make cost() method pure virtual

Author: alexbruy

python/analysis/network/qgsstrategy.sip

@@ -46,5 +46,5 @@ class QgsStrategy
     /**
      * Return edge cost
      */
-    virtual QVariant cost( double distance, const QgsFeature &f ) const;
+    virtual QVariant cost( double distance, const QgsFeature &f ) const = 0;
 };

src/analysis/network/qgsstrategy.h

@@ -51,12 +51,7 @@ class ANALYSIS_EXPORT QgsStrategy
     /**
      * Return edge cost
      */
-    virtual QVariant cost( double distance, const QgsFeature &f ) const
-    {
-      Q_UNUSED( distance );
-      Q_UNUSED( f );
-      return QVariant();
-    }
+    virtual QVariant cost( double distance, const QgsFeature &f ) const = 0;
 };
 
 #endif // QGSSTRATERGY_H