size_t 跟 unsigned int 不一样

size_t 和 unsigned int 不一样
在64位机器上
sizeof(size_t) = 8
sizeof(unsigned int) = 4

size_t a = 4;
boost::any b = (a);
boost::any_cast<unsigned int>(b);

会抛出 异常信息
Detail: boost::bad_any_cast: failed conversion using boost::any_cast