http://stackoverflow.com/questions/16772842/what-is-the-difference-between-cout-cerr-clog-of-iostream-header-in-c-when

cerr for errors

cout for normal output

clog for logging

The major difference is that std::cerr is not buffered like the other two. 

cout 会将输出信息先写入缓存，而cerr会将消息直接输出到console

【C++】cerr，cout，clog