QPluginLoader doesn't load plugin

[BeS]

Hello,

i try to load a plugin into my application. I have followed the "Text Area" example from the book "C++ GUI Programming with Qt4" but my application refuses to load the plugin.

That's how i have defined the plugin interface:

#ifndef INTERFACE_H
#define INTERFACE_H
 
#include <QtPlugin>
 
class PluginInterface
{
public:
    virtual ~PluginInterface() { }
 
    virtual QString pluginName();
};
 
Q_DECLARE_INTERFACE(PluginInterface, "cops.demo.PluginInterface/1.0")
 
#endif

#ifndef INTERFACE_H
#define INTERFACE_H

#include <QtPlugin>

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

    virtual QString pluginName();
};

Q_DECLARE_INTERFACE(PluginInterface, "cops.demo.PluginInterface/1.0")

#endif

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Here is the code of the plugin:

header-file:

#ifndef MYPLUGIN_H
#define MYPLUGIN_H
 
#include <QString>
 
#include "myPlugin.h"
#include "../pluginInterface.h"
 
 
class MyPlugin : public QObject, public PluginInterface
{
 
    Q_OBJECT
    Q_INTERFACES(PluginInterface)
 
public:
    QString pluginName();
 
};
 
#endif

#ifndef MYPLUGIN_H
#define MYPLUGIN_H

#include <QString>

#include "myPlugin.h"
#include "../pluginInterface.h"


class MyPlugin : public QObject, public PluginInterface
{
    
    Q_OBJECT
    Q_INTERFACES(PluginInterface)
            
public:
    QString pluginName();
  
};

#endif

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and source file:

#include <QtGui>
#include <iostream>
 
#include "myPlugin.h"
 
QString MyPlugin::pluginName()
{
    return "My Plugin Name";
}
 
Q_EXPORT_PLUGIN2(myplugin, MyPlugin)

#include <QtGui>
#include <iostream>

#include "myPlugin.h"

QString MyPlugin::pluginName()
{
    return "My Plugin Name";
}

Q_EXPORT_PLUGIN2(myplugin, MyPlugin)

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I can compile the plugin and get a libmyPlugin.so in my plugin directory.

Here you can see how i try to load the plugin in my application:

void MainWindow::loadPlugins()
{
    QDir pluginDir(QApplication::applicationDirPath());
 
    if (!pluginDir.cd("plugins"))
        return;
 
    foreach (QString fileName, pluginDir.entryList(QDir::Files)) {
        std::cout << qPrintable(fileName)  << "\n";
        QPluginLoader loader(pluginDir.absoluteFilePath(fileName));
        if (PluginInterface *interface = qobject_cast<PluginInterface *>(loader.instance())) {
            interfaces.append(interface);
            std::cout << qPrintable(interface->pluginName()) << "loaded \n";
        } else {
            std::cout << "not loaded!\n";
        }
    }
}

void MainWindow::loadPlugins()
{
    QDir pluginDir(QApplication::applicationDirPath());
    
    if (!pluginDir.cd("plugins"))
        return;

    foreach (QString fileName, pluginDir.entryList(QDir::Files)) {
        std::cout << qPrintable(fileName)  << "\n";
        QPluginLoader loader(pluginDir.absoluteFilePath(fileName));
        if (PluginInterface *interface = qobject_cast<PluginInterface *>(loader.instance())) {
            interfaces.append(interface);
            std::cout << qPrintable(interface->pluginName()) << "loaded \n";
        } else {
            std::cout << "not loaded!\n";
        }
    }
}

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The program finds the plugin ("std::cout << qPrintable(fileName) << "\n";" shows the filename of the plugin) but the if statement is always false so that i end in the else-arm.

Do you have an idea what's wrong here?

Thanks a lot!

[wysota]

What does QPluginLoader::isLoaded() return? What does QPluginLoader::errorString() return?

[BeS]

QPluginLoader::isLoaded() returns false and QPluginLoader::errorString() says:

QLibrary::load_sys: Cannot load /home/foo/src/demo/plugins/libmyPlugin.so (/home/foo/src/demo/plugins/libmyPlugin.so: undefined symbol: _ZTI15PluginInterface)

[wysota]

The method in the interface should be pure virtual (=0) so that you don't have to have a constructor.

[jacek]

That symbol has something to do with RTTI. Did you compile the plugin with the same options as Qt? Also add = 0 after pluginName() in PluginInterface.

[BeS]

Thank you! The missing "= 0" was the problem, now it works.

Maybe a dump question but why do i need "= 0" at the end of the method?

[jacek]

Maybe a dump question but why do i need "= 0" at the end of the method?

"= 0" means that the method is abstract. I.e. it has no implementation in this class and that this implementation will be provided by derived classes.

Without = 0, if you won't provide some implementation of that method, you will get linker error when you try to use it.