Cocos2d-x is good for C++ multi-platform games. It's not for a novice though. The strength is the same code runs on almost all device platforms. While Unity is very popular, it is not free to use like Cocos2d-x. Cocos2d-x is a great 2D game engine to work with and it's well maintained. If you are a single programmer, who has no money, but does have some real world programming experience, it is the best option around. If you have a little money and no programming experience, GameSalad and BuildBox are drag and drop game designers. The support multiple platforms also.