How to create a plugin

[tbscope]

This is weird. I don't see the problem. QMake should find it automatically.

[MTK358]

$ ls
Makefile  myplugin.cpp  myplugin.h  myplugin.pro
$ cat myplugin.pro
######################################################################
# Automatically generated by qmake (2.01a) Fri Aug 27 08:29:32 2010
######################################################################
 
QT += gui
TEMPLATE = lib
CONFIG = plugin
TARGET = $$qtLibraryTarget(myplugin)
DEPENDPATH += .
INCLUDEPATH += .
 
# Input
HEADERS += myplugin.h
SOURCES += myplugin.cpp
$ qmake
$ make
g++ -c -m64 -pipe -march=x86-64 -mtune=generic -O2 -pipe -fPIC  -I/usr/share/qt/mkspecs/linux-g++-64 -I. -I. -o myplugin.o myplugin.cpp
In file included from myplugin.cpp:1:0:
myplugin.h:1:17: fatal error: QtGui: No such file or directory
compilation terminated.
make: *** [myplugin.o] Error 1

$ ls
Makefile  myplugin.cpp  myplugin.h  myplugin.pro
$ cat myplugin.pro
######################################################################
# Automatically generated by qmake (2.01a) Fri Aug 27 08:29:32 2010
######################################################################

QT += gui
TEMPLATE = lib
CONFIG = plugin
TARGET = $$qtLibraryTarget(myplugin)
DEPENDPATH += .
INCLUDEPATH += .

# Input
HEADERS += myplugin.h
SOURCES += myplugin.cpp
$ qmake
$ make
g++ -c -m64 -pipe -march=x86-64 -mtune=generic -O2 -pipe -fPIC  -I/usr/share/qt/mkspecs/linux-g++-64 -I. -I. -o myplugin.o myplugin.cpp
In file included from myplugin.cpp:1:0:
myplugin.h:1:17: fatal error: QtGui: No such file or directory
compilation terminated.
make: *** [myplugin.o] Error 1

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[tbscope]

As you can see in the following line:

g++ -c -m64 -pipe -march=x86-64 -mtune=generic -O2 -pipe -fPIC  -I/usr/share/qt/mkspecs/linux-g++-64 -I. -I. -o myplugin.o myplugin.cpp

g++ -c -m64 -pipe -march=x86-64 -mtune=generic -O2 -pipe -fPIC  -I/usr/share/qt/mkspecs/linux-g++-64 -I. -I. -o myplugin.o myplugin.cpp

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The following include paths are at least missing:

-I/usr/include/.../QtCore -I/usr/include/.../QtGui

-I/usr/include/.../QtCore -I/usr/include/.../QtGui

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These are pseudo paths, I don't know where they are on your system.

This is mighty strange as, like I said, when QT += gui and QT += core (which are default, you don't generally have to add those manually to the .pro file) are added to your .pro file, the include paths should be set by qmake.

One thing I can think of is to manually delete the Makefile, and try qmake and make again. But I guess this won't solve it either.

Edit: try this too:

CONFIG += plugin

CONFIG += plugin

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Note the +
When you do not add the +, you erase all the configs

[MTK358]

Another error:

$ make
/usr/bin/qmake -unix -o Makefile myplugin.pro
g++ -c -m64 -pipe -march=x86-64 -mtune=generic -O2 -pipe -Wall -W -D_REENTRANT -fPIC -DQT_NO_DEBUG -DQT_PLUGIN -DQT_GUI_LIB -DQT_CORE_LIB -DQT_SHARED -I/usr/share/qt/mkspecs/linux-g++-64 -I. -I/usr/include/QtCore -I/usr/include/QtGui -I/usr/include -I. -I. -o myplugin.o myplugin.cpp
myplugin.cpp: In function ‘QObject* qt_plugin_instance()’:
myplugin.cpp:18:1: error: cannot allocate an object of abstract type ‘MyPlugin’
myplugin.h:6:1: note: because the following virtual functions are pure within ‘MyPlugin’:
/home/michael/junk/QPanel/qpanelappletinterface.h:13:18: note: virtual void QPanelAppletInterface::showSettingsDialog()
make: *** [myplugin.o] Error 1
The interface:

#ifndef QPANELAPPLETINTERFACE_H
#define QPANELAPPLETINTERFACE_H
 
#include <QtPlugin>
 
class QPanelAppletInterface
{
public:
    virtual ~QPanelAppletInterface() {}
 
    virtual QWidget *getWidget() = 0;
 
    virtual void showSettingsDialog() = 0;
};
 
Q_DECLARE_INTERFACE(QPanelAppletInterface, "com.trolltech.PlugAndPaint.QPanelAppletInterface/1.0")
 
#endif // QPANELAPPLETINTERFACE_H

#ifndef QPANELAPPLETINTERFACE_H
#define QPANELAPPLETINTERFACE_H

#include <QtPlugin>

class QPanelAppletInterface
{
public:
    virtual ~QPanelAppletInterface() {}

    virtual QWidget *getWidget() = 0;

    virtual void showSettingsDialog() = 0;
};

Q_DECLARE_INTERFACE(QPanelAppletInterface, "com.trolltech.PlugAndPaint.QPanelAppletInterface/1.0")

#endif // QPANELAPPLETINTERFACE_H

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[tbscope]

Yes, you need to add the void showSettingsDialog() function to MyPlugin

[MTK358]

My main application now "unexpectedly finishes" if I uncomment the commented out line:

dirIter.next();
        QPluginLoader loader(dirIter.filePath());
        QPanelAppletInterface *plugin = qobject_cast<QPanelAppletInterface*>(loader.instance());
        //ui->appletLayout->addWidget(plugin->getWidget());

dirIter.next();
        QPluginLoader loader(dirIter.filePath());
        QPanelAppletInterface *plugin = qobject_cast<QPanelAppletInterface*>(loader.instance());
        //ui->appletLayout->addWidget(plugin->getWidget());

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[tbscope]

Most probably, plugin is 0 resulting in the crash.

To prevent the crash, but not solve it do this:

if (plugin)
    ui->appletLayout->addWidget(plugin->getWidget());

if (plugin)
    ui->appletLayout->addWidget(plugin->getWidget());

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Now, to solve the plugin problem.
1. check if loader does point to the correct file. This means that dirIter.filePath() should be the same as "/home/you/where/the/plugin/is/myplugin.so"
2. Check if the cast can actually be performed. Therefor, the instance returned by the plugin loader needs to be correct at least.

[MTK358]

It finally works!!!

The problem is that it was trying to load the files "." and "..".

[tbscope]

Nice, well done.