如何翻译这个给定代码中的 ::* 运算符和member_fn?

它的意思正是您在第一段中描述的内容：

// cb contains a shared pointer to an instance of Handler and a member function of Hander
std::pair<boost::shared_ptr<Handler>, void (Handler::*)()> cb = job_queue.sync_pop(); 
// assign the member function stored in cb.second to a local variable    
void (Handler::*member_fn)() = cb.second; 
(cb.first.get()->*member_fn)(); // call the function stored in member_fn on 
// the object held by cb.first

如果这还不够干净，你可以在这里找到一个使用标准库类的最小示例，它做同样的事情

class Handler;
std::pair<                     // a pair consisting of first and second
boost::shared_ptr<Handler>, // first  - a shared pointer to Handler object
void (Handler::*)()>        // second - a pointer to the member function of Handler class
cb = job_queue.sync_pop();     // sync_pop() returns such pair, stores it in cb variable
// varaible "member_fn" stores pointer to a member function from the pair
void (Handler::*member_fn)() = cb.second; 
// cb.first.get()->  - dereference pointer to Handler object, say handlerObj
// *member_fn        - dereference the pointer to member function
// (cb.first.get()->.*member_fn())()  - invoke the function stored in member_fn variable
//                                      on handlerObj (just like handlerObj.member_fn();)
(cb.first.get()->*member_fn)();

这将更具可读性：

auto pair = job_queue.sync_pop();
auto function = std::bind(pair.second, pair.first.get());
function();