Re: [Boost-users] boost::asio::asyn_accept handler

Subject: Re: [Boost-users] boost::asio::asyn_accept handler
From: Gavin Lambert (gavinl_at_[hidden])
Date: 2017-03-28 22:29:59

On 29/03/2017 03:28, Christopher Pisz via Boost-users wrote:
> If I followed what you wrote in your post correctly, I would end up with:
>
>     void start_accept()
>     {
>         tcp_connection::pointer new_connection =
>             tcp_connection::create(acceptor_.get_io_service());
>
>         //auto callback = std::bind(&tcp_server::handle_accept, this,
> new_connection, std::placeholders::_1);
>         const std::function<void(tcp_connection::pointer, const
> boost::system::error_code &amp;)> callback =
>             std::bind(&tcp_server::handle_accept, this, new_connection,
> std::placeholders::_1);
>
>         acceptor_.async_accept(new_connection->socket(), std::bind(callback,
> this, boost::asio::placeholders::error));
>     }
>
>
> Which does not compile. I don't know why I'd bind something that is already
> bound or what this would mean in the context of the second bind.

All that bind does is to return a functor that has fewer parameters than 
another functor, by "binding" some specific values to some parameters 
while letting others pass through from the caller.  You can bind as many 
times as you like (although there's a slight performance hit due to the 
indirection).

(Though in this context there's no point in doing two binds -- it's 
always possible to write that as a single bind, or as a lambda.  You'd 
only bind multiple times if it needed to happen in different contexts.)

Now, have a look at the code you wrote:

     const std::function<void(tcp_connection::pointer, const
        boost::system::error_code&)> callback =
           std::bind(&tcp_server::handle_accept, this, new_connection,
                     std::placeholders::_1);

You are declaring a function that takes a tcp_connection::pointer and a 
const boost::system::error_code& as parameters -- that's two parameters. 
  But you're only using one placeholder in the bind, which means that 
the second parameter doesn't go anywhere, which is probably not your 
intention.

Meanwhile in the second bind you're trying to mix boost binding with std 
binding.  Either use boost::bind for the async_accept call or use 
std::placeholders::_1 instead of the boost::asio::placeholders::error. 
Also, "this" is probably not a tcp_connection::pointer, so it's not 
valid to try to bind that to the first parameter of callback.

(You can mix binds to a certain extent -- a boost bind can be stored in 
a std::function and vice versa, and you can boost bind a std::function 
and vice versa, but you can't use std:: placeholders in a boost:: bind 
and vice versa.)

In any case, this is completely different code from what you had in your 
previous post.  I already showed you what to use for your previous code.