'Undefined interface' when including from /usr/include

[xarxer]

Hello all!

I have an interface defined like this (minimal example):
@
#ifndef IPLUGIN_H_
#define IPLUGIN_H_

class IPlugin
{
public:
virtual ~IPlugin() { }
virtual int getVersion() = 0; //Arbitrary, isn't needed to recreate error
};

QT_BEGIN_NAMESPACE
Q_DECLARE_INTERFACE(IPlugin, "com.xarxer.IPlugin")
QT_END_NAMESPACE

#endif
@

The IPlugin header file is placed in /usr/include/c++/4.7.2/MyStuff/
In my .pro file, I do this:
@
INCLUDEPATH += /usr/include/c++/4.7.2/MyStuff
@

I've also tried variants of INCLUDEPATH such as "-I /usr/include/[...]" etc.

And in my plugin library (Shared library project) I do this:
@
#ifndef MYPLUGIN_H_
#define MYPLUGIN_H_

#include <QObject>
#include <IPlugin> //Tried variants such as <MyStuff/IPlugin>, <MyStuff/IPlugin.h> (with and without .h in the extension on the file etc.)

class MyPlugin : public QObject, public IPlugin
{
Q_OBJECT
Q_INTERFACES(IPlugin) //This is what generates the error
//Also tried with and without Q_PLUGIN_METADATA

public:
~MyPlugin() { }
virtual int getVersion() { return 1; }
};

#endif
@

But when I compile I get the error "Undefined interface".

However, if I move the file from /usr/include/[...] to my actual project directory, then the compiler recognizes the interface.
This really seems like a bug in the compiler or something. Any ideas?

Thanks in advance!

[leon.anavi]

INCLUDEPATH points to the location of the header files and LIBS points to the location of the share library. Make sure that you have set a correct path for LIBS. Read the following wiki article for details and examples of both linking shared library and loading it with QLibrary: "How to create a library with Qt and use it in an application":http://qt-project.org/wiki/How_to_create_a_library_with_Qt_and_use_it_in_an_application

[xarxer]

Well, the issue is that the shared library project doesn't compile at all. However, I tried the exact same code on Windows today, and it works fine. I shall experiment a bit with permissions on the include folder as soon as I get home, altough I don't think that's the issue at hand..

EDIT: Nope, exact same code under Linux does not compile, compiler gives the error 'Undefined interface'. Any help greatly appreciated! :-(

[xarxer]

Hello again. I've been experimenting some.

If I do this in my .pro file:
@
INCLUDEPATH += /usr/include/c++/4.7.2/MyStuff
@

and this in my header file:
@
#include <IPlugin.h>
@

It does not compile, and the compiler says "Undefined interface".
If I omit the INCLUDEPATH, it doesn't compile either, and this really should be omittable since /usr/include is included per default.
However, if I do this in my .pro file:
@
INCLUDEPATH += /usr
@

and this in my header file:
@
#include <include/c++/4.7.2/MyStuff/IPlugin.h>
@

It compiles nicely!
The former configuration does not generate moc_ files if that matters.

Again, any help at all in understanding what the problem is is greatly appreciated!