用endl，std::endl, "\n"都不好用，不输出换行，怎么回事？

【解决方法】

If you're writing a text file, you shouldn't be using the binary flag.

fout.open ("bodyfat.txt", ios::out | ios::app | ios::binary);

Remove the ios::binary. In text mode iostream will corectly map endl to \r\n for you.