Question

My question is , can my interface inherit from QObject and how to do that ? Well , i know that interfaces in C++ are simply classes that contains only virtual methods , and normally a class can inherit from a superclass. But if i do so i get an error due to ambiguous QObject references. I need to inherit QObject to add signals /slot feature to my plugins.

My interface

#ifndef LABELINTERFACE_H
#define LABELINTERFACE_H

#include <QLabel>
#include <QObject>

class LabelInterface : public QObject {

public :

    virtual ~LabelInterface() {}
    virtual QLabel* newLabel() = 0;

public slots:
    virtual void setLabelText() = 0;
};

Q_DECLARE_INTERFACE (LabelInterface,"com.stefan.Plugin.LabelInterface/1.0")

#endif // LABELINTERFACE_H

Plugin header file

#ifndef LABELPLUGIN_H
#define LABELPLUGIN_H

#include "labelinterface.h"

class LabelPlugin : public LabelInterface

{
    Q_OBJECT
    Q_INTERFACES(LabelInterface)

public:

    QLabel* label;
    QLabel* newLabel();
     LabelPlugin() {}
    ~LabelPlugin() {}

public slots:
    void setTextForLabel();
};

#endif // LABELPLUGIN_H

Implementation file

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

QLabel* LabelPlugin::newLabel() {

    label = new QLabel("This plugin works");

    return label;
}

void LabelPlugin::setTextForLabel() {

    label->setText("This plugin works fine");

}

// Exporta plugin-ul
Q_EXPORT_PLUGIN2 (labelplugin,LabelPlugin)

I get error

labelplugin.cpp:18: error: cannot allocate an object of abstract type ‘LabelPlugin’

Answer 1

You forgot to implement

virtual void setLabelText() = 0;

You implemented

void setTextForLabel();

was that a typo? To instantiate a class you need to override and implement all pure virtual methods in the base class. Since you’re not doing that, you class remains abstract.