlags
get_flags (Gtk:Tree:Model *self (check null type))
/* Here would be the implementation */
return (GtkTreeModelFlags)0;

If you want to implement multiple interfaces just list more class flag lines as follows:

class Some:Object from G:Object
(interface Gtk:Tree:Model)
(interface Gtk:Editable)
/* ... */


If you want to build a BonoboObject class gob2 has direct support for these. Just create
a new object that derives from Bonobo:Object. Then use a "BonoboObject" class flag with
the interface name as an argument. The interface name should be as you would type it in
C, that is with underscores as namespace separators. Then you add the methods (using
exact same names as in the idl file) and prepend those methods with a BonoboObject
keyword. For example imagine you have an interface GNOME/Foo/SomeInterface, with a method
fooBar that takes a single string:

class Foo:Some:Interface from Bonobo:Object
(BonoboObject GNOME_Foo_SomeInterface) {

private void
fooBar (PortableServer_Servant servant,
const CORBA_char *string,
CORBA_Environment *ev)
Self *self = SELF (bonobo_object_from_servant (servant));

/* your code here */

/* rest of class */

Note that the implementation method can be private, in fact that´s probably a good idea to
do. It won´t work to make this a signal, it can however be a virtual. Note that the
method prototype must match the one from the interface header file, or you will get a bad
assignment warning. You should check the header file generated by orbit-idl and see the
epv structure for the correct prototypes if you can´t figure them out from the idl itself.
Also note that the first argument is not "self", but the servant and you must use
bonobo_object_from_servant function to get the actual object pointer.


Gob can simplify writing a libglade class. Just create a new object that derives from a
GtkContainer widget. Then use a "GladeXML" class flag with the glade file name, root
widget and optional domain as arguments between double quotes. For example:

class My:Glade from Gtk:Window (GladeXML "gob-libglade.glade" "root")

Note however that then "gob-libglade.glade" would have to be in the current directory.
You could specify a path, but that may not work for all installations. You can replace
the glade filename with a token to be used in the generated .c file and you can then have
a macro with the filename, as follows:

class My:Glade from Gtk:Window (GladeXML GLADE_FILE "root")

And somewhere in your header files you would have

#define GLADE_FILE "/path/to/file.glade"

You can declare widgets as data members by adding a 'GladeXML' to the definition.

private Gtk:Button * button1 GladeXML;

This will automatically set the "button1" from the GladeXML file.

All signals created with glade are automatically connected if you defined those class
methods in your class. For example suppose in glade that we set the "connect" signal on
button1 to go to on_button1_clicked, then in our gob file we can just write:

public void
on_button1_clicked(self, GtkButton * button)

See the examples directory for a full example. Note that this feature requires version at
least 2.0.12.


Gob will need to define some local variables and functions in the generated files, so you
need to take some precaution not to conflict with these. The general rule of thumb is
that all of these start with three underscores. There is one, "parent_class" which
doesn´t because it´s intended for use in your code. For virtuals or signals, you cannot
use the identifier __parent__ which is used for the parent of the object. You should
actually never access __parent__ either as it not guaranteed that it will stay named this
way. Data members cannot be named __parent__ nor _priv. For methods, you cannot use the
identifiers "init" or "class_init" unless you mean the constructor methods. You shouldn´t
generally use 3 underscores even in override method argument lists and virtual and signal
method names as it might confuse the PARENT_HANDLER macro. In fact avoiding all names
with three underscores is the best policy when working with gob.

There are a couple of defines which you shouldn´t be redefining in the code or other

As for types, there are Self and SelfClass types which are only defined in your source
files. Their generation (just like the generation of the SELF macros) can be turned off,
see command line options.


If you want to use gtk-doc style inline documentation for your objects, you can do one of
two things. First, you could include the inline documentation comments in your %{ %}
section which will then be put verbatim into the output source file. This is the way you
should use for functions you define outside of the class.

For class methods, you should use a gtk+ style comment, however it can be indented any
number of tabs or spaces and you can use the short method name without the type prefix.
Gob will automatically try to extract these and translate to full names and put them in
the output source file. An example would be:

class Gtk:Button:Example from Gtk:Button {
* new:
* Makes a new #GtkButtonExample widget
* Returns: a new widget
GtkWidget *
return (GtkWidget *)GET_NEW;

If the function you are documenting is a signal or a virtual then it will be documenting
the wrapper that starts that virtual function or emits that signal.


Sometimes you may need to use an object of type MyObjectA in the MyObjectB class and vice
versa. Obviously you can´t include headers for both. So you need to just declare the
typedef in the header of A for B, and the other way around as well. The headers generated
include a protecting define before it declares the typedef. This define is the
__TYPEDEF_<upper case object name>__. So inside my-object-a.h there will be this:

typedef struct _MyObjectA MyObjectA;

Now instead of including my-object-a.h in the header section of my-object-b.gob, just copy
the above code there and you´re set for using MyObjectA as a type in the method parameters
and public types.

Another way to get out of this problem is if you can use those types only in the private
members, in which case they won´t be in the generated public header.


If you are using normal makefiles, what you need to do is to add a generic rule for .gob
files. So you would include the following in the Makefile and then just use the .c and .h
files as usual (make sure the space before the ´gob2´ is a tab, not spaces):

%.c %.h %-private.h: %.gob
gob2 $<


This is a little bit more involved. Basically the first thing to do is to check for GOB2
in your configure.in file. You can use the supplied m4 macro which will also check the
version of gob. Basically you include this:


This will replace @GOB2@ in your makefiles with the full path of gob2. Thus when adding
the generic rule to your Makefile.am file, it should look like:

%.c %.h %-private.h: %.gob
@GOB2@ $<

For Makefile.am you have to set up a couple more things. First you have to include the
generated .c and .h files into BUILT_SOURCES variable. You have to include both the .gob
and the .c and .h files in the SOURCES for your program.


When nothing has changed you might not really want to rebuild everything and gob provides
options --no-touch (since 2.0.13) and --no-touch-headers to avoid this. When working with
build systems such as automake you have to be more careful as just using those options can
cause automake to get confused and you will need to use something like the following:

foo_SOURCES = foo.gob foo.gob.stamp foo.c foo.h foo-private.h
BUILT_SOURCES = foo.gob.stamp

%.gob.stamp: %.gob
@GOB2@ --no-touch $<
@touch $@


GOB does several things to make debugging the code easier. First it adds preprocessor
commands into the output c file that point to the correct places in your .gob input file.
However sometimes there might be some bigger confusion and this is just not helpful. In
this case you will probably want to have gcc point you directly at the generated files.
For this use the --no-lines command line option. You should also note that these commands
are not generated for the public header file at all. If there is an error which points
you to the public header file, make sure you fix this error in the .gob file, otherwise
your changes will not have any effect after gob recompiles the sources again.

Sometimes you might want to know which method you are in for some debugging output. GOB
will define __GOB_FUNCTION__ macro, which is just a string constant with a pretty name of
the method.


It is possible to have your .gob file also preprocessed by m4. This is useful if you have
a lot of files and you´d like to have some preprocessor put in some common features. All
you have to do is add --m4 to the command line of gob2 and gob2 will first run your file
through m4. You can print the directory that is searched for m4 files by running "gob2

All the arguments after --m4 will be passed to m4 itself, so it has to be the last gob2
argument on the command line. This way you can specify arbitrary options to pass to m4.

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