Re: [boost] [ASIO] Do I have to use shared_ptrs with ASIO?

22 Sep 2021

      wt., 21 wrz 2021 o 17:22 Дмитрий Архипов via Boost 
napisał(a):
...
I haven't tested it, but this should probably work:
class session {
public:
  session(tcp::socket socket)
    : session(std::make_unique(std::move(socket)))
  {}
void start() { do_read(); }
private:
  session(std::unique_ptr impl) : impl_(std::move(impl)) {}
void do_read()
  {
    auto& impl = *impl_;
    impl.socket.async_read_some(
      boost::asio::buffer(impl.data, max_length),
      [impl = std::move(impl_)](boost::system::error_code ec,
std::size_t length) mutable {
        if (ec) { return; }
        session(std::move(impl)).do_write(length);
      });
  }
void do_write(std::size_t length) { ... }
auto constexpr max_length = 1024;
  struct session_impl {
    session_impl(tcp::socket socket) : socket_(std::move(socket)) {}
    tcp::socket socket_;
    char data_[max_length];
  };
};
...
session(std::move(socket)).start();
Thank you for this example. I haven't compiled it yet, but my superficial
analysis tells me that it should work.
I asked the question, you gave the right answer, so I should only be
thankful, I guess.
But having seen the answer, I now realize that I asked the wrong question.
The question that I really wanted to ask is:
do I have to heap-allocate "IO" objects in order to use tem with
continuation-passing style completion handlers?

My observation about the example code:

  void do_read()
  {
    auto self(shared_from_this());
    *socket_              *// <-- socket passed in one place
    .async_read_some(boost::asio::buffer(data_, max_length),
        [this, *self*]     // <-- socket passed in another place
        (boost::system::error_code ec, std::size_t length)
        {
          if (!ec)
          {
            do_write(length);
          }
        });
  }

is that I have to pass my socket in two places in one function invocation.
Whereas, at least at high level, I could imagine an interface when I only
pass it once (by movig) to function async_read_some, and after the function
has made the read it passes (via a move) the socket argument to the
handler. Is there something fundamental that makes this impossible?

Regards,
&rzej;
...
вт, 21 сент. 2021 г. в 18:02, Andrzej Krzemienski via Boost
:
...
wt., 21 wrz 2021 o 16:50 Vinnie Falco via Boost 
napisał(a):
...
On Tue, Sep 21, 2021 at 7:34 AM Andrzej Krzemienski via Boost
 wrote:
...
I am not trying to solve any specific problem. It just strikes me
that a
...
...
...
shared_ptr is used in a demo example for the library. I was always
of the
opinion that a shared_ptr is often abused as a way of doing an
ad-hoc GC.
In theory it should work since the echo protocol is half-duplex. What
happens when you switch it to unique_ptr? Move-only handlers should
work, but it is possible that Chris missed a place that is still doing
a copy.
It breaks when I pass a callback (completion handler), for instance in:
void do_read()
  {
    auto self(shared_from_this());
    socket_.async_read_some(boost::asio::buffer(data_, max_length),
        [this, *self*](boost::system::error_code ec, std::size_t length)
        {
          if (!ec)
          {
            do_write(length);
          }
        });
  }
I would need to move the data inside the lambda capture, but if I do
it, the subsequent call to socket_.async_read_some() is UB.
In order for this to work, the function `async_read_some()` would have
to pass the socket back to my handler after it has performed the read.
Regards,
&rzej;
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