This is a virtual function that the inheriting class does not implement:

The problem function call is a pure virtual that QoccSnap doesn't implement the inheriting class does:

bool QoccSnap::Snap(QPoint& pos, V3d_Coordinate& X, V3d_Coordinate& Y, V3d_Coordinate& Z)
{
	AIS_ListOfInteractive aList;
	myContext->DisplayedObjects( aList );
	AIS_ListIteratorOfListOfInteractive aListIterator;
	
	for ( aListIterator.Initialize( aList ); aListIterator.More(); aListIterator.Next() )
	{
		if (aListIterator.Value ()->IsKind (STANDARD_TYPE (AIS_Shape)))
		{
			Handle(AIS_Shape) aisShape = Handle(AIS_Shape)::DownCast(aListIterator.Value());
			const TopoDS_Shape& aShape = aisShape->Shape();
		
			if(aShape.ShapeType() == mySignature)
			{
----Problem is here------>	if(SnapLocation(aisShape, X, Y, Z))
				{
				
				}
			}
		}
		else
			cout << "Not a Shape" << endl;
	}
	
	return false;
}