从朋友方法使用async_read

Question

I've overloaded the shift operator in a class to provide input. I do a sync asio::write() in that method then do an async asio::async_read() immediately after. My problem is that the shift overload needs to be a friend of my class.

If I supply this to async_read:

void operator>>(const vector<unsigned char> input, Socket &socket) {
      const size_t size = input.size();
      const size_t bytes = asio::write(socket.connection_socket, asio::buffer(input, size));
      if (bytes != size) {
        const std::error_code ec;
        throw std::system_error(ec, fmt::format("Tried to send {0} bytes but sent {1} instead.", size, bytes));
      }
      asio::async_read(socket.connection_socket,
                       asio::buffer(socket.read_buffer),
                       std::bind(&Socket::handle_async_read,
                                 this,
                                 std::placeholders::_1));
    }

I get the error:

error: invalid use of 'this' outside of a non-static member function

If I pass the reference to socket:

void operator>>(const vector<unsigned char> input, Socket &socket) {
      const size_t size = input.size();
      const size_t bytes = asio::write(socket.connection_socket, asio::buffer(input, size));
      if (bytes != size) {
        const std::error_code ec;
        throw std::system_error(ec, fmt::format("Tried to send {0} bytes but sent {1} instead.", size, bytes));
      }
      asio::async_read(socket.connection_socket,
                       asio::buffer(socket.read_buffer),
                       std::bind(&Socket::handle_async_read,
                                 socket,
                                 std::placeholders::_1));
    }

I get the error:

error: call to implicitly-deleted copy constructor of 'std::__1::__bind<void
      (databaseclient::internal::Socket::*)(std::__1::error_code &, unsigned long), databaseclient::internal::Socket &, std::__1::placeholders::__ph<1> &>'
  ASIO_READ_HANDLER_CHECK(ReadHandler, handler) type_check;
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Answer 1

You're binding to a copy of the socket, which is illegal.

this is better:

asio::async_read(socket.connection_socket,
                       asio::buffer(socket.read_buffer),
                       std::bind(&Socket::handle_async_read,
                                 std::ref(socket),
                                 std::placeholders::_1));

this is even better (since bind is anachronistic):

asio::async_read(socket.connection_socket,
                       asio::buffer(socket.read_buffer),
                       [&socket](auto const& ec, auto transferred) 
                       {
                         handle_async_read(socket, ec);
                       });