How to get result of async_read() in boost::beast

Question

I have been trying to implement a client using the boost::beast websocket example from https://www.boost.org/doc/libs/develop/libs/beast/example/websocket/client/async/websocket_client_async.cpp

Looking as the main() of this example:

#include <boost/beast/core.hpp>
#include <boost/beast/websocket.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 websocket = beast::websocket; // from <boost/beast/websocket.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";
}

// Sends a WebSocket message and prints the response
class session : public std::enable_shared_from_this<session>
{
    tcp::resolver resolver_;
    websocket::stream<beast::tcp_stream> ws_;
    beast::flat_buffer buffer_;
    std::string host_;
    std::string text_;

public:
    // Resolver and socket require an io_context
    explicit
    session(net::io_context& ioc)
        : resolver_(net::make_strand(ioc))
        , ws_(net::make_strand(ioc))
    {
    }

    // Start the asynchronous operation
    void
    run(
        char const* host,
        char const* port,
        char const* text)
    {
        // Save these for later
        host_ = host;
        text_ = text;

        // 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 the timeout for the operation
        beast::get_lowest_layer(ws_).expires_after(std::chrono::seconds(30));

        // Make the connection on the IP address we get from a lookup
        beast::get_lowest_layer(ws_).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 ep)
    {
        if(ec)
            return fail(ec, "connect");

        // Turn off the timeout on the tcp_stream, because
        // the websocket stream has its own timeout system.
        beast::get_lowest_layer(ws_).expires_never();

        // Set suggested timeout settings for the websocket
        ws_.set_option(
            websocket::stream_base::timeout::suggested(
                beast::role_type::client));

        // Set a decorator to change the User-Agent of the handshake
        ws_.set_option(websocket::stream_base::decorator(
            [](websocket::request_type& req)
            {
                req.set(http::field::user_agent,
                    std::string(BOOST_BEAST_VERSION_STRING) +
                        " websocket-client-async");
            }));

        // Update the host_ string. This will provide the value of the
        // Host HTTP header during the WebSocket handshake.
        // See https://tools.ietf.org/html/rfc7230#section-5.4
        host_ += ':' + std::to_string(ep.port());

        // Perform the websocket handshake
        ws_.async_handshake(host_, "/",
            beast::bind_front_handler(
                &session::on_handshake,
                shared_from_this()));
    }

    void
    on_handshake(beast::error_code ec)
    {
        if(ec)
            return fail(ec, "handshake");
        
        // Send the message
        ws_.async_write(
            net::buffer(text_),
            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");
        
        // Read a message into our buffer
        ws_.async_read(
            buffer_,
            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");

        // Close the WebSocket connection
        ws_.async_close(websocket::close_code::normal,
            beast::bind_front_handler(
                &session::on_close,
                shared_from_this()));
    }

    void
    on_close(beast::error_code ec)
    {
        if(ec)
            return fail(ec, "close");

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

        // The make_printable() function helps print a ConstBufferSequence
        std::cout << beast::make_printable(buffer_.data()) << std::endl;
    }
};

//------------------------------------------------------------------------------
int main(int argc, char** argv)
{
    // Check command line arguments.
    if(argc != 4)
    {
        std::cerr <<
            "Usage: websocket-client-async <host> <port> <text>\n" <<
            "Example:\n" <<
            "    websocket-client-async echo.websocket.org 80 \"Hello, world!\"\n";
        return EXIT_FAILURE;
    }
    auto const host = argv[1];
    auto const port = argv[2];
    auto const text = argv[3];

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

    // I WOULD LIKE THE RESPONSE TO BE SAVED HERE
    beast::flat_buffer myBuffer;
    
    // Launch the asynchronous operation
    std::make_shared<session>(ioc)->run(host, port, text);

    // Run the I/O service. The call will return when
    // the socket is closed.
    ioc.run();

    return EXIT_SUCCESS;
}

How do I actually return, outside of the session object, the value received by the on_read() call inside std::make_shared(ioc)->run(host, port, text)? Say I have a beast::flat_buffer myBuffer in the main() function, to which I want to save the response. How do I do that instead of just printing it in on_close()?

Answer 1

The on_read gets called as a completion handler. So once the deferred read is completed, you'll either have an error_code (ec) or there will be bytes_transferred data put into the buffer_.

So, you could pass your myBuffer, by reference into the session, and copy the bytes into it. You could also do the async_read into the myBuffer directly.

To do exactly what you're asking.

#include <boost/beast/core.hpp>
#include <boost/beast/websocket.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 websocket = beast::websocket; // from <boost/beast/websocket.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";
}

// Sends a WebSocket message and prints the response
class session : public std::enable_shared_from_this<session>
{
    tcp::resolver resolver_;
    websocket::stream<beast::tcp_stream> ws_;
    beast::flat_buffer &buffer_;
    std::string host_;
    std::string text_;

public:
    // Resolver and socket require an io_context
    explicit
    session(net::io_context& ioc, beast::flat_buffer &buffer)
        : resolver_(net::make_strand(ioc))
        , ws_(net::make_strand(ioc)), buffer_(buffer)
    {
    }

    // Start the asynchronous operation
    void
    run(
        char const* host,
        char const* port,
        char const* text)
    {
        // Save these for later
        host_ = host;
        text_ = text;

        // 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 the timeout for the operation
        beast::get_lowest_layer(ws_).expires_after(std::chrono::seconds(30));

        // Make the connection on the IP address we get from a lookup
        beast::get_lowest_layer(ws_).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 ep)
    {
        if(ec)
            return fail(ec, "connect");

        // Turn off the timeout on the tcp_stream, because
        // the websocket stream has its own timeout system.
        beast::get_lowest_layer(ws_).expires_never();

        // Set suggested timeout settings for the websocket
        ws_.set_option(
            websocket::stream_base::timeout::suggested(
                beast::role_type::client));

        // Set a decorator to change the User-Agent of the handshake
        ws_.set_option(websocket::stream_base::decorator(
            [](websocket::request_type& req)
            {
                req.set(http::field::user_agent,
                    std::string(BOOST_BEAST_VERSION_STRING) +
                        " websocket-client-async");
            }));

        // Update the host_ string. This will provide the value of the
        // Host HTTP header during the WebSocket handshake.
        // See https://tools.ietf.org/html/rfc7230#section-5.4
        host_ += ':' + std::to_string(ep.port());

        // Perform the websocket handshake
        ws_.async_handshake(host_, "/",
            beast::bind_front_handler(
                &session::on_handshake,
                shared_from_this()));
    }

    void
    on_handshake(beast::error_code ec)
    {
        if(ec)
            return fail(ec, "handshake");
        
        // Send the message
        ws_.async_write(
            net::buffer(text_),
            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");
        
        // Read a message into our buffer
        ws_.async_read(
            buffer_,
            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");

        // Close the WebSocket connection
        ws_.async_close(websocket::close_code::normal,
            beast::bind_front_handler(
                &session::on_close,
                shared_from_this()));
    }

    void
    on_close(beast::error_code ec)
    {
        if(ec)
            return fail(ec, "close");

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

//------------------------------------------------------------------------------
int main(int argc, char** argv)
{
    // Check command line arguments.
    if(argc != 4)
    {
        std::cerr <<
            "Usage: websocket-client-async <host> <port> <text>\n" <<
            "Example:\n" <<
            "    websocket-client-async echo.websocket.org 80 \"Hello, world!\"\n";
        return EXIT_FAILURE;
    }
    auto const host = argv[1];
    auto const port = argv[2];
    auto const text = argv[3];

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

    // I WOULD LIKE THE RESPONSE TO BE SAVED HERE
    beast::flat_buffer myBuffer;
    
    // Launch the asynchronous operation
    std::make_shared<session>(ioc, myBuffer)->run(host, port, text);

    // Run the I/O service. The call will return when
    // the socket is closed.
    ioc.run();

    // The make_printable() function helps print a ConstBufferSequence
    std::cout << beast::make_printable(myBuffer.data()) << std::endl;

    return EXIT_SUCCESS;
}