Interfaces usage in library is like class usage. We need to define a interface struct, but no object struct is needed:

NOTE: PLEASE READ ALL COMMENT CAREFULLY.

Then we register the interface using g_type_register_static() with G_TYPE_INTERFACE as first parameter. For interfaces, we only need to assign base_init() and base_finalize()callbacks.

As described in the official document, we should allocate dynamic memebers of class struct in base_init(). Otherwise, all copies of the class struct share only one copy of dynamic members. This leads to problems.

Let’s define the type which implements the interface:

Note the naming convention I used here. Our FakeDesktop class will implement the FakeIServer interface and another FakeIClient interface. This time do not use corresponding interface struct as the first members of FakeDesktop and FakeDesktopClass. Interface info will be added dynamically when initialize a real instance of FakeDesktop. Let’s move to the *.c code:

Note the g_type_add_interface_static() function call to add interface info. The interface info is defined in a GInterfaceInfo struct. We just make use of the interface_init() callback. In it, we assign function pointers of corresponding interface to our implementation function. We can add multiple interface infos to implement them.

Finally, the test code:

In runtime, if your classed type implements an interface, it will be considered as the interface type (is-a).

All source code is available in my skydrive: http://cid-481cbe104492a3af.office.live.com/browse.aspx/share/dev/TestOO. In the TestGObject-{date}.zip/TestGObject6 folder.