Getting Physical – Box2D - Cocos2d-x by Example Beginner's Guide - page 289

Creating edges
If a shape can only be hit on one side, an edge is probably what you need. Here is how we
create edges in our game:
b2BodyDef tableBodyDef;
tableBodyDef.position.Set(0, 0);
b2Body* tableBody = _world->CreateBody(&tableBodyDef);
// Define the table edges
b2EdgeShape tableBox;
// bottom edge
tableBox.Set(b2Vec2(_screenSize.width * 0.14f/PTM_RATIO,
_screenSize.height * 0.09f/PTM_RATIO),
b2Vec2(_screenSize.width * 0.86f/PTM_RATIO,
_screenSize.height * 0.09f/PTM_RATIO));
tableBody->CreateFixture(&tableBox,0);
// top edge
tableBox.Set(b2Vec2(_screenSize.width * 0.14f/PTM_RATIO,
_screenSize.height * 0.91f/PTM_RATIO),
b2Vec2(_screenSize.width * 0.86f/PTM_RATIO,
_screenSize.height * 0.91f/PTM_RATIO));
tableBody->CreateFixture(&tableBox,0);
So the same b2Body object can have as many edges as you need. You set an edge with its
start and end points (in this case, the b2Vec2 structures) and add it as a fixture to the body,
with a density of 0 .