How to separate the reading of an HTTP message header and body into two operations.

http_client_async.cpp

//
// Copyright (c) 2016-2019 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 http://www.boost.org/LICENSE_1_0.txt)
//
// Official repository: https://github.com/boostorg/beast
//

//------------------------------------------------------------------------------
//
// Example: HTTP client, asynchronous
//
//------------------------------------------------------------------------------

#include <boost/beast/core.hpp>
#include <boost/beast/http.hpp>
#include <boost/beast/version.hpp>
#include <boost/asio/strand.hpp>
#include <cstdlib>
#include <functional>
#include <iostream>
#include <memory>
#include <string>

namespace beast = boost::beast;         // from <boost/beast.hpp>
namespace http = beast::http;           // from <boost/beast/http.hpp>
namespace net = boost::asio;            // from <boost/asio.hpp>
using tcp = boost::asio::ip::tcp;       // from <boost/asio/ip/tcp.hpp>

//------------------------------------------------------------------------------

// Report a failure
void
fail(beast::error_code ec, char const* what)
{
    std::cerr << what << ": " << ec.message() << "\n";
}

// Performs an HTTP GET and prints the response
class session : public std::enable_shared_from_this<session>
{
    tcp::resolver resolver_;
    beast::tcp_stream stream_;
    beast::flat_buffer buffer_; // (Must persist between reads)
    http::response_parser<http::string_body> parser_; // (Must persist between reads)
    http::request<http::empty_body> req_;

public:
    // Objects are constructed with a strand to
    // ensure that handlers do not execute concurrently.
    explicit
    session(net::io_context& ioc)
        : resolver_(net::make_strand(ioc))
        , stream_(net::make_strand(ioc))
    {
    }

    // Start the asynchronous operation
    void
    run(
        char const* host,
        char const* port,
        char const* target,
        int version)
    {
        // Set up an HTTP GET request message
        req_.version(version);
        req_.method(http::verb::get);
        req_.target(target);
        req_.set(http::field::host, host);
        req_.set(http::field::user_agent, BOOST_BEAST_VERSION_STRING);

        // Look up the domain name
        resolver_.async_resolve(
            host,
            port,
            beast::bind_front_handler(
                &session::on_resolve,
                shared_from_this()));
    }

    void
    on_resolve(
        beast::error_code ec,
        tcp::resolver::results_type results)
    {
        if(ec)
            return fail(ec, "resolve");

        // Set a timeout on the operation
        stream_.expires_after(std::chrono::seconds(30));

        // Make the connection on the IP address we get from a lookup
        stream_.async_connect(
            results,
            beast::bind_front_handler(
                &session::on_connect,
                shared_from_this()));
    }

    void
    on_connect(beast::error_code ec, tcp::resolver::results_type::endpoint_type)
    {
        if(ec)
            return fail(ec, "connect");

        // Set a timeout on the operation
        stream_.expires_after(std::chrono::seconds(30));

        // Send the HTTP request to the remote host
        http::async_write(stream_, req_,
            beast::bind_front_handler(
                &session::on_write,
                shared_from_this()));
    }

    void
    on_write(
        beast::error_code ec,
        std::size_t bytes_transferred)
    {
        boost::ignore_unused(bytes_transferred);

        if(ec)
            return fail(ec, "write");
        
        // Receive the HTTP response header
        http::async_read_header(stream_, buffer_, parser_,
            beast::bind_front_handler(
                &session::on_read_header,
                shared_from_this()));
    }

    void
    on_read_header(
        beast::error_code ec,
        std::size_t bytes_transferred)
    {
        boost::ignore_unused(bytes_transferred);

        if(ec)
            return fail(ec, "read_header");

        // Write the headers to standard out
        std::cout << parser_.get() << std::endl;

        // Receive the HTTP response body
        http::async_read(stream_, buffer_, parser_,
            beast::bind_front_handler(
                &session::on_read,
                shared_from_this()));
    }

    void
    on_read(
        beast::error_code ec,
        std::size_t bytes_transferred)
    {
        boost::ignore_unused(bytes_transferred);

        if(ec)
            return fail(ec, "read");

        // Write the body to standard out
        std::cout << parser_.get().body() << std::endl;

        // Gracefully close the socket
        stream_.socket().shutdown(tcp::socket::shutdown_both, ec);

        // not_connected happens sometimes so don't bother reporting it.
        if(ec && ec != beast::errc::not_connected)
            return fail(ec, "shutdown");

        // If we get here then the connection is closed gracefully
    }
};

//------------------------------------------------------------------------------

int main(int argc, char** argv)
{
    // Check command line arguments.
    if(argc != 4 && argc != 5)
    {
        std::cerr <<
            "Usage: http-client-async <host> <port> <target> [<HTTP version: 1.0 or 1.1(default)>]\n" <<
            "Example:\n" <<
            "    http-client-async www.example.com 80 /\n" <<
            "    http-client-async www.example.com 80 / 1.0\n";
        return EXIT_FAILURE;
    }
    auto const host = argv[1];
    auto const port = argv[2];
    auto const target = argv[3];
    int version = argc == 5 && !std::strcmp("1.0", argv[4]) ? 10 : 11;

    // The io_context is required for all I/O
    net::io_context ioc;

    // Launch the asynchronous operation
    std::make_shared<session>(ioc)->run(host, port, target, version);

    // Run the I/O service. The call will return when
    // the get operation is complete.
    ioc.run();

    return EXIT_SUCCESS;
}