[SOLVED] Simple plugin fails to load

[alaink]

Hello,

I tried to make a simple program with plugins, similar to the "Echo Plugin Example":http://qt-project.org/doc/qt-5/qtwidgets-tools-echoplugin-example.html . However, my program gives the following error:

"Cannot load library /home/alain/qt/plugins/libtest_plugin.so: (/home/alain/qt/plugins/libtest_plugin.so: undefined symbol: _ZN11Test_plugin4echoEv)"
Segmentation fault

Why doesn't it find that function? Did I forget something? These are my files:

@+qt/
- test_app/
main.cpp
mainwindow.cpp
mainwindow.h
test_interface.h
test_app.pro
- test_plugin/
test_plugin.cpp
test_plugin.h
test_plugin.pro
- plugins/
libtest_plugin.so
@

main.cpp:
@#include "mainwindow.h"
#include <QApplication>

int main(int argc, char *argv[])
{
QApplication a(argc, argv);
MainWindow w;
w.show();

return a.exec();

}
@

mainwindow.h:
@#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>
#include <QDebug>
#include <QPluginLoader>
#include "test_interface.h"

namespace Ui {
class MainWindow;
}

class MainWindow : public QMainWindow
{
Q_OBJECT

public:
explicit MainWindow(QWidget *parent = 0);
~MainWindow();

private:
Ui::MainWindow *ui;
};

#endif // MAINWINDOW_H
@

mainwindow.cpp:
@#include "mainwindow.h"
#include "ui_mainwindow.h"

MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);

QPluginLoader pluginLoader("/home/alain/qt/plugins/libtest_plugin.so");
QObject *plugin = pluginLoader.instance();
qDebug() << pluginLoader.errorString();
qDebug() << qobject_cast<test_interface*>(plugin)->echo();

}

MainWindow::~MainWindow()
{
delete ui;
}
@

test_interface.h:
@#ifndef TEST_INTERFACE_H
#define TEST_INTERFACE_H

#include <QString>
#include <QObject>
#include <QtPlugin>

class test_interface : public QObject
{
Q_OBJECT

public:
test_interface();
virtual QString echo(void) = 0;
};

Q_DECLARE_INTERFACE(test_interface, "something.unique")

#endif // TEST_INTERFACE_H
@

test_plugin.h:
@#ifndef TEST_PLUGIN_H
#define TEST_PLUGIN_H

#include "test_interface.h"

class Test_plugin : public test_interface
{
Q_OBJECT
Q_PLUGIN_METADATA(IID "something.unique")
Q_INTERFACES(test_interface)

public:
Test_plugin();
QString echo(void);
};

#endif // TEST_PLUGIN_H
@

test_plugin.cpp:
@#include "test_plugin.h"

Test_plugin::Test_plugin()
{
}

QString echo(void)
{
return QString("Hallo");
}
@

test_app.pro:
@QT += core gui

greaterThan(QT_MAJOR_VERSION, 4): QT += widgets

TARGET = test_app
TEMPLATE = app

SOURCES += main.cpp
mainwindow.cpp

HEADERS += mainwindow.h
test_interface.h

FORMS += mainwindow.ui
@

test_plugin.pro:
@CONFIG += plugin
TARGET = $$qtLibraryTarget(test_plugin)
TEMPLATE = lib
INCLUDEPATH += ../test_app
DESTDIR = ../plugins
SOURCES += test_plugin.cpp
HEADERS += test_plugin.h ../test_app/test_interface.h
@

I am using Qt 5.3.1 and Qt Creator 3.2.0 on Arch Linux 64bit.

Thanks

[t3685]

You did not properly implement your Test_plugin::echo() function. You forgot to add Test_plugin:: in front of the implementation in the cpp file.

[dheerendra]

Basic requirement. Interface should not inherit from QObject. Remove QObject inheritance from Interface. Also try to put destructor.

class test_interface : public QObject

Also as "t3685" identified, it is issue with implementation as well.

[alaink]

Thanks, I overlooked that. I also added a destructor to test_interface and Test_plugin, removed the QObject from the test_interface and let Test_plugin inherit from QObject instead. Unfortunately, it still doesn't work. Now I get that:

./test_app: symbol lookup error: home/alain/qt/plugins/libtest_plugin.so: undefined symbol: _ZN14test_interfaceC2Ev

I ran qmake and rebuilt everything, but it still can't find it.

And why shouldn't the Interface inherit from QObject? If I, for example, would want to make an interface for a widget, shouldn't the interface inherit from QWidget and through that from QObject? What would be the alternative?

Thanks

[SGaist]

Hi,

It should not, take this case: you create two interfaces each one deriving from QObject. Now you want to implement these two interfaces in only one class -> you can't because you are trying to derive multiple times from QObject and it's not possible. Same problem if you try to implement one of these .interface on a QWidget

Take a look at the Plug and Paint and it's friends examples, they provide also a good starting point.

[alaink]

Thanks, I see now that the interface doesn't have to inherit from QWidget to provide it's functions. However, I still can't see why my program fails.

[alaink]

I found the problem. I didn't implement the constructor in the interface. Changing this:

@public:
test_interface();@

to this

@public:
test_interface(){}@

or removing the constructor completely solved my problem.