问题

什么时候需要使用asio_handler_invoke来实现仅通过包装处理程序无法完成的操作？

一个规范的示例说明需要asio_handler_invoke的情况将是理想的。

背景

boost asio文档包含一个如何使用asio_handler_invoke here的示例，但是我认为这不是为什么要使用调用处理程序的引人注目的示例。在该示例中，您似乎可以进行如下更改(并删除asio_handler_invoke)并获得相同的结果:

template <typename Arg1>
void operator()(Arg1 arg1)
{
  queue_.add(priority_, std::bind(handler_, arg1));
}

最佳答案

简而言之，包装处理程序和asio_handler_invoke可完成两项不同的任务:

template <typename Handler>
struct custom_handler
{
  void operator()(...); // Customize invocation of handler_.
  Handler handler_;
};

// Customize invocation of Function within context of custom_handler.
template <typename Function>
void asio_handler_invoke(Function function, custom_handler* context);

// Invoke custom invocation of 'perform' within context of custom_handler.
void perform() {...}
custom_handler handler;
using boost::asio::asio_handler_invoke;
asio_handler_invoke(std::bind(&perform), &handler);

When asynchronous operations are composed from other asynchronous operations, all intermediate handlers should be invoked using the same method as the final handler. This is required to ensure that user-defined objects are not accessed in a way that may violate the guarantees. This [asio_handler_invoke] hooking function ensures that the invoked method used for the final handler is accessible at each intermediate step.

template <typename Handler>
class counting_handler
{
  void operator()(...)
  {
    // invoke handler
  } 
  Handler handler_;
};

template <typename Function>
void asio_handler_invoke(Function function, counting_handler* context)
{
  // increment counter
  // invoke function
}

counting_handler handler(&handle_read);
boost::asio::async_read(socket, buffer, handler);

namespace detail {

/// @brief counting_handler is a handler that counts the number of
///        times a handler is invoked within its context.
template <class Handler>
class counting_handler
{
public:
  counting_handler(Handler handler, std::size_t& count)
    : handler_(handler),
      count_(count)
  {}

  template <class... Args>
  void operator()(Args&&... args)
  {
    handler_(std::forward<Args>(args)...);
  }

  template <typename Function>
  friend void asio_handler_invoke(
    Function intermediate_handler,
    counting_handler* my_handler)
  {
    ++my_handler->count_;
    // Support chaining custom strategies incase the wrapped handler
    // has a custom strategy of its own.
    using boost::asio::asio_handler_invoke;
    asio_handler_invoke(intermediate_handler, &my_handler->handler_);
  }

private:
  Handler handler_;
  std::size_t& count_;
};

} // namespace detail

/// @brief Auxiliary class used to wrap handlers that will count
///        the number of functions invoked in their context.
class operation_counter
{
public:

  template <class Handler>
  detail::counting_handler<Handler> wrap(Handler handler)
  {
    return detail::counting_handler<Handler>(handler, count_);
  }

  std::size_t count() { return count_; }

private:
  std::size_t count_ = 0;
};

...

operation_counter counter;
boost::asio::async_read(socket, buffer, counter.wrap(&handle_read));
io_service.run();
std::cout << "Count of async_read_some operations: " <<
             counter.count() << std::endl;

template <class Handler>
class counting_handler
{
public:
  ...

  template <class... Args>
  void operator()(Args&&... args)
  {
    ++count_;
    handler_(std::forward<Args>(args)...);
  }

  // No asio_handler_invoke implemented.
};

#include <functional> // std::bind
#include <iostream>   // std::cout, std::endl
#include <utility>    // std::forward
#include <boost/asio.hpp>

// This example is not interested in the handlers, so provide a noop function
// that will be passed to bind to meet the handler concept requirements.
void noop() {}

namespace detail {

/// @brief counting_handler is a handler that counts the number of
///        times a handler is invoked within its context.
template <class Handler>
class counting_handler
{
public:
  counting_handler(Handler handler, std::size_t& count)
    : handler_(handler),
      count_(count)
  {}

  template <class... Args>
  void operator()(Args&&... args)
  {
    handler_(std::forward<Args>(args)...);
  }

  template <typename Function>
  friend void asio_handler_invoke(
    Function function,
    counting_handler* context)
  {
    ++context->count_;
    // Support chaining custom strategies incase the wrapped handler
    // has a custom strategy of its own.
    using boost::asio::asio_handler_invoke;
    asio_handler_invoke(function, &context->handler_);
  }

private:
  Handler handler_;
  std::size_t& count_;
};

} // namespace detail

/// @brief Auxiliary class used to wrap handlers that will count
///        the number of functions invoked in their context.
class operation_counter
{
public:

  template <class Handler>
  detail::counting_handler<Handler> wrap(Handler handler)
  {
    return detail::counting_handler<Handler>(handler, count_);
  }

  std::size_t count() { return count_; }

private:
  std::size_t count_ = 0;
};

int main()
{
  using boost::asio::ip::tcp;
  operation_counter all_operations;

  // Create all I/O objects.
  boost::asio::io_service io_service;
  tcp::acceptor acceptor(io_service, tcp::endpoint(tcp::v4(), 0));
  tcp::socket socket1(io_service);
  tcp::socket socket2(io_service);

  // Connect the sockets.
  // operation 1: acceptor.async_accept
  acceptor.async_accept(socket1, all_operations.wrap(std::bind(&noop)));
  // operation 2: socket2.async_connect
  socket2.async_connect(acceptor.local_endpoint(),
      all_operations.wrap(std::bind(&noop)));
  io_service.run();
  io_service.reset();

  // socket1 and socket2 are connected.  The scenario below will:
  // - write bytes to socket1.
  // - initiate a composed async_read operaiton to read more bytes
  //   than are currently available on socket2.  This will cause
  //   the operation to  complete with multple async_read_some 
  //   operations on socket2.
  // - write more bytes to socket1.

  // Write to socket1.
  std::string write_buffer = "demo";
  boost::asio::write(socket1, boost::asio::buffer(write_buffer));

  // Guarantee socket2 has received the data.
  assert(socket2.available() == write_buffer.size());

  // Initiate a composed operation to more data than is immediately
  // available.  As some data is available, an intermediate async_read_some
  // operation (operation 3) will be executed, and another async_read_some 
  // operation (operation 4) will eventually be initiated.
  std::vector<char> read_buffer(socket2.available() + 1);
  operation_counter read_only;
  boost::asio::async_read(socket2, boost::asio::buffer(read_buffer),
    all_operations.wrap(read_only.wrap(std::bind(&noop))));

  // Write more to socket1.  This will cause the async_read operation
  // to be complete.
  boost::asio::write(socket1, boost::asio::buffer(write_buffer));

  io_service.run();
  std::cout << "total operations: " << all_operations.count() << "\n"
               "read operations: " << read_only.count() << std::endl;
}

total operations: 4
read operations: 2

boost::asio::async_read(..., all_operations.wrap(read_only.wrap(...)));

template <typename Function>
void asio_handler_invoke(
  Function function,
  counting_handler* context)
{
  ++context->count_;
  // Support chaining custom strategies incase the wrapped handler
  // has a custom strategy of its own.
  using boost::asio::asio_handler_invoke;
  asio_handler_invoke(function, &context->handler_);
}

template <typename Function>
void asio_handler_invoke(
  Function function,
  counting_handler* context)
{
  ++context->count_;
  function(); // No chaining.
}

// Operations will not be counted.
boost::asio::async_read(..., std::bind(all_operations.wrap(...)));    

boost::asio::async_read(socket, ..., std::bind(strand.wrap(&handle_read)));
boost::asio::async_write(socket, ..., std::bind(strand.wrap(&handle_write)));

关于c++ - 何时使用 `asio_handler_invoke`？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/32857101/