c++ - 为什么 Boost.Asio 处理程序必须是可复制构造的？

Question

根据 http://www.boost.org/doc/libs/1_61_0/doc/html/boost_asio/reference/Handler.html ，提供给 io_service::post 的处理程序必须是可复制构造的。

但是，这不包括接受套接字并 move 响应处理程序的情况，向我保证只有一个处理程序可以处理该作业:

auto socket = std::make_unique<socket>();
accepter.accept(*socket);

service.post([s{std::move(socket)}] {
  asio::write(*s, buffer("response"), ignored_err);
});

那么为什么这个复制构造要求？

-- 编辑--

澄清:我想让我的代码确保只有一个处理程序实例存在。这使得对其进行推理变得容易得多。

对于很多程序来说，CopyConstructible要求太严格，MoveConstructible更合适。

    auto i = std::make_unique<int>();
    auto handler_w_resource = [i{std::move(i)}]{ ++(*i);};
    // auto copied = handler_w_resource; --> +1: lambda can't be copied!
    auto moved = std::move(handler_w_resource);

因此我很惊讶它不能搬进来:

    service.post(std::move(moved)); // :( post accepts no rvalue

Answer 1

我无法找到任何 Material 来解释为什么要求处理程序是 CopyConstructible 的原因，但即使在 C++11 支持的情况下也明确指出了这一点。 Movable Handlers文档说明:

As an optimisation, user-defined completion handlers may provide move constructors, and Boost.Asio's implementation will use a handler's move constructor in preference to its copy constructor. In certain circumstances, Boost.Asio may be able to eliminate all calls to a handler's copy constructor. However, handler types are still required to be copy constructible.

当不满足类型要求时，Asio 执行的类型检查允许更友好的编译器错误消息。此类型检查发生在调用堆栈的较早位置，并且不以使用该对象是否会产生编译器错误为前提。例如，当启用类型检查时，类型检查将为没有复制构造函数的处理程序发出编译器错误，即使对处理程序的复制构造函数的所有调用都已消除。通过定义 BOOST_ASIO_DISABLE_HANDLER_TYPE_REQUIREMENTS，可以禁用显式类型检查，并允许编译器错误在发生时从实现中更深层的调用点浮出水面。 History注意这个选项:

Asio 1.6.0 / Boost 1.47

• ...
• Added friendlier compiler errors for when a completion handler does not meet the necessary type requirements. When C++0x is available (currently supported for g++ 4.5 or later, and MSVC 10), static_assert is also used to generate an informative error message. This checking may be disabled by defining BOOST_ASIO_DISABLE_HANDLER_TYPE_REQUIREMENTS.

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这是一个完整的例子demonstrating这个功能。在其中，由 unique_ptr 管理的套接字的所有权通过 std::move() 转移到处理程序:

#include <functional> // std::bind
#include <memory>     // std::unique_ptr
#include <string>     // std::string
#define BOOST_ASIO_DISABLE_HANDLER_TYPE_REQUIREMENTS
#include <boost/asio.hpp>

const auto noop = std::bind([]{});

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

  // Create all I/O objects.
  boost::asio::io_service io_service;
  tcp::acceptor acceptor(io_service, tcp::endpoint(tcp::v4(), 0));
  auto socket1 = std::make_unique<tcp::socket>(std::ref(io_service));
  tcp::socket socket2(io_service);

  // Connect the sockets.
  acceptor.async_accept(*socket1, noop);
  socket2.async_connect(acceptor.local_endpoint(), noop);
  io_service.run();
  io_service.reset();

  // Move ownership of socket1 to a handler that will write to the
  // socket.
  const std::string expected_message = "test message";
  io_service.post([socket1{std::move(socket1)}, &expected_message] {
    boost::asio::write(*socket1, boost::asio::buffer(expected_message));
  });
  io_service.run();

  // Read from socket2.
  std::vector<char> actual_message(socket2.available());
  boost::asio::read(socket2, boost::asio::buffer(actual_message));

  // Verify message.
  assert(std::equal(
    begin(expected_message), end(expected_message),
    begin(actual_message), end(actual_message)));
}