Compilation Error due to Boost::Asio::Spawn function

Question

RE: https://www.boost.org/doc/libs/1_80_0/libs/beast/example/websocket/client/coro-ssl/websocket_client_coro_ssl.cpp

I tried following the above example and ran into a compilation error; namely, code C2039: 'type': is not a member of 'boost::asio::result_of<Function (boost::asio::basic_yield_context<Executor>)>'.

The modifications to the example provided above are attached here:

Header:

namespace NetTools {
class ClientSession
    : public std::enable_shared_from_this<ClientSession>
{
private:
    std::string auth_token;
    std::shared_ptr<spdlog::logger> m_pLog;

public:
    ClientSession(
        std::string auth_token, std::shared_ptr<spdlog::logger> log)
        : auth_token(std::move(auth_token))
        , m_pLog(std::move(log))
    {
    }

    int StartSession();

    void do_session(
        std::string host,
        std::string const& port,
        std::string const& text,
        net::io_context& ioc,
        ssl::context& ctx,
        net::yield_context yield);
};

do_session modification:

// Sends a WebSocket message and prints the response
void NetTools::ClientSession::do_session(
    std::string host,
    std::string const& port,
    std::string const& text,
    net::io_context& ioc,
    ssl::context& ctx,
    net::yield_context yield)
{
    beast::error_code ec;

    // These objects perform our I/O
    // ... etc

main function replacement:

int NetTools::ClientSession::StartSession()
{
    auto const host = "echo.websocket.events";
    auto const port = "https";
    auto const text = "Hello world!";

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

    // The SSL context is required, and holds certificates
    ssl::context ctx{ssl::context::tlsv12_client};

    // This holds the root certificate used for verification
    load_root_certificates(ctx);

    // Launch the asynchronous operation
    net::spawn(
        ioc,
        std::bind(
            &NetTools::ClientSession::do_session,
            std::string(host),
            std::string(port),
            std::string(text),
            std::ref(ioc),
            std::ref(ctx),
            std::placeholders::_1));

The function bodies are unmodified; I replaced the main function with a StartSession function which I call in a test file (not shown). I am not sure why this error occurs and prevents my code from compiling.

CPP Version: 17

Boost (and all related beast packages) Version: 1.8.0

OS: Windows 11 x64

Tried: Running code above as described.

Expectation: Compiles and runs (though at minimum compiles!)

Result: Does not compile with error code C2039: 'type': is not a member of 'boost::asio::result_of<Function (boost::asio::basic_yield_context)>'.

Answer 1

You made do_session a non-static member function of a class. This means that you need an instance of the ClientSession object to invoke it on. You don't pass one in the std::bind() expression, which leads to confused error messages because there is no valid async_result<> specialization for your handler type. In fact, your handler type is simply invalid.

Make an instance, and use it:

auto cs = std::make_shared<NetTools::ClientSession>(
    "no_auth_token", nullptr);

cs->StartSession();

Then inside StartSession you can use the current instance, this:

// Launch the asynchronous operation
net::spawn(ioc, std::bind(
    &NetTools::ClientSession::do_session,
    this, // <-- HERE
    std::string(host),
    std::string(port),
    std::string(text),
    std::ref(ioc),
    std::ref(ctx),
    std::placeholders::_1));

All code rolled into one:

Live On Coliru

#include <boost/asio.hpp>
#include <boost/asio/spawn.hpp>
#include <boost/asio/ssl.hpp>
#include <memory>

#include "example/common/root_certificates.hpp"

#include <boost/asio/spawn.hpp>
#include <boost/beast/core.hpp>
#include <boost/beast/ssl.hpp>
#include <boost/beast/websocket.hpp>
#include <boost/beast/websocket/ssl.hpp>
#include <functional>
#include <iostream>
#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>
namespace ssl = boost::asio::ssl; // from <boost/asio/ssl.hpp>
using tcp = boost::asio::ip::tcp; // from <boost/asio/ip/tcp.hpp>

namespace spdlog { struct logger { }; }; // namespace spdlog

static auto constexpr s_target_path = "/.ws"; // for COLIRU demo, you had "/"

namespace NetTools {
class ClientSession
    : public std::enable_shared_from_this<ClientSession>
{
private:
    std::string auth_token;
    std::shared_ptr<spdlog::logger> m_pLog;

public:
    ClientSession(
        std::string auth_token, std::shared_ptr<spdlog::logger> log)
        : auth_token(std::move(auth_token))
        , m_pLog(std::move(log))
    {
    }

    int StartSession();

    void do_session(
        std::string host,
        std::string const& port,
        std::string const& text,
        net::io_context& ioc,
        ssl::context& ctx,
        net::yield_context yield);
};

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

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

// Sends a WebSocket message and prints the response
void NetTools::ClientSession::do_session(
    std::string host,
    std::string const& port,
    std::string const& text,
    net::io_context& ioc,
    ssl::context& ctx,
    net::yield_context yield)
{
    beast::error_code ec;

    // These objects perform our I/O
    tcp::resolver resolver(ioc);
    websocket::stream<beast::ssl_stream<beast::tcp_stream>> ws(
        ioc, ctx);

    // Look up the domain name
    auto const results =
        resolver.async_resolve(host, port, yield[ec]);
    if(ec)
        return fail(ec, "resolve");

    // Set a timeout on 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
    auto ep =
        beast::get_lowest_layer(ws).async_connect(results, yield[ec]);
    if(ec)
        return fail(ec, "connect");

    // Set SNI Hostname (many hosts need this to handshake
    // successfully)
    if(! SSL_set_tlsext_host_name(
           ws.next_layer().native_handle(), host.c_str()))
    {
        ec = beast::error_code(
            static_cast<int>(::ERR_get_error()),
            net::error::get_ssl_category());
        return fail(ec, "connect");
    }

    // 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());

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

    // 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-coro");
        }));

    // Perform the SSL handshake
    ws.next_layer().async_handshake(
        ssl::stream_base::client, yield[ec]);
    if(ec)
        return fail(ec, "ssl_handshake");

    // 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));

    // Perform the websocket handshake
    ws.async_handshake(host, s_target_path, yield[ec]);
    if(ec)
        return fail(ec, "handshake");

    // Send the message
    ws.async_write(net::buffer(std::string(text)), yield[ec]);
    if(ec)
        return fail(ec, "write");

    // This buffer will hold the incoming message
    beast::flat_buffer buffer;

    // Read a message into our buffer
    ws.async_read(buffer, yield[ec]);
    if(ec)
        return fail(ec, "read");

    // Close the WebSocket connection
    ws.async_close(websocket::close_code::normal, yield[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 NetTools::ClientSession::StartSession()
{
    auto const host = "echo.websocket.events";
    auto const port = "https";
    auto const text = "Hello world!";

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

    // The SSL context is required, and holds certificates
    ssl::context ctx{ssl::context::tlsv12_client};

    // This holds the root certificate used for verification
    load_root_certificates(ctx);

    // Launch the asynchronous operation
    net::spawn(
        ioc,
        std::bind(
            &NetTools::ClientSession::do_session,
            this,
            std::string(host),
            std::string(port),
            std::string(text),
            std::ref(ioc),
            std::ref(ctx),
            std::placeholders::_1));

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

    return 0;
}

int main()
{
    auto cs = std::make_shared<NetTools::ClientSession>(
        "no_auth_token", nullptr);

    cs->StartSession();
}

Running locally against the https://echo.websocket.events/.ws service: