Multiple instance of Plugin object

[mcosta]

Hi,

can I instantiate multiple object of a plugin class?
I show you a prototype of code I'd like to use

QPluginLoader loader (pluginDir.absoluteFilePath (fileName));
 
QObject *instance = loader.instance ();
QObject *newObj = 0;
QMetaObject *mo = 0;
if (instance) {
    IExample *plugin = qobject_cast<IExample*>(instance);
    if (plugin) {
        qDebug() << instance->metaObject ()->className () << "plugin says:"
                        << plugin->messageString ();
 
        newObj = instance->metaObject ()->newInstance (0);
        if (0 != newObj)
            qDebug () << "Create New Instance OK";
        else
            qDebug () << "Create New Instance Failed";
    }
}

QPluginLoader loader (pluginDir.absoluteFilePath (fileName));

QObject *instance = loader.instance ();
QObject *newObj = 0;
QMetaObject *mo = 0;
if (instance) {
    IExample *plugin = qobject_cast<IExample*>(instance);
    if (plugin) {
        qDebug() << instance->metaObject ()->className () << "plugin says:"
                        << plugin->messageString ();

        newObj = instance->metaObject ()->newInstance (0);
        if (0 != newObj)
            qDebug () << "Create New Instance OK";
        else
            qDebug () << "Create New Instance Failed";
    }
}

To copy to clipboard, switch view to plain text mode 

where IExample is defined

class IExample
{
public:
    virtual ~IExample () {}
 
    virtual QString messageString () const = 0;
};
 
Q_DECLARE_INTERFACE (IExample, "local.costa.IExample/1.0")

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

    virtual QString messageString () const = 0;
};

Q_DECLARE_INTERFACE (IExample, "local.costa.IExample/1.0")

To copy to clipboard, switch view to plain text mode 

and this is the loaded plugin

class HelloWorldPlugin: public QObject, IExample
{
    Q_OBJECT
    Q_INTERFACES (IExample)
 
public:
    HelloWorldPlugin(QObject *parent = 0);
 
    QString messageString () const;
};

class HelloWorldPlugin: public QObject, IExample
{
    Q_OBJECT
    Q_INTERFACES (IExample)

public:
    HelloWorldPlugin(QObject *parent = 0);

    QString messageString () const;
};

To copy to clipboard, switch view to plain text mode 

HelloWorldPlugin::HelloWorldPlugin(QObject *parent)
    : QObject (parent)
{
}
 
QString HelloWorldPlugin::messageString() const
{
    return QString("Hello World");
}
 
Q_EXPORT_PLUGIN2 (HelloWorldPlugin, HelloWorldPlugin)

HelloWorldPlugin::HelloWorldPlugin(QObject *parent)
    : QObject (parent)
{
}

QString HelloWorldPlugin::messageString() const
{
    return QString("Hello World");
}

Q_EXPORT_PLUGIN2 (HelloWorldPlugin, HelloWorldPlugin)

To copy to clipboard, switch view to plain text mode 

the output is

HelloWorldPlugin plugin says: "Hello World"
Create New Instance Failed
Found 1 entries

HelloWorldPlugin plugin says: "Hello World"
Create New Instance Failed
Found 1 entries

To copy to clipboard, switch view to plain text mode 

[joyer83]

It says in Qt's documentation at QMetaObject::newInstance() this:

Note that only constructors that are declared with the Q_INVOKABLE modifier are made available through the meta-object system.

I don't see this Q_INVOKABLE macro used in your source code. Try adding it and see what happens.

[wysota]

Usually you make the plugin a factory that can return instances of interfaces you need:

class MyPluginIface : ... {
 // ...
public:
  virtual MyRealIface * create() = 0;
}

class MyPluginIface : ... {
 // ...
public:
  virtual MyRealIface * create() = 0;
}

To copy to clipboard, switch view to plain text mode 

Then you can create as many instances of MyRealIface as you want.

[mcosta]

Thank you for the answer.

Your solution is more elegant than than mine.
I was courious about QMetaObject::newInstance().

I solved using joyer83 tips but for a real solution I'll use the factory.