为什么没有strand::wrap(）等价于strand::post(）

关于为什么strand.post的strand.wrap等价物不存在的纯粹猜测：

• 我找不到哪里有人正式提出在功能请求中需要strand.wrap()等效物的情况
• 基于strand.wrap()最常见的用法，它可能根据strand.dispatch()来实现，以优化组合操作的中间处理程序。用户的完成处理程序可以在满足完成条件后立即调用，而不必为延迟调用发布完成处理程序

最简单的解决方案可能是将链与a和b处理程序一起传递到my_great_function。如果my_great_function需要处理程序调用的特定顺序，那么让my_great_function保证顺序似乎是可以接受的，而不是将责任推给调用者，后者可能会忽略必要的顺序。另一方面，如果my_great_function是相当通用的，处理程序之间需要特定的调用顺序，那么可以考虑以直接或间接暗示排序的结构将处理程序一起传递，例如std::tuple。

虽然这两种解决方案都不提供通用的可重复使用的解决方案，但它可能是最简单的解决方案。官方支持的解决方案是使用自定义处理程序类型，此外还通过ADL提供asio_handler_invoke函数，以说明操作的中间和完成处理程序。以下是一个完整的示例，主要基于细节/wrapped_handler.hp:

#include <iostream>
#include <boost/asio.hpp>
/// @brief Custom handler wrapper type that will post into its dispatcher.
template <typename Dispatcher,
          typename Handler>
class post_handler
{
public:
  typedef void result_type;
  post_handler(Dispatcher dispatcher, Handler handler)
    : dispatcher_(dispatcher),
      handler_(handler)
  {}
  void operator()()
  {
    dispatcher_.post(handler_);
  }
  template <typename Arg1>
  void operator()(Arg1 arg1)
  {
    dispatcher_.post(boost::bind(handler_, arg1));
  }
  template <typename Arg1, typename Arg2>
  void operator()(Arg1 arg1, Arg2 arg2)
  {
    dispatcher_.post(boost::bind(handler_, arg1, arg2));
  }
  Dispatcher dispatcher_;
  Handler handler_;
};
// Custom invocation hooks for post_handler.  These must be declared in 
// post_handler's associated namespace for proper resolution.
template <typename Function, typename Dispatcher, typename Handler>
inline void asio_handler_invoke(Function& function,
    post_handler<Dispatcher, Handler>* this_handler)
{
  this_handler->dispatcher_.post(
      boost::asio::detail::rewrapped_handler<Function, Handler>(
        function, this_handler->handler_));
}
template <typename Function, typename Dispatcher, typename Handler>
inline void asio_handler_invoke(const Function& function,
    post_handler<Dispatcher, Handler>* this_handler)
{
  this_handler->dispatcher_.post(
      boost::asio::detail::rewrapped_handler<Function, Handler>(
        function, this_handler->handler_));
}
/// @brief Factory function used to create handlers that post through the
///        dispatcher.
template <typename Dispatcher, typename Handler>
post_handler<Dispatcher, Handler>
wrap_post(Dispatcher dispatcher, Handler handler)
{
  return post_handler<Dispatcher, Handler>(dispatcher, handler);
}
/// @brief Convenience factory function used to wrap handlers created from
///        strand.wrap.
template <typename Dispatcher, typename Handler>
post_handler<Dispatcher, 
             boost::asio::detail::wrapped_handler<Dispatcher, Handler> >
wrap_post(boost::asio::detail::wrapped_handler<Dispatcher, Handler> handler)
{
  return wrap_post(handler.dispatcher_, handler);
}
boost::asio::io_service io_service;
boost::asio::strand strand(io_service);
boost::asio::deadline_timer timer(io_service);
void a() { std::cout << "a" << std::endl; }
void b() { std::cout << "b" << std::endl; }
void c() { std::cout << "c" << std::endl; }
void d() { std::cout << "d" << std::endl; }
void noop() {}
void my_great_function()
{
  std::cout << "++my_great_function++" << std::endl;
  // Standard dispatch.
  strand.dispatch(&a);
  // Direct wrapping.
  wrap_post(strand, &b)();
  // Convenience wrapping.
  wrap_post(strand.wrap(&c))();
  // ADL hooks.
  timer.async_wait(wrap_post(strand.wrap(boost::bind(&d))));
  timer.cancel();
  std::cout << "--my_great_function--" << std::endl;
}
int main()
{
  // Execute my_great_function not within a strand.  The noop
  // is used to force handler invocation within strand.
  io_service.post(&my_great_function);
  strand.post(&noop);
  io_service.run();
  io_service.reset();
  // Execute my_great_function within a strand.
  std::cout << std::endl;
  io_service.post(strand.wrap(&my_great_function));
  strand.post(&noop);
  io_service.run();
}

它产生以下输出：

++my_great_function++--my_great_function--一bcd++my_great_function++一--my_great_function--bcd

取决于实现细节的一个稍微简单的解决方案是调整detail::wrapped_handler的Dispatcher类型参数。这种方法允许在Boost.Asio.的其余部分中透明地使用具有自适应Dispatcher类型的wrapped_handler

/// @brief Class used to adapter the wrapped_handler's Dispatcher type
///        requirement to post handlers instead of dispatching handlers.
template <typename Dispatcher>
struct post_adapter
{
  post_adapter(Dispatcher& dispatcher)
    : dispatcher_(dispatcher)
  {}
  template <typename Handler>
  void dispatch(const Handler& handler)
  {
    dispatcher_.post(handler);
  }
  Dispatcher dispatcher_;
};
/// @brief Factory function used to create handlers that post through an
///        adapted dispatcher.
template <typename Dispatcher, typename Handler>
boost::asio::detail::wrapped_handler<post_adapter<Dispatcher>, Handler>
wrap_post(Dispatcher& dispatcher, Handler handler)
{
  typedef post_adapter<Dispatcher> adapter_type;
  return boost::asio::detail::wrapped_handler<
    adapter_type, Handler>(adapter_type(dispatcher), handler);
}
/// @brief Convenience factory function used to wrap handlers created from
///        strand.wrap.
template <typename Dispatcher, typename Handler>
boost::asio::detail::wrapped_handler<
  post_adapter<Dispatcher>, 
  boost::asio::detail::wrapped_handler<Dispatcher, Handler> >
wrap_post(boost::asio::detail::wrapped_handler<Dispatcher, Handler> handler)
{
  return wrap_post(handler.dispatcher_, handler);
}

两种CCD_ 27解决方案都可能引入关于处理程序调用的定义顺序的复杂度。