title: Undefined symbols "vtable for ..." and "typeinfo for..."?
body: <p>Nearly the final step but still some strange erros....</p>
  
  <pre><code>bash-3.2$ make
  g++ -Wall -c -g Myworld.cc
  g++ -Wall -g solvePlanningProblem.o Position.o AStarNode.o PRM.o PRMNode.o World.o SingleCircleWorld.o Myworld.o RECTANGLE.o CIRCLE.o -o solvePlanningProblem
  Undefined symbols:
    "vtable for Obstacle", referenced from:
        Obstacle::Obstacle()in Myworld.o
    "typeinfo for Obstacle", referenced from:
        typeinfo for RECTANGLEin RECTANGLE.o
        typeinfo for CIRCLEin CIRCLE.o
  ld: symbol(s) not found
  collect2: ld returned 1 exit status
  make: *** [solvePlanningProblem] Error 1
  </code></pre>
  
  <p>What's the meaning of vtable and typeinfo?</p>