Abstract plugin inheritance error

Defohin

I'm trying to create a plugin system and I have an interface for the plugin and also an abstract plugin that others plugin will inherit however I'm getting a lot of errors:

Before the errors, here is how my structure looks like:

|-- myproject
    |-- myproject.pro
    |-- plugins
    |   |-- plugins.pro
    |   |-- pluginfoo
    |   |   |-- pluginfoo.cpp
    |   |   |-- pluginfoo.hpp
    |   |   |-- pluginfoo.json
    |   |   |-- pluginfoo.pro
    |-- myproject
        |-- myproject.pro
        |-- gui
        |   |-- myprojectwindow.cpp
        |   |-- myprojectwindow.hpp
        |-- src
            |-- main.cpp
            |-- plugininterface.hpp
            |-- abstractplugin.cpp
            |-- abstractplugin.hpp

myproject.pro

TEMPLATE = subdirs
SUBDIRS  = myproject \
           plugins

plugins.pro

TEMPLATE = subdirs
SUBDIRS  = pluginfoo

pluginfoo.cpp

#include "pluginfoo.hpp"

pluginfoo.hpp

#ifndef PLUGINFOO_HPP
#define PLUGINFOO_HPP

#include "abstractplugin.hpp"

class PluginFoo : public AbstractPlugin
{
    Q_OBJECT
    Q_PLUGIN_METADATA(IID PluginInterface_iid FILE "pluginfoo.json")
    Q_INTERFACES(PluginInterface)
};

#endif // PLUGINFOO_HPP

pluginfoo.json
{}

pluginfoo.pro

QT          += core gui
TARGET       = pluginfoo
TEMPLATE     = lib
CONFIG      += plugin
DESTDIR      = ../../bin/plugins
DEFINES     += QT_DEPRECATED_WARNINGS
INCLUDEPATH += ../../myproject/src
SOURCES     += pluginfoo.cpp
HEADERS     += pluginfoo.hpp
DISTFILES   += pluginfoo.json

myproject.pro

QT      += core gui widgets xml network
TARGET   = myproject
TEMPLATE = app
DESTDIR  = ../bin
DEFINES += QT_DEPRECATED_WARNINGS
SOURCES += src/main.cpp \
           gui/myprojectwindow.cpp
           src/abstractplugin.hpp
HEADERS += gui/myprojectwindow.hpp \
           src/plugininterface.hpp
           src/abstractplugin.hpp

myprojectwindow.cpp

#include "myprojectwindow.hpp"

MyProjectWindow::MyProjectWindow(QWidget *parent) : QMainWindow(parent)
{
}

MyProjectWindow::~MyProjectWindow()
{

}

myprojectwindow.hpp

#ifndef MYPROJECTWINDOW_HPP
#define MYPROJECTWINDOW_HPP

#include <QMainWindow>

class MyProjectWindow : public QMainWindow
{
    Q_OBJECT

public:
    MyProjectWindow(QWidget *parent = 0);
    ~MyProjectWindow();
};

#endif // MYPROJECTWINDOW_HPP

main.cpp

#include "gui/myprojectwindow.hpp"
#include <QApplication>

int main(int argc, char *argv[])
{
    QApplication application(argc, argv);

    MyProjectWindow window;
    window.show();

    return application.exec();
}

plugininterface.hpp

#ifndef PLUGININTERFACE_HPP
#define PLUGININTERFACE_HPP

class PluginInterface
{
public:
    virtual ~PluginInterface() {}
};

#define PluginInterface_iid "PluginInterface/1.0"

Q_DECLARE_INTERFACE(PluginInterface, PluginInterface_iid)

#endif // PLUGININTERFACE_HPP

abstractplugin.cpp

#include "abstractplugin.hpp"

abstractplugin.hpp

#ifndef ABSTRACTPLUGIN_HPP
#define ABSTRACTPLUGIN_HPP

#include <QtPlugin>
#include "plugininterface.hpp"

class AbstractPlugin : public QObject, public virtual PluginInterface
{
    Q_OBJECT
    Q_INTERFACES(PluginInterface)
};

#endif // ABSTRACTPLUGIN_HPP

And I get this when compiling:

moc_pluginfoo.obj:-1: error: LNK2001: unresolved external symbol "public: static struct QMetaObject const AbstractPlugin::staticMetaObject" (?staticMetaObject@AbstractPlugin@@2UQMetaObject@@B)

moc_pluginfoo.obj:-1: error: LNK2001: unresolved external symbol "public: virtual struct QMetaObject const * __cdecl AbstractPlugin::metaObject(void)const " (?metaObject@AbstractPlugin@@UEBAPEBUQMetaObject@@XZ)

moc_pluginfoo.obj:-1: error: LNK2019: unresolved external symbol "public: virtual void * __cdecl AbstractPlugin::qt_metacast(char const *)" (?qt_metacast@AbstractPlugin@@UEAAPEAXPEBD@Z) referenced in function "public: virtual void * __cdecl PluginFoo::qt_metacast(char const *)" (?qt_metacast@PluginFoo@@UEAAPEAXPEBD@Z)

moc_pluginfoo.obj:-1: error: LNK2019: unresolved external symbol "public: virtual int __cdecl AbstractPlugin::qt_metacall(enum QMetaObject::Call,int,void * *)" (?qt_metacall@AbstractPlugin@@UEAAHW4Call@QMetaObject@@HPEAPEAX@Z) referenced in function "public: virtual int __cdecl PluginFoo::qt_metacall(enum QMetaObject::Call,int,void * *)" (?qt_metacall@PluginFoo@@UEAAHW4Call@QMetaObject@@HPEAPEAX@Z)

....\bin\plugins\pluginfoo.dll:-1: error: LNK1120: 4 unresolved externals

kshegunov

You haven't exported any of your classes.
Also is the virtual inheritance really necessary? I shouldn't think so.

Defohin

@kshegunov It's not an export problem, I forgot to type the src/plugininterface.hpp in the pro file here on the example. I edited the question.
The virtual inheritance is to avoid the diamond problem.

kshegunov

@Defohin said in Abstract plugin inheritance error:

It's not an export problem

I would argue that it is. You're trying to use a class that's defined in the application project in the library. The Q_OBJECT macro generates members(!) that have to be accessible to the linker, and in your case they aren't. This is moot when dealing with interfaces, but it's significant when dealing with classes like AbstractPlugin.

The virtual inheritance is to avoid the diamond problem.

If you have a diamond somewhere, then you've done something very, very, very wrong. I have had a need to deal with it only a couple of times in my 10 years of experience with C++. Virtual inheritance is evil, avoid it at (almost) any cost!

Defohin

@kshegunov Thank you, for real, but what you think I should do? The entire code is here, but still I have no idea what is happening.

kshegunov

Simply remove the AbstractPlugin class, it's not necessary anyway, it does nothing. Instead for each plugin derive from QObject and the interface as you've done. That should fix it for the most part. Namely:

class PluginFoo : public QObject, public PluginInterface
{
    Q_OBJECT
    Q_PLUGIN_METADATA(IID PluginInterface_iid FILE "pluginfoo.json")
    Q_INTERFACES(PluginInterface)
};

Then (as you're working on windows) you'll probably get a linker error for the plugin class itself, exporting it from the dll should fix that as well.

Defohin

@kshegunov I will need the abstract class to add a few common methods. It's just an example code.

kshegunov

@Defohin said in Abstract plugin inheritance error:

I will need the abstract class to add a few common methods. It's just an example code.

Then you must provide the linker with the appropriate binary. Meaning you must, in this case, move the abstract plugin class in a separate library to which your plugins will link (~~look up the docs, there's examples how to make an ordinary dynamic library~~ look here). In this library you need to export the AbstractPlugin class (through Q_DECL_EXPORT and Q_DECL_IMPORT again you can look how exactly in the docs). The interface can stay either in the application project or in the new library, that's up to you and is a matter of convenience.

Defohin

@kshegunov I tried to add this:

myprojectglobal.h

#ifndef MYPROJECTGLOBAL_HPP
#define MYPROJECTGLOBAL_HPP

#include <QtCore/qglobal.h>

#if defined(MYPROJECT_LIBRARY)
#  define MYPROJECT_EXPORT Q_DECL_EXPORT
#else
#  define MYPROJECT_EXPORT Q_DECL_IMPORT
#endif

#endif // MYPROJECTGLOBAL_HPP

myproject.pro

DEFINES += MYPROJECT_LIBRARY

plugininterface.hpp

#ifndef PLUGININTERFACE_HPP
#define PLUGININTERFACE_HPP

#include "myprojectglobal.hpp"
#include <QtPlugin>

class MYPROJECT_EXPORT PluginInterface
{
public:
    virtual ~PluginInterface() {}
};

#define PluginInterface_iid "PluginInterface/1.0"

Q_DECLARE_INTERFACE(PluginInterface, PluginInterface_iid)

#endif // PLUGININTERFACE_HPP

abstractplugin.hpp

#ifndef ABSTRACTPLUGIN_HPP
#define ABSTRACTPLUGIN_HPP

#include "plugininterface.hpp"

class MYPROJECT_EXPORT AbstractPlugin : public QObject, public PluginInterface
{
    Q_OBJECT
    Q_INTERFACES(PluginInterface)
};

#endif // ABSTRACTPLUGIN_HPP

And now I'm getting these errors:

moc_pluginfoo.obj:-1: error: LNK2019: unresolved external symbol "__declspec(dllimport) public: virtual __cdecl PluginInterface::~PluginInterface(void)" (_imp??1PluginInterface@@UEAA@XZ) referenced in function "int public: __cdecl PluginFoo::PluginFoo(void)'::1'::dtor$0" (?dtor$0@?0???0PluginFoo@@QEAA@XZ@4HA)

moc_pluginfoo.obj:-1: error: LNK2019: unresolved external symbol "__declspec(dllimport) public: __cdecl PluginInterface::PluginInterface(void)" (_imp??0PluginInterface@@QEAA@XZ) referenced in function "public: __cdecl PluginFoo::PluginFoo(void)" (??0PluginFoo@@QEAA@XZ)

moc_pluginfoo.obj:-1: error: LNK2019: unresolved external symbol "__declspec(dllimport) public: virtual void * __cdecl AbstractPlugin::qt_metacast(char const *)" (_imp?qt_metacast@AbstractPlugin@@UEAAPEAXPEBD@Z) referenced in function "public: virtual void * __cdecl PluginFoo::qt_metacast(char const *)" (?qt_metacast@PluginFoo@@UEAAPEAXPEBD@Z)

moc_pluginfoo.obj:-1: error: LNK2019: unresolved external symbol "__declspec(dllimport) public: virtual int __cdecl AbstractPlugin::qt_metacall(enum QMetaObject::Call,int,void * *)" (_imp?qt_metacall@AbstractPlugin@@UEAAHW4Call@QMetaObject@@HPEAPEAX@Z) referenced in function "public: virtual int __cdecl PluginFoo::qt_metacall(enum QMetaObject::Call,int,void * *)" (?qt_metacall@PluginFoo@@UEAAHW4Call@QMetaObject@@HPEAPEAX@Z)

moc_pluginfoo.obj:-1: error: LNK2019: unresolved external symbol "__declspec(dllimport) public: __cdecl AbstractPlugin::AbstractPlugin(void)" (_imp??0AbstractPlugin@@QEAA@XZ) referenced in function "public: __cdecl PluginFoo::PluginFoo(void)" (??0PluginFoo@@QEAA@XZ)

moc_pluginfoo.obj:-1: error: LNK2019: unresolved external symbol "__declspec(dllimport) public: virtual __cdecl AbstractPlugin::~AbstractPlugin(void)" (_imp??1AbstractPlugin@@UEAA@XZ) referenced in function "public: virtual __cdecl PluginFoo::~PluginFoo(void)" (??1PluginFoo@@UEAA@XZ)

moc_pluginfoo.obj:-1: error: LNK2001: unresolved external symbol "__declspec(dllimport) public: static struct QMetaObject const AbstractPlugin::staticMetaObject" (_imp?staticMetaObject@AbstractPlugin@@2UQMetaObject@@B)

....\bin\plugins\pluginfoo.dll:-1: error: LNK1120: 7 unresolved externals

jsulm

@Defohin Do you link your pluginfoo plugin against the lib containing AbstractPlugin and PluginInterface?

Defohin

@jsulm What you mean? The whole code I have is here.

SGaist

Hi,

The usual setup when building a plugin based application is to have the plugin interfaces definitions and related code in a library that you will link to both the plugin and the application.

That's what is suggested here.

Defohin

@SGaist I'm really struggling to understand, can you provide an example, please?

SGaist

Roughly :
pluginfoo.pro

QT          += core gui
TARGET       = pluginfoo
TEMPLATE     = lib
CONFIG      += plugin
DESTDIR      = $$OUTPWD/bin/plugins
LIBS += -L$$OUTPWD/lib -lmycoollib
DEFINES     += QT_DEPRECATED_WARNINGS
INCLUDEPATH += ../mycoollib
SOURCES     += pluginfoo.cpp
HEADERS     += pluginfoo.hpp
DISTFILES   += pluginfoo.json

mycoollib.pro

QT          += core gui
TARGET       = mycoollib
TEMPLATE  = lib
DESTDIR      = $$OUTPWD/lib
INCLUDEPATH += .
SOURCES     += \
    abstractplugin.cpp
HEADERS     += \
    plugininterface.hpp
    abstractplugin.hpp

myproject.pro

QT      += core gui widgets xml network
TARGET   = myproject
TEMPLATE = app
DESTDIR  = $$OUTPWD/bin
LIBS += -L$$OUTPWD/lib -lmycoollib
INCLUDEPATH += ../mycoollib
DEFINES += QT_DEPRECATED_WARNINGS
SOURCES += src/main.cpp \
           gui/myprojectwindow.cpp

HEADERS += gui/myprojectwindow.hpp

Project structure

mycoolproject \
---- mycoollib
---- pluginfoo
---- myproject

mycoolproject being a SUBDIR project building first mycoollib and then pluginfoo and myproject.

Defohin

@SGaist Do I have to link with -l every single plugin I have? Or I can just provide a path like -L and it's going to work?

I will try to modify here based on your example.

SGaist

You don't link the plugins. You link your plugins against that common library.

Yes, you have to use both -land -L.

-L just tells the linker where to look at.

Defohin

@SGaist It works, but I have another question now.

I named the "commonlib" to "pluginmanager" this class will hold information about plugins, load, unload and so on using QPluginLoader, but... when I try to use it on the main application including the header pluginmanager.hpp it says that it cannot open the file, even if I have the INCLUDEPATH += ../pluginmanager. On Qt Creator it does work, I'm able to ctrl+click, etc, but when I try to compile I get this error.

jsulm

@Defohin Did you run qmake again after changing the pro file? After qmake you should do a complete rebuild.

Defohin

@jsulm It works, but now I'm getting unresolved external symbol on the main project when trying to use PluginManager manager on the MyProjectWindow, and I have LIBS += -L../libs -lpluginmanager on the pro file.

myprojectwindow.obj:-1: error: LNK2019: unresolved external symbol "public: __cdecl PluginManager::PluginManager(class QObject *)" (??0PluginManager@@QEAA@PEAVQObject@@@Z) referenced in function "public: __cdecl MyProjectWindow::MyProjectWindow(class QWidget *)" (??0MyProjectWindow@@QEAA@PEAVQWidget@@@Z)

jsulm

@Defohin ../libs is a relative path, are you sure it is correct?
Can you post the compiler/linker call just before the error message?
As suggested by @SGaist : LIBS += -L$$OUTPWD/lib -lmycoollib (pluginmanager instead of mycoollib)