是这个结构，doing为补足语。
补语分宾语和主补
宾补
适用宾补的句型：
主语+谓语+宾语+宾补（S+V+O+OC）
宾补对宾语加以解释或描述。
以下成分可以充当宾补：
名词，代词，形容词，副词，ing形式，ed分词，不定式，介词短语，名词性从句。
名词作宾补
I consider him a gentleman.
代词作宾补
Whom do you think me?
（Whom是宾补）
形容词作宾补
Wash your hands clean.
副词作宾补
I found him out.
ing形式作宾补
They left me wait.
ed分词作宾补
I heard my name called.
不定式作宾补
I thought him to be a good man.
介词短语作宾补
Make yourself at home.
名词性从句作宾补
His wife has made him what he is.
主语+动词+形容词（双谓语）
接在动词之后对主语起着解释或描述作用的成分，叫作主补。
形容词作主补
They married young.
他们结婚时还年轻。
He died happy.
他怀着幸福的心情死去。
No man is born wise.
没有生而知之。
ed分词作主补
He came in drunk.
他走进来的时候醉醺醺的。
名词作主补
He died a poor man.
他死时很穷。
He died a millionaire.
他死的时候是个百万富翁。
表语（又叫作主补）
（香港，台湾学界把表语叫作主补）
Africa is a big continent.
非洲是个大洲。
That remains a puzzle to me.
这对我还是个难题。
What’s your fax number?
你的传真号是多少？
I feel much better today.
我今天感觉好多了。
由宾补变成主补
改为被动语态之后，原来的宾补就变成了主补。
如果主动句是“主语－动词－宾语－宾补”（SVOC）的结构，变为被动句时，原来的宾语变做主语，原来的宾补变成主补。
例如：
They named the child Tom. 他们给孩子命名为汤姆。
>>> The child was named Tom. 那孩子被命名为汤姆。
They painted the wall blue. 他们把墙壁漆成蓝色。
>>> The wall was painted blue. 墙壁被漆成蓝色。
We regard him as the best dentist in town. 我们认为他是城里最好的牙医。
>>> He is regarded as the best dentist in town. 他被公认为城里最好的牙医。
We know him to be a good teacher. 我们知道他是一位好老师。
>>> He is known to be a good teacher. 众所周知他是以为好老师。
We thought the matter to have been settled. 我们以为问题已经解决。
>>> The mattered was thought to have been settled. 问题被认为已经解决。
参考资料：http://www.sinoya.com/search_1.asp