如果我将shared_ptr’a’移动到shared_ptr’b’是’a’保证为null？
任何标准类的状态是否被移动指定？
解决方法
一般17.6.5.15/1适用：
Objects of types defined in the C++ standard library may be moved from (12.8). Move operations may be explicitly specified or implicitly generated. Unless otherwise specified,such moved-from objects shall be placed in a valid but unspecified state.
如果指定,则适用what @Xeo said.
