Cannot store bind_front_handler return value in variable

Question

With these present in other parts of my codebase,

namespace net = boost::asio;
using boost::asio::ip::tcp;

boost::asio::io_context& io_context_;
tcp::acceptor acceptor_;
void server::on_accept(boost::beast::error_code ec, boost::asio::ip::tcp::socket socket);

I have noticed that this piece of code compiles:

auto strand = net::make_strand(io_context_);
std::shared_ptr<server> this_pointer = shared_from_this();

acceptor_.async_accept(
    strand,
    boost::beast::bind_front_handler(&server::on_accept, this_pointer)
);

whereas this does not:

auto strand = net::make_strand(io_context_);
std::shared_ptr<server> this_pointer = shared_from_this();

auto call_next = boost::beast::bind_front_handler(&server::on_accept, this_pointer);

acceptor_.async_accept(
    strand,
    call_next
);

and it fails with the error

/usr/include/boost/beast/core/detail/bind_handler.hpp:251:45: error: cannot convert ‘boost::beast::detail::bind_front_wrapper<void (server::*)(boost::system::error_code, boost::asio::basic_stream_socket<boost::asio::ip::tcp>), std::shared_ptr<server> >’ to ‘void (server::*)(boost::system::error_code, boost::asio::basic_stream_socket<boost::asio::ip::tcp>)’ in initialization
  251 |         , args_(std::forward<Args_>(args)...)

I am very curious why passing the value returned from bind_front_handler directly to the async_accept would work but storing that value in a variable and then passing that variable would not work.

I also understand very little about Boost and Beast right now, but here it appears to me like I am forgetting something very basic about C++ itself. Why are both of those piece of code not equivalent?

Answer 1

Indeed, you should not be doing that. The bind-front wrapper wants to be a temporary (in that it is move only). You could "fix" it by doing

    acceptor_.async_accept(strand, std::move(call_next));

(after which you will have to remember that call_next may not be used again because it has been moved-from).

I would personally go the other way - as this helper was clearly intended - and write the idiomatic

    acceptor_.async_accept(
        make_strand(io_context_),
        bind_front_handler(&server::on_accept, shared_from_this()));

Which replaces the entire function.

Demo

Live On Coliru

#include <boost/beast.hpp>
#include <boost/asio.hpp>
#include <iostream>
namespace net = boost::asio;
namespace beast = boost::beast;
using boost::system::error_code;
using net::ip::tcp;

struct server : std::enable_shared_from_this<server> {
    server() {
        acceptor_.listen();
    }

    void start(){
        using beast::bind_front_handler;
        acceptor_.async_accept(
            make_strand(io_context_),
            bind_front_handler(&server::on_accept, shared_from_this()));
    }

    void wait() {
        work_.reset();

        if (thread_.joinable())
            thread_.join();
    }

  private:
    void on_accept(error_code ec, tcp::socket&& socket) {
        std::cout << "Accepted connection from " << socket.remote_endpoint() << "\n";
        //// loop to accept more:
        // start();
    }

    net::io_context io_context_;
    tcp::acceptor   acceptor_{io_context_, {{}, 9999}};

    net::executor_work_guard<net::io_context::executor_type> work_{
        io_context_.get_executor()};
    std::thread thread_{[this] { io_context_.run(); }};
};

int main()
{
    auto s = std::make_shared<server>();
    s->start();
    s->wait();
}

With

g++ -std=c++20 -O2 -Wall -pedantic -pthread main.cpp
./a.out& sleep .5; nc 127.0.0.1 9999 <<<'hello world'; wait

Prints e.g.

Accepted connection from 127.0.0.1:36402