双重锁: http://www.cs.wustl.edu/~schmidt/PDF/DC-Locking.pdf ACCU上的讨论: http://accu.org/index.php/journals/1328 http://accu.org/index.php/journals/1306 ACE与Boost中singleton的不同实现: http://www.cppblog.com/Fox/archive/2009/09/22/96898.html