OOP Using GObject (2) – A Classed Type

In last article, we defined a fundamental type. But nothing can be done with it. Now, we will extend it to be a classed type, say adding class info into our fundamental type. To do this, we should define a class struct, which can be regard as the meta info of a C++ class:


GTypeClass should be the first member of a class struct. You can image the i field to be the version of the class. And we can add a string field to hold the author of this class. There’s also a function pointer bar(). As you may already know, it is used to implement polymorphism, which can be regard as virtual function of a C++ class.

When registering our fundamental type, additional field in GTypeInfo and GTypeFundamentalInfo are filled:

GTypeInfo is the key data structure of GObject type system. It defines how a classed type should be initialized and finalized. Here, we just assigned the class_init() callback. It is called when our FooClass needs initialization. For fundamental and static types, their class_finalize() are never called. We will demo the usage of this callback when introducing dynamic types. Please also note the G_TYPE_FLAG_CLASSED flag passed into GTypeFundamentalInfo struct.

Now, let’s implement our foo_class_init() function. This function is used to initialize fields and assign virtual functions in most time:

Now, we’ve finished our definition of the class struct. Let’s see how to use it:

See? We use g_type_class_ref() and g_type_class_unref() to ref/unref a class, and invoke a function. But its function is still limited. We can just get/set its meta info. It still cannot be instantiated. This will be discussed in the next article.

All source code is available in my skydrive: http://cid-481cbe104492a3af.office.live.com/browse.aspx/share/dev/TestOO. In the TestGObject-{date}.zip/TestGObject2 folder.

Leave a Reply

Your email address will not be published. Required fields are marked *