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Gtk+/Gnome Application Development
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Object Arguments

Arguments are one of the most interesting features of GtkObject. Arguments are a mechanism for handling what CORBA's Interface Definition Language (IDL) calls an attribute: a value with a "getter" and a "setter." In concrete terms, object arguments pair a key (which is a string) with a value (represented as a GtkArg). Each GtkObject subclass can register permissible keys and the GtkTypes of their associated values.

Using object arguments, one can discover at runtime what attributes an object has, and then get or set their values. This is very useful for people implementing GUI builders, since some of the widget configuration dialogs can be automated. Similarly, it makes it much easier to write GTK+ bindings for scripting languages. It can also be convenient for programmers, since they can avoid writing all the get/set functions---the GnomeCanvas, for example, uses object arguments for almost all of its API. Finally, object arguments may be configurable via the gtkrc configuration mechanism in a future version of GTK+, making it possible for users to extensively customize GTK+ software.

Setting Object Arguments

Most commonly, arguments are used as an API to set attributes of widgets. However, not all of the GTK+ API has been exported via arguments, so it is not always possible.

To set widget attributes, the most convenient interface is gtk_object_set(). Here's an example:

                  "GtkContainer::border_width", (gulong) 10,

The above code is identical in effect to:

   gtk_container_set_border_width(GTK_CONTAINER(vbox), 10);

It's up to you which to use; it depends on the context. Typically, you would use the argument mechanism if you have a reason to, i.e. if you are using its dynamic, runtime-oriented features. However, if you are setting several attributes, it may be easier to type and read.

gtk_object_set() takes a GtkObject as the first argument, followed by any number of key-value pairs. If a key is not defined for the object you pass in, a runtime error will be triggered. The list of key-value pairs must be terminated with a NULL key. When a GtkObject registers itself with GTK+, it tells GTK+ what type of value to expect after each key. For the aggregate fundamental types gtk_object_set() will expect more than one C function argument after the key. For example, first a signal function and then a user data pointer will be expected after GTK_TYPE_SIGNAL arguments. (Table 1 in the section called GtkArg and the Type System gives the types of the expected arguments.)

It is permissible to leave off the object class portion of an argument name---"GtkContainer::border_width" can be simply "border_width":

                  "border_width", (gulong) 10,

If you do not specify the class name as part of the argument name, GTK+ will start with the real type of the object and look up the argument name in the argument table for each superclass until it finds the right one (GtkContainer in this case). If you do specify the class name, GTK+ will only look for the argument in the specified class's argument table.

Since gtk_object_set() uses C variable argument lists, it has limited type safety. This can be a real problem in your code. You may have noticed the cast to gulong in the sample call to gtk_object_set(). The argument GtkContainer::border_width has type GTK_TYPE_ULONG. GTK+ will extract sizeof(gulong) bytes from the argument list when it encounters this argument. If you leave out the cast, C will probably pass only sizeof(gint) bytes to the function. As you might imagine, this causes memory corruption on many platforms. A similar problem arises with arguments of type GTK_TYPE_DOUBLE; if you type 5 instead of 5.0, C will pass an integer to gtk_object_set(). These bugs are very hard to find, once you introduce them.

gtk_object_set() is syntactic sugar for a more fundamental function call, gtk_object_setv(). gtk_object_setv() takes a vector of GtkArg (gtk_object_set() converts each key-value pair in its argument list to GtkArg internally).

   GtkArg args[1];
   args[0].name = "GtkContainer::border_width";
   args[0].type = GTK_TYPE_ULONG;
   GTK_VALUE_ULONG(args[0]) = 10;

The second argument to gtk_object_setv() is the length of the array of GtkArg. gtk_object_set() is plainly easier to use when you are typing the code in manually, but gtk_object_setv() can be passed a dynamically-constructed argument array---which is convenient if you're exporting GTK+ functionality to an interpreted environment.

It is also possible to set object arguments when objects are created. You can create most objects using the gtk_object_new() function, and most widgets with the gtk_widget_new() function. The routines take a GtkType as their first argument, and create an object or widget of that type. They then take a list of argument-value pairs, just as gtk_object_set() does. There are also gtk_object_newv() and gtk_widget_newv() variants.

Gtk+/Gnome Application Development
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