C++ Coding Standards - Prefer composition to inheritance.

Herb Sutter and Andrei Alexandrescu 的理由是：Avoid inheritance taxes: Inheritance is the second-tightest coupling relationship in C++, second only to friendship. 除了 tight-coupling 外，一個重要的原因是 OO-modeling 哲學：兒子不能超過爸爸。兒子指的是 derived class，爸爸當然是 base class.

之前寫的 BubbleBall Level One 是錯誤示範，兒子超過爸爸了。改寫後如下：

class Box2dLevelOne {
 public:
  Box2dLevelOne() { 
    physics_.initWorld(); 
    physics_.setEdgeBackground(...);
    physics_.setPlayer(...);  
    physics_.setGoal(...);
  }
  ~Box2dLevelOne() { 
    physics_.deinitWorld(); 
  }

  void setTriangle(...) {
    physics_.setTriangle(...);
  }

  bool isGoaledAfter(int seconds) {
    float time_step = 1.0f/60.0f;
    for (int i = 0; i < seconds; ++i) {
      physics_.simulate(time_step);
      if (physics_.isGoaled()) {
        return true;
      }
    }
    return false;
  }

 private:
  Box2dPhysics physics_;
};

TEST(Box2dLevelOne, StressTest) {
  const TestItem item[] = { ... };

  const int size = arraysize(item);
  for (int i = 0; i != size; ++i) {
    Box2dLevelOne level;
    level.setTriangle(...);
    EXPECT_EQ(item[i].is_goaled, level.isGoaledAfter(60 * 10));
  }
}

以上。當然我還犯其他錯誤 (過早最佳化，沒有利用 assert 來說明內部假設與不變性)，接下來會修掉這些錯誤。繼續加油吧！