How to use Glib::Interface derived interfaces in custom classes?

[Krzesimir Nowak <qdlacz gmail com>]

Hi,

I'm trying to write a sort of a test of completion feature in
gtksourceviewmm. Or rather port the one written in C in gtksourceview-3
to C++. I have pushed a nonworking version to gtksourceviewmm repo. [1]

In short - there is a class TestProvider [2], which inherits from
Glib::Object and Gsv::SourceCompletionProvider [3]. The latter is an
interface.

There is a SourceCompletion's method named add_provider() which is used
in three places. [4][5][6]. The [4] call works, [5][6] - don't: I'm
getting a critical from gtksourceview:

(test:17365): GtkSourceView-CRITICAL **:
gtk_source_completion_add_provider: assertion
`GTK_IS_SOURCE_COMPLETION_PROVIDER (provider)' failed

(test:17365): GtkSourceView-CRITICAL **:
gtk_source_completion_add_provider: assertion
`GTK_IS_SOURCE_COMPLETION_PROVIDER (provider)' failed


That means that TestProvider::gobj() does not return a GObject
deriving/implementing a C GtkSourceCompletionProvider class. What I am
doing wrong?

Thanks for help,
Krzesimir

[1]
http://git.gnome.org/browse/gtksourceviewmm/tree/tests/completion/main.cc
[2]
http://git.gnome.org/browse/gtksourceviewmm/tree/tests/completion/main.cc#n14
[3]
http://git.gnome.org/browse/gtksourceviewmm/tree/gtksourceview/src/sourcecompletionprovider.hg
[4]
http://git.gnome.org/browse/gtksourceviewmm/tree/tests/completion/main.cc#n176
[5]
http://git.gnome.org/browse/gtksourceviewmm/tree/tests/completion/main.cc#n184
[6]
http://git.gnome.org/browse/gtksourceviewmm/tree/tests/completion/main.cc#n190