Boost C++ Libraries of the most highly regarded and expertly designed C++ library projects in the world. Herb Sutter and Andrei Alexandrescu, C++ Coding Standards

This is the documentation for an old version of Boost. Click here to view this page for the latest version.


// Copyright (c) 2016-2017 Vinnie Falco (vinnie dot falco at gmail dot com)
// Distributed under the Boost Software License, Version 1.0. (See accompanying
// file LICENSE_1_0.txt or copy at
// Official repository:

#include <boost/beast/core.hpp>
#include <boost/asio.hpp>
#include <cstddef>
#include <iostream>
#include <memory>
#include <utility>


    class AsyncStream,
    class CompletionToken>
async_echo(AsyncStream& stream, CompletionToken&& token)

    -> BOOST_ASIO_INITFN_RESULT_TYPE(CompletionToken, void(boost::beast::error_code));


/** Asynchronously read a line and echo it back.

    This function is used to asynchronously read a line ending
    in a carriage-return ("CR") from the stream, and then write
    it back. The function call always returns immediately. The
    asynchronous operation will continue until one of the
    following conditions is true:

    @li A line was read in and sent back on the stream

    @li An error occurs.

    This operation is implemented in terms of one or more calls to
    the stream's `async_read_some` and `async_write_some` functions,
    and is known as a <em>composed operation</em>. The program must
    ensure that the stream performs no other operations until this
    operation completes. The implementation may read additional octets
    that lie past the end of the line being read. These octets are
    silently discarded.

    @param The stream to operate on. The type must meet the
    requirements of @b AsyncReadStream and @AsyncWriteStream

    @param token The completion token to use. If this is a
    completion handler, copies will be made as required.
    The equivalent signature of the handler must be:
    void handler(
        error_code ec       // result of operation
    Regardless of whether the asynchronous operation completes
    immediately or not, the handler will not be invoked from within
    this function. Invocation of the handler will be performed in a
    manner equivalent to using `boost::asio::io_context::post`.
    class AsyncStream,
    class CompletionToken>
BOOST_ASIO_INITFN_RESULT_TYPE(          /*< `BOOST_ASIO_INITFN_RESULT_TYPE` customizes the return value based on the completion token >*/
    void(boost::beast::error_code))     /*< This is the signature for the completion handler >*/
    AsyncStream& stream,
    CompletionToken&& token);



// This composed operation reads a line of input and echoes it back.
template<class AsyncStream, class Handler>
class echo_op
    // This holds all of the state information required by the operation.
    struct state
        // The stream to read and write to
        AsyncStream& stream;

        // Indicates what step in the operation's state machine
        // to perform next, starting from zero.
        int step = 0;

        // The buffer used to hold the input and output data.
        // We use a custom allocator for performance, this allows
        // the implementation of the io_context to make efficient
        // re-use of memory allocated by composed operations during
        // a continuation.
        boost::asio::basic_streambuf<typename std::allocator_traits<
            boost::asio::associated_allocator_t<Handler> >::
                template rebind_alloc<char> > buffer;

        // handler_ptr requires that the first parameter to the
        // contained object constructor is a reference to the
        // managed final completion handler.
        explicit state(Handler& handler, AsyncStream& stream_)
            : stream(stream_)
            , buffer((std::numeric_limits<std::size_t>::max)(),

    // The operation's data is kept in a cheap-to-copy smart
    // pointer container called `handler_ptr`. This efficiently
    // satisfies the CopyConstructible requirements of completion
    // handlers with expensive-to-copy state.
    // `handler_ptr` uses the allocator associated with the final
    // completion handler, in order to allocate the storage for `state`.
    boost::beast::handler_ptr<state, Handler> p_;

    // Boost.Asio requires that handlers are CopyConstructible.
    // In some cases, it takes advantage of handlers that are
    // MoveConstructible. This operation supports both.
    echo_op(echo_op&&) = default;
    echo_op(echo_op const&) = default;

    // The constructor simply creates our state variables in
    // the smart pointer container.
    template<class DeducedHandler, class... Args>
    echo_op(AsyncStream& stream, DeducedHandler&& handler)
        : p_(std::forward<DeducedHandler>(handler), stream)

    // Associated allocator support. This is Asio's system for
    // allowing the final completion handler to customize the
    // memory allocation strategy used for composed operation
    // states. A composed operation should use the same allocator
    // as the final handler. These declarations achieve that.

    using allocator_type =

    get_allocator() const noexcept
        return boost::asio::get_associated_allocator(p_.handler());

    // Executor hook. This is Asio's system for customizing the
    // manner in which asynchronous completion handlers are invoked.
    // A composed operation needs to use the same executor to invoke
    // intermediate completion handlers as that used to invoke the
    // final handler.

    using executor_type = boost::asio::associated_executor_t<
        Handler, decltype(std::declval<AsyncStream&>().get_executor())>;

    executor_type get_executor() const noexcept
        return boost::asio::get_associated_executor(
            p_.handler(), p_->stream.get_executor());

    // The entry point for this handler. This will get called
    // as our intermediate operations complete. Definition below.
    void operator()(boost::beast::error_code ec, std::size_t bytes_transferred);



// echo_op is callable with the signature void(error_code, bytes_transferred),
// allowing `*this` to be used as both a ReadHandler and a WriteHandler.
template<class AsyncStream, class Handler>
void echo_op<AsyncStream, Handler>::
operator()(boost::beast::error_code ec, std::size_t bytes_transferred)
    // Store a reference to our state. The address of the state won't
    // change, and this solves the problem where dereferencing the
    // data member is undefined after a move.
    auto& p = *p_;

    // Now perform the next step in the state machine
    switch(ec ? 2 : p.step)
        // initial entry
        case 0:
            // read up to the first newline
            p.step = 1;
            return boost::asio::async_read_until(, p.buffer, "\r", std::move(*this));

        case 1:
            // write everything back
            p.step = 2;
            // async_read_until could have read past the newline,
            // use buffers_prefix to make sure we only send one line
            return boost::asio::async_write(,
                boost::beast::buffers_prefix(bytes_transferred,, std::move(*this));

        case 2:

    // Invoke the final handler. The implementation of `handler_ptr`
    // will deallocate the storage for the state before the handler
    // is invoked. This is necessary to provide the
    // destroy-before-invocation guarantee on handler memory
    // customizations.
    // If we wanted to pass any arguments to the handler which come
    // from the `state`, they would have to be moved to the stack
    // first or else undefined behavior results.



template<class AsyncStream, class Handler>
class echo_op;

// Read a line and echo it back
template<class AsyncStream, class CompletionToken>
BOOST_ASIO_INITFN_RESULT_TYPE(CompletionToken, void(boost::beast::error_code))
async_echo(AsyncStream& stream, CompletionToken&& token)
    // Make sure stream meets the requirements. We use static_assert
    // to cause a friendly message instead of an error novel.
        "AsyncStream requirements not met");

    // This helper manages some of the handler's lifetime and
    // uses the result and handler specializations associated with
    // the completion token to help customize the return value.
    boost::asio::async_completion<CompletionToken, void(boost::beast::error_code)> init{token};

    // Create the composed operation and launch it. This is a constructor
    // call followed by invocation of operator(). We use BOOST_ASIO_HANDLER_TYPE
    // to convert the completion token into the correct handler type,
    // allowing user-defined specializations of the async_result template
    // to be used.
            CompletionToken, void(boost::beast::error_code))>{
                init.completion_handler}(boost::beast::error_code{}, 0);

    // This hook lets the caller see a return value when appropriate.
    // For example this might return std::future<error_code> if
    // CompletionToken is boost::asio::use_future, or this might
    // return an error code if CompletionToken specifies a coroutine.
    return init.result.get();


int main(int, char** argv)
    using socket_type = boost::asio::ip::tcp::socket;
    using endpoint_type = boost::asio::ip::tcp::endpoint;

    // Create a listening socket, accept a connection, perform
    // the echo, and then shut everything down and exit.
    boost::asio::io_context ioc;
    socket_type sock{ioc};
    boost::asio::ip::tcp::acceptor acceptor{ioc};
    endpoint_type ep{boost::asio::ip::make_address(""), 0};;
        [&](boost::beast::error_code ec)
                std::cerr << argv[0] << ": " << ec.message() << std::endl;
    return 0;