学步园

boost::polymorphic_cast
boost::polymorphic_downcast

shared_ptr支持的转型有：static_pointer_cast<T>、const_pointer_cast<T>、dynamic_pointer_cast<T>，返回的结果是shared_ptr，并能保证这些指针的引用计数正确

boost::lexical_cast
eg: int a = boost::lexical_cast<int>("123");