Re: [Vala] Fail to generate C code for following vala code, is this a bug?

On Sat, Mar 27, 2010 at 02:37:37AM +0800, PCMan wrote:
On Sat, Mar 27, 2010 at 12:17 AM, Julian Andres Klode <jak jak-linux org> wrote:
Well, than f_finish() should probably not be there in the Volume
interface; so I think that's a bug.
So how can I fix this? Ship with a custom glib vapi file and remove
*_finish from GVolume? Will this work? Or, can you make automatic
generation of *_finish optional in Vala, or make the automatically
generated *_finish override the abstract method in parent interface if
its found?
It's a bug:

For the second question, if I want to return a unowned GList, it has
to be owned by the class and cannot be a local variable, right? Would
you please add this to vala tutorial? I found it not documented and
this behavior is not easy to figure out. In addition, this is not
consistent with GObject with C. In C we can create a new GList and
return it without the need to make it owned by anyone. The callee of
the function is responsible to free the list.

Because the list is unowned in can not be local, as it would go
away once the function returns. If you want to return a locally
created object, the return value can only be owned.
This is quite different from C and C++ and is hard to understand for a
C programmer I think.
No, it's not. The difference is just that instead of manually freeing
the returned pointer, the return values are marked as unowned/owned,
and Vala then adds code to free owned values automatically.

In short, if you have to free a return value in C, it is owned; if
you don't have to free it, it is unowned.

However, I don't get it. Let take GVolumeMonitor::get_volumes as  example.
It returns unowned GLib.List. So if I want to implement that, I should
create a list and make it owned by the object instance and then return
it unowned. However this is slightly different from the behavior of
glib in C. In C GVolumeMonitor::get_volumes returns a
"newly-allocated" GList containing a list of GObjects. The caller is
responsible to g_list_foreach(list, g_object_unref, NULL); and then
So, the problem is that the function return values are falsely
marked as unowned in the VAPI. I reported a bug for it at

However, with Vala, how can I make vala methods return a "newly
allocated GList" with all objects in it having increased ref_count?
Automatic ref couting management in Vala is handy if you only use it
in Vala but I find it difficult to make things right when interfacing
with C code. I need to check the generated C code to see if it does
what I really want. Any better guide for this? Already read Vala
tutorial, but I still haven't get it.
Just don't mark the list as unowned.

Julian Andres Klode  - Debian Developer, Ubuntu Member

See and

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