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

From: Дмитрий Архипов (grisumbras_at_[hidden])
Date: 2021-09-21 15:22:08

I haven't tested it, but this should probably work:

class session {
public:
  session(tcp::socket socket)
    : session(std::make_unique<session_impl>(std::move(socket)))
  {}

  void start() { do_read(); }

private:
  session(std::unique_ptr<session_impl> 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();

вт, 21 сент. 2021 г. в 18:02, Andrzej Krzemienski via Boost
<boost_at_[hidden]>:
>
> wt., 21 wrz 2021 o 16:50 Vinnie Falco via Boost <boost_at_[hidden]>
> napisał(a):
>
> > On Tue, Sep 21, 2021 at 7:34 AM Andrzej Krzemienski via Boost
> > <boost_at_[hidden]> 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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