Inheritance of plugins with QPluginLoader not working?

[tallia1]

Hello everybody,

I am having some major issues with inheritance in between plugins and the ability
to load/cast them properly at runtime. Let me describe the problem. I will also provide
the whole project tree so you can try it out yourself.

I have two interfaces: Parent/ChildInterface, linked by a natural inheritance relation:
@
class ParentInterface{};
Q_DECLARE_INTERFACE(ParentInterface, "ParentInterface/1.0")
class ChildInterface : public ParentInterface{};
Q_DECLARE_INTERFACE(ChildInterface, "ChildInterface/1.0")
@

Then I have two plugins (PluginChild/Parent), each respectively implementing the
corresponding interface:
@
class PluginChild : public QObject, public ChildInterface{ //...
class PluginParent : public QObject, public ParentInterface{ //...
@

Now I load the plugins at runtime using what follows:
@
QObject *plugin = loader.instance();
if( plugin ){
qDebug() << "==> Loaded correct QPlugin";

        ChildInterface*  iC = qobject_cast<ChildInterface*>(plugin);
        if(iC) qDebug() << "==> ChildInterface CASTED";
        else   qDebug() << "==> ChildInterface FAILED";            
        
        ParentInterface* iP = qobject_cast<ParentInterface*>(plugin);
        if(iP) qDebug() << "==> ParentInterface CASTED";
        else   qDebug() << "==> ParentInterface FAILED";
    } else
        qDebug() << "==> Load incorrect QPlugin";

@

And this is the output I get:
@
Loading: "/Users/ata2/workspace/starlab/tester/plugins/libplugin_child.dylib"
==> Loaded correct QPlugin
==> ChildInterface CASTED
==> ParentInterface FAILED
Loading: "/Users/ata2/workspace/starlab/tester/plugins/libplugin_parent.dylib"
==> Loaded correct QPlugin
==> ChildInterface FAILED
==> ParentInterface CASTED
@

The question is simple: why libplugin_child fails during casting ParentInterface?
The full source tree is available in a compressed zip "HERE":http://www.sfu.ca/~ata2/tester.zip

Thank you so much for your input

Andrea Tagliasacchi

[lgeyer]

You are most probably missing a Q_INTERFACES(ParentInterface) in your PluginChild.

qobject_cast() relies on the information provided by the meta object system, which requires you to specify any implemented interfaces using Q_INTERFACES() (and all QObject classes to specify Q_OBJECT). ParentInterface and ChildInterface are not QObjects and so there is no meta object information (and thus qobject_cast() cannot work).

However, ParentInterface is still a parent class of ChildInterface (in terms of C++ inheritance). So once you have cast a plugin to ChildInterface using qobject_cast(), you can cast it to ParentInterface implicitly or explicitly using static_cast().
@
ChildInterface* iC = qobject_cast<ChildInterface*>(plugin);
ParentInterface* iP = static_cast<ParentInterface*>(iC);
@

[tallia1]

Thanks Lukas, I understand the problem. The source of it was in the full project.
In there I had the following

@
class SurfaceMeshIO : public ModelIO{
Q_INTERFACES(ModelIO)
...
@

@ // Source of error
class io_surfacemesh_off : public QObject, public SurfaceMeshIO{
Q_OBJECT
Q_INTERFACES(SurfaceMeshIO)
....
@

I was assuming that since "io_surfacemesh_off" inherited from SurfaceMeshIO, the Q_INTERFACE(ModelIO) was already taken care of from its superclass. I have to admit, it's kind of weird that I have to do what follows:

@ // Correction
class io_surfacemesh_off : public QObject, public SurfaceMeshIO{
Q_OBJECT
Q_INTERFACES(SurfaceMeshIO)
Q_INTERFACES(ModelIO)
....
@