Made couple of functions pure virtual in backend.h.

src/backends/null/nullbackend.cpp

@@ -57,5 +57,32 @@ Cantor::Backend::Capabilities NullBackend::capabilities() const
     return Cantor::Backend::Nothing;
 }
 
+QWidget* NullBackend::settingsWidget(QWidget* parent) const
+{
+    Q_UNUSED(parent)
+    return nullptr;
+}
+
+KConfigSkeleton* NullBackend::config() const
+{
+    return nullptr;
+}
+
+QUrl NullBackend::helpUrl() const
+{
+    return QUrl();
+}
+
+QString NullBackend::version() const
+{
+    return QLatin1String();
+}
+
+bool NullBackend::requirementsFullfilled(QString* const reason) const
+{
+    Q_UNUSED(reason);
+    return true;
+}
+
 K_PLUGIN_FACTORY_WITH_JSON(nullbackend, "nullbackend.json", registerPlugin<NullBackend>();)
 #include "nullbackend.moc"

src/backends/null/nullbackend.h

@@ -34,7 +34,11 @@ class NullBackend : public Cantor::Backend
 
     Cantor::Session *createSession() override;
     Cantor::Backend::Capabilities capabilities() const override;
-
+    QWidget* settingsWidget(QWidget* parent) const override;
+    KConfigSkeleton* config() const override;
+    bool requirementsFullfilled(QString* const reason) const override;
+    QUrl helpUrl() const override;
+    QString version() const override;
 };
 
 

src/backends/python2/python2backend.cpp

@@ -86,5 +86,11 @@ KConfigSkeleton* Python2Backend::config() const
     return PythonSettings::self();
 }
 
+bool Python2Backend::requirementsFullfilled(QString* const reason) const
+{
+    const QString& path = QStandardPaths::findExecutable(QLatin1String("cantor_python2server"));
+    return Cantor::Backend::checkExecutable(QLatin1String("Cantor Python2 Server"), path, reason);
+}
+
 K_PLUGIN_FACTORY_WITH_JSON(python2backend, "python2backend.json", registerPlugin<Python2Backend>();)
 #include "python2backend.moc"

src/backends/python2/python2backend.h

@@ -36,7 +36,7 @@ class Python2Backend : public PythonBackend
     Cantor::Backend::Capabilities capabilities() const override;
     QUrl helpUrl() const override;
     QString description() const override;
-
+    bool requirementsFullfilled(QString* const reason = nullptr) const override;
     KConfigSkeleton* config() const override;
 };
 

src/backends/python3/python3backend.cpp

@@ -82,5 +82,11 @@ KConfigSkeleton* Python3Backend::config() const
     return PythonSettings::self();
 }
 
+bool Python3Backend::requirementsFullfilled(QString* const reason) const
+{
+    const QString& path = QStandardPaths::findExecutable(QLatin1String("cantor_python3server"));
+    return Cantor::Backend::checkExecutable(QLatin1String("Cantor Python3 Server"), path, reason);
+}
+
 K_PLUGIN_FACTORY_WITH_JSON(python3backend, "python3backend.json", registerPlugin<Python3Backend>();)
 #include "python3backend.moc"

src/backends/python3/python3backend.h

@@ -35,7 +35,7 @@ class Python3Backend : public PythonBackend
     Cantor::Backend::Capabilities capabilities() const override;
     QUrl helpUrl() const override;
     QString description() const override;
-
+    bool requirementsFullfilled(QString* const reason = nullptr) const override;
     KConfigSkeleton* config() const override;
 };
 

src/lib/backend.cpp

@@ -71,21 +71,11 @@ QString Backend::icon() const
     return d->icon;
 }
 
-QString Backend::version() const
-{
-    return QLatin1String();
-}
-
 QString Backend::url() const
 {
     return d->url;
 }
 
-QUrl Backend::helpUrl() const
-{
-    return QUrl();
-}
-
 bool Backend::isEnabled() const
 {
     return d->enabled && requirementsFullfilled();
@@ -168,17 +158,6 @@ Backend* Backend::getBackend(const QString& name)
     return nullptr;
 }
 
-QWidget* Backend::settingsWidget(QWidget* parent) const
-{
-    Q_UNUSED(parent)
-    return nullptr;
-}
-
-KConfigSkeleton* Backend::config() const
-{
-    return nullptr;
-}
-
 QStringList Backend::extensions() const
 {
     QList<Extension*> extensions = findChildren<Extension*>(QRegularExpression(QLatin1String(".*Extension")));
@@ -193,12 +172,6 @@ Extension* Backend::extension(const QString& name) const
     return findChild<Extension*>(name);
 }
 
-bool Backend::requirementsFullfilled(QString* const reason) const
-{
-    Q_UNUSED(reason);
-    return true;
-}
-
 bool Backend::checkExecutable(const QString& name, const QString& path, QString* reason)
 {
     if (path.isEmpty())

src/lib/backend.h

@@ -83,6 +83,7 @@ class CANTOR_EXPORT Backend : public QObject, public KXMLGUIClient
      * Destructor. Doesn't anything.
      */
     ~Backend() override;
+
   public:
 
     /**
@@ -103,7 +104,7 @@ class CANTOR_EXPORT Backend : public QObject, public KXMLGUIClient
      * @return @c false some requirements are missing. e.g. the maxima executable can not be found
      * @see Capability
     */
-    virtual bool requirementsFullfilled(QString* const reason = nullptr) const;
+    virtual bool requirementsFullfilled(QString* const reason = nullptr) const = 0;
 
     /**
      * Returns a unique string to identify this backend.
@@ -116,7 +117,7 @@ class CANTOR_EXPORT Backend : public QObject, public KXMLGUIClient
      * Returns the recommended version of the backend supported by Cantor
      * @return the recommended version of the backend
      */
-    virtual QString version() const;
+    virtual QString version() const = 0;
 
     //Stuff extracted from the .desktop file
     /**
@@ -148,7 +149,7 @@ class CANTOR_EXPORT Backend : public QObject, public KXMLGUIClient
      *   "http://docs.kde.org/stable/en/kdeedu/kalgebra/");
      * @return Url of the help
      */
-    virtual QUrl helpUrl() const;
+    virtual QUrl helpUrl() const = 0;
     /**
      * Returns if the backend should be enabled (shown in the Backend dialog)
      * @return @c true, if the enabled flag is set to true, and the requirements are fulfilled
@@ -172,13 +173,13 @@ class CANTOR_EXPORT Backend : public QObject, public KXMLGUIClient
      * Returns a Widget for configuring this backend
      * @return Widget for usage in the Settings dialog
      */
-    virtual QWidget* settingsWidget(QWidget* parent) const;
+    virtual QWidget* settingsWidget(QWidget* parent) const = 0;
     /**
      * Returns a KConfig object, containing all the settings,
      * the backend might need
      * @return a KConfigSkeleton object, for configuring this backend
      */
-    virtual KConfigSkeleton* config() const;
+    virtual KConfigSkeleton* config() const = 0;
     /**
      * Returns a list of the names of all the Extensions supported by this backend
      * @return a list of the names of all the Extensions supported by this backend
@@ -190,7 +191,7 @@ class CANTOR_EXPORT Backend : public QObject, public KXMLGUIClient
      * if the Backend doesn't have an extension with this name.
      * @return Pointer to the Extension object with the given name
      */
-    Extension * extension(const QString& name) const;
+    Extension* extension(const QString& name) const;
 
     /**
      * Returns a list of the names of all the installed and enabled backends
@@ -215,6 +216,7 @@ class CANTOR_EXPORT Backend : public QObject, public KXMLGUIClient
      * In case the requrements are not fullfilled, the reason is written to @c reason.
      */
     static bool checkExecutable(const QString& name, const QString& path, QString* reason);
+
   private:
     BackendPrivate* d;
 };