Obama was elected president of the US.
president是表示头衔的名词,在此作主语（Obama）的补足语.
（　主动语态中的宾语补足语,可以转化为被动语态中的主语补足语.
　　eg.They caught boy stealing.（stealing 作为宾语补足语）
　　转化为被动语态 The boy was caught stealing.（stealing转化为主语补足语））
The central committee appointed Bo boss of Chongqing.
Bo是宾语,boss of Chongqing是宾语补足语.
上述两例,president 和 boss都是表头衔的名词.