Plugin implementation question

[JPNaude]

Ahhh, so the plugin and widget should be seperate. I think that makes sense and it would make things a little bit easier. Thanks!

So the application can just add a new subwindow to the MDI area using something like the code below, without the main application needing to know the definition of the varMatrixReal class?

mdiArea->addSubWindow(interface->loadViewer("Real Matrix,"table_name",active_db_file));

mdiArea->addSubWindow(interface->loadViewer("Real Matrix,"table_name",active_db_file));

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[JPNaude]

Reading through my last post again, that is not going to work... Is it?

[wysota]

It should work provided that the plugin never returns a null pointer from loadViewer and interface contains an instance of the plugin.

[JPNaude]

I've tried your suggestions and I am trying to understand why it would work. I've separated the plugin and the QWidget now and my loadViewer function in the plugin (matrixViewers) returns a matrixReal instance now as shown below:

matrixReal* matrixViewers::loadViewer(const QString &view_type, QString dbTable, QString dbFile) {
	if (view_type == "Real") {
		matrixReal* newViewer = new matrixReal;
		newViewer->setupVUI(dbTable,dbFile);
		return newViewer;
	} else {
		// Create a default window here which says the plugin failed to load
	}
}

matrixReal* matrixViewers::loadViewer(const QString &view_type, QString dbTable, QString dbFile) {
	if (view_type == "Real") {
		matrixReal* newViewer = new matrixReal;
		newViewer->setupVUI(dbTable,dbFile);
		return newViewer;
	} else {
		// Create a default window here which says the plugin failed to load
	}
}

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This makes sense but the loadViewer function is a virtual function in the interface base class. Now the question: How is the interface base class going to know anything about the matrixReal class? Isn't the main program using this interface without knowledge of the classes inside the plugin (matrixReal for example)?

The viewerInterface is shown below:

class viewerInterface {
public:
	virtual ~viewerInterface() { }
	virtual QStringList views() const = 0;
	virtual matrixReal* loadViewer(const QString &view_type, QString dbTable, QString dbFile) = 0;
};
Q_DECLARE_INTERFACE(viewerInterface,"com.software_name.app-name.viewerInterface/0.1")

class viewerInterface {
public:
	virtual ~viewerInterface() { }
	virtual QStringList views() const = 0;
	virtual matrixReal* loadViewer(const QString &view_type, QString dbTable, QString dbFile) = 0;
};
Q_DECLARE_INTERFACE(viewerInterface,"com.software_name.app-name.viewerInterface/0.1")

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Is it possible to do it in this way?

[wysota]

How is the interface base class going to know anything about the matrixReal class?

Why should it know anything? It knows that it implements the interface and this should be enough.

Isn't the main program using this interface without knowledge of the classes inside the plugin (matrixReal for example)?

Yes, that's correct.

The viewerInterface is shown below:

Is it possible to do it in this way?

If your loadViewer() returns only subclasses of matrixReal objects than yes.

[JPNaude]

Thanks again for the reply. My problem is that the interface class is used when compiling the plugin library as well as compiling the main application. In both the return type of loadViewer() need to be specified. For the plugin it makes sense to use the return type of matrixReal as shown in my previous posts.

However for the main program, it can't use matrixReal because it does not know anything about it. This is what I don't understand... Should I use QWidget instead in the interface used to build the main program? I would think that the interface classes should be exactly the same.

[wysota]

However for the main program, it can't use matrixReal because it does not know anything about it.

So the interface should return a QWidget pointer.

class Iface {
public:
  QWidget *getWidget() = 0;
};
 
class Implementation : public QObject, Iface {
  Q_OBJECT
public:
  QWidget *getWidget() { return new matrixReal; }
};

class Iface {
public:
  QWidget *getWidget() = 0;
};

class Implementation : public QObject, Iface {
  Q_OBJECT
public:
  QWidget *getWidget() { return new matrixReal; }
};

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[JPNaude]

Ok that helps, thanks!. I've tried this before but it did not work so I thought there was an error on my side. So I assume when you return a matrixReal pointer to a QWidget pointer it does some sort of reinterpret_cast<QWidget *> automatically.

I'm still not 100% sure why the main program can use the QWidget pointer as a matrixReal pointer without doing a qobject_cast<matrixReal*> on it. However I believe you.

For the moment, I am stuck where the plugin dll return a 0x1 pointer from the loadViewer() function and this pointer is useless since it bombs the program when I use it. So, I'm not sure if this is due to the way I've done my plugin or due to something else in the main program where I use the pointer.

[wysota]

've tried this before but it did not work so I thought there was an error on my side.

Maybe you forgot to include matrixreal.h.

So I assume when you return a matrixReal pointer to a QWidget pointer it does some sort of reinterpret_cast<QWidget *> automatically.

Not really. As matrixReal inherits QWidget, you can always cast to the base class, so this cast is purely implicit and based on C++ rules.

I'm still not 100% sure why the main program can use the QWidget pointer as a matrixReal pointer without doing a qobject_cast<matrixReal*> on it. However I believe you.

This is one of C++ (or rather OO-programming) paradigms.

For the moment, I am stuck where the plugin dll return a 0x1 pointer from the loadViewer() function and this pointer is useless since it bombs the program when I use it. So, I'm not sure if this is due to the way I've done my plugin or due to something else in the main program where I use the pointer.

0x1 is... 1. So you must be returning an improper object (not allocated with new).

[JPNaude]

Thanks alot for the help! I've checked and my return pointer is initialized correctly, so something else is wrong. I would need to debug it but ran out of time now. The only trace I can find at the moment is a bunch of disassembly messages like this:

0x7c90eaf0 <ntdll!LdrDisableThreadCalloutsForDll+4>: mov (%esp),%ebx

I've built both the library and the main app in debug mode. I'm calling the interface like this:

foreach (viewerInterface *interface, viewerInterfaces) {
         		QWidget * viewer = new QWidget(this);
         		viewer = interface->loadViewer("Real",dropEvent->mimeData()->text(),active_db_file);
         		try {
         			viewer->show();
         		}

foreach (viewerInterface *interface, viewerInterfaces) {
         		QWidget * viewer = new QWidget(this);
         		viewer = interface->loadViewer("Real",dropEvent->mimeData()->text(),active_db_file);
         		try {
         			viewer->show();
         		}

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And my return function in the library looks something like this, just to test it:

QWidget* matrixViewers::loadViewer(const QString &view_type, QString dbTable, QString dbFile) {
	/*if (view_type == "Real") {
		matrixReal* newViewer = new matrixReal(0);
		newViewer->setupVUI(dbTable,dbFile);
		return newViewer;
	} else {*/
		// Create a default window here which says the plugin failed to load
		QLabel *label = new QLabel(tr("Viewer loading failed..."));
		QWidget *failWindow = new QWidget;
		QHBoxLayout *topLeftLayout = new QHBoxLayout;
		topLeftLayout->addWidget(label);
		failWindow->setLayout(topLeftLayout);
		failWindow->show();
		return failWindow;
	//}
}

QWidget* matrixViewers::loadViewer(const QString &view_type, QString dbTable, QString dbFile) {
	/*if (view_type == "Real") {
		matrixReal* newViewer = new matrixReal(0);
		newViewer->setupVUI(dbTable,dbFile);
		return newViewer;
	} else {*/
		// Create a default window here which says the plugin failed to load
		QLabel *label = new QLabel(tr("Viewer loading failed..."));
		QWidget *failWindow = new QWidget;
		QHBoxLayout *topLeftLayout = new QHBoxLayout;
		topLeftLayout->addWidget(label);
		failWindow->setLayout(topLeftLayout);
		failWindow->show();
		return failWindow;
	//}
}

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My knowledge of library usage is limited so I need to spend some time on it probably, if there is nothing obvious wrong with the above...

Thanks again...

[wysota]

Instead of returning a "fail widget", return 0 and let the main application handle the fails.