clone函数

http://blog.csdn.net/caianye/article/details/5947282

http://wenku.baidu.com/link?url=qnq7laYDYm1V8tlgrkwNZg4nl58_x9wQ0j1UlT-ews8GEvGZHsMDkJHp7StBkqVYGgbQOAMBO5jMN1_f09_-Bq4u7mbwDi2e37asxERZxGS