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#ifndef MAINWINDOW_H
#define MAINWINDOW_H
#include <QMainWindow>
#include "interface.h" //без этого не работает, а надо без него
QT_BEGIN_NAMESPACE
QT_END_NAMESPACE
class MainWindow : public QMainWindow
{
    Q_OBJECT
public:
    MainWindow();
#endif // MAINWINDOW_H

#ifndef INTERFACE_H
#define INTERFACE_H
class MyModuleInterface
{
public:
    virtual ~MyModuleInterface() {}
    virtual QString run() = 0;
};
Q_DECLARE_INTERFACE(MyModuleInterface, "mywww.My.MyModuleInterface/1.0")
#endif // INTERFACE_H

#include "mainwindow.h"
#include <QtGui>

MainWindow::MainWindow()
{
QPluginLoader loader( ".../libplugin.so" );
    if ( !loader.load() )
    {
        MainWindow::setWindowTitle("This is not Qt plugin or plugin not match out Qt Libraries" );
      return;
    }
    MyModuleInterface * i = qobject_cast<MyModuleInterface*>( loader.instance() );
    if ( !i )
    {
      MainWindow::setWindowTitle( "This is not plugin for interface MyInterface" );
      return;
    }
    MainWindow::setWindowTitle( i->run());

#include <QApplication>
#include "mainwindow.h"
int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    w.show();
    return a.exec();
}

QT       += core gui
TARGET = project
TEMPLATE = app
SOURCES += main.cpp mainwindow.cpp
HEADERS  += mainwindow.h interface.h

#ifndef INTERFACE_H
#define INTERFACE_H
class MyModuleInterface
{
public:
    virtual ~MyModuleInterface() {}
    virtual QString run() = 0;
};
Q_DECLARE_INTERFACE(MyModuleInterface, "mywww.My.MyModuleInterface/1.0")
#endif // INTERFACE_H

#ifndef PLUGIN_H
#define PLUGIN_H
#include <QObject>
#include "interface.h"
class MyModulePlugin : public QObject, public MyModuleInterface
{
    Q_OBJECT
    Q_INTERFACES(MyModuleInterface)
public:
    QString run() { return QString("Ready!"); };
};
#endif // PLUGIN_H

#include <QtGui>
#include "plugin.h"
Q_EXPORT_PLUGIN2(mymoduleplugin, MyModulePlugin)

TEMPLATE = lib
CONFIG += plugin
VERSION = 1.0.0
DEFINES += D_SHARED_LIB
TARGET = plugin
HEADERS += interface.h plugin.h
SOURCES += plugin.cpp

typedef Qstring (*MyPrototype)();
     MyPrototype myFunction = (MyPrototype) QLibrary::resolve(".../libplugin.so", "run");
     if (myFunction) MainWindow::setWindowTitle(myFunction());

extern "C" MY_EXPORT int avg(int a, int b)
 {
     return (a + b) / 2;
 }

#ifdef Q_WS_WIN
 #define MY_EXPORT __declspec(dllexport)
 #else
 #define MY_EXPORT
 #endif

#ifndef PLUGIN_H
#define PLUGIN_H

#ifdef Q_WS_WIN
 #define MY_EXPORT __declspec(dllexport)
 #else
 #define MY_EXPORT
 #endif

#include <QObject>
#include <QString>
class MyModulePlugin : public QObject
{
    Q_OBJECT
public:
    extern "C" MY_EXPORT QString run() { return QString("Ready!"); };
};
#endif // PLUGIN_H

#include <QtGui>
Q_EXPORT_PLUGIN2(mymoduleplugin, MyModulePlugin)

extern "C" MY_EXPORT int run2()
{
    MyModulePlugin*myPlugin = new MyModulePlugin;
    int result = myPlugin->run();
    qDebug()<<result;//12
    return 13;               //13
}