无法设置＆quot; setAsEdge＆rdquo;在Box2D（iphone）。简单但不明白

// Define the ground body.
        b2BodyDef groundBodyDef;
        groundBodyDef.position.Set(0,0); // bottom-left corner

        // Call the body factory which allocates memory for the ground body
        // from a pool and creates the ground box shape (also from a pool).
        // The body is also added to the world.
        b2Body* groundBody = world->CreateBody(&groundBodyDef);

        // Define the ground box shape.
        b2PolygonShape groundBox;       

        // bottom
        groundBox.SetAsEdge(b2Vec2(0,0), b2Vec2(screenSize.width/PTM_RATIO,0));
        groundBody->CreateFixture(&groundBox,0);

        // top
        groundBox.SetAsEdge(b2Vec2(0,screenSize.height/PTM_RATIO), b2Vec2(screenSize.width/PTM_RATIO,screenSize.height/PTM_RATIO));
        groundBody->CreateFixture(&groundBox,0);

        // left
        groundBox.SetAsEdge(b2Vec2(0,screenSize.height/PTM_RATIO), b2Vec2(0,0));
        groundBody->CreateFixture(&groundBox,0);

        // right
        groundBox.SetAsEdge(b2Vec2(screenSize.width/PTM_RATIO,screenSize.height/PTM_RATIO), b2Vec2(screenSize.width/PTM_RATIO,0));
        groundBody->CreateFixture(&groundBox,0);