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. Answers to this question are eligible for a +50 reputation bounty. bmahf wants to to this question:

I really need an answer to this question guys. Please take a look at the code I have added and let me know if you can see my error. Thanks in advance.

I have a boost asio project that I am at wits end with. I have defined a TCPConnect class which is inherited by both a TCPSession and a TCPClient class. The reason for this is because I would like both the server and client side of the connection to be directly used for sending and receiving messages over full duplex TCP/IP. Only the TCPSession class listens for new connections, and only the TCPClient class makes an outgoing connection.

When the client connects to the server it sends a handshake message and blocks for the response. The server receives the message and sends a handshake ack back. On sending the ack, the server considers the connection completed. On receiving the ack, the client considers the connection completed.

The problem that I am having is that only the client side (the TCPClient object) can call its inherited TCPConnect::Send() and have it be received by the remote TCPSession object. If the server side (the TCPSession object) calls TCPConnect::Send(), it puts the message on the line without a problem, but the message is never received by the remote TCPClient object.

I must say I am a total beginner at working with boost. I have looked into this issue, trying to word and reword my search query, but have not found a solution. On both sides I have the TCPSession and TCPClient objects sitting on an async_read(), but on the client side the callback for the async_read() does not get called when the server side sends a message. I am assuming this has to do with how the io_service is set up on the TCPSession object's side, and possibly with threading.

So, a TCPServer is started inside a TCPWorker::run() function, this function being run in its own thread. The TCPWorker class owns the io_service object. The threaded run() function instantiates a TCPServer object and then calls io_service::run(). The TCPServicer object is responsible for creating TCPSession objects for new connections. For each new connection, the TCPServer creates a new TCPSession object with the io_service pointer and calls async_accept on its acceptor. Once a new connection is accepted, the TCPSession's socket is set for an async_read(). So it is known that io_service and the multiple sockets that can be created to use it are in one thread.

On the TCPClient side, when Connect() is called, if an io_service doesn't exist and/or a thread does not yet exist for the io_service, they are created using the following lines:

 if (:m_pClientServiceThread) { if (:m_pclient_io_service) // if no io_service has yet been created m_pclient_io_service = new boost::asio;:io_service: m_pClientServiceThread = new boost::thread(boost::bind(&boost::asio::io_service,;run, m_pclient_io_service)); }

So the service is being run in a thread. Then a new resolver and a new boost::asio::ip::tcp::socket are created given the io_service pointer, and boost::asio::connect() is called given the socket and a valid resolved endpoint. So the client seems to have its io_service running in a thread. Having successfully made the connection, I then send a handshake message using boost::asio::read(), and sit with boost::asio::read() waiting on the handshare response. One receiving a valid response, I pass the socket to an async_read() to wait for incoming messages.

I have looked at this for so long now without figuring out why the client side async_read() does no receive a message sent from the server side, even though the server side receives the message that is sent from the client side.

Please help me to figure this out. I am quite sure there is something simple I am not seeing, but as I have said, I am not a boost expert, so I am not sure what it is.

Added code:

TcpConnect class:

class TcpConnect : public boost::enable_shared_from_this<TcpConnect>
{
    TcpConnect(boost::asio::io_service* pio_service, ConnType connType, std::string sHostIp, int iHostPort)
        : m_pio_service(pio_service)
        , eConnType(connType)
        , m_strHostIp(sHostIp)
        , m_iHostPort(iHostPort)
    {
    }
    virtual ~TcpConnect() { /* does what is needed - this works */ }

    bool SendBlocked(CmdPacket& msg, CmdPacket& rsp)
    {
        // Function used for handling connection handshake response
        
        std::size_t write_length = boost::asio::write( *m_pSocket, boost::asio::buffer(msg.Serialize(), (std::size_t)msg.SerializedLength()));
        if (msg.SerializedLength() != write_length)
        {
            return false;
        }

        boost::asio::streambuf sbuff;
        boost::system::error_code error;
        size_t reply_length(0);

        // read header for message body length
        byte* buffer = rsp.CreateBuffer(MSG_HEADER_LENGTH);
        reply_length = boost::asio::read(*m_pSocket, boost::asio::buffer(buffer, MSG_HEADER_LENGTH), boost::asio::transfer_exactly(MSG_HEADER_LENGTH), error); 
        if (error || !rsp.ReadMessageLength())
        {
            /* error handled here */
            return false;
        }

        // read message body
        int expectedlen = rsp.BodyLength();
        buffer = rsp.CreateBuffer(expectedlen);
        reply_length = boost::asio::read(*m_pSocket, boost::asio::buffer(buffer, expectedlen), boost::asio::transfer_exactly(expectedlen), error);
        if (error)
        {
            /* error handled here */
            return false;
        }

        if (!rsp.Deserialize() || reply_length != rsp.BodyLength())
        {
            /* error handled here */
            return false;
        }

        return true;
    }
    bool Send(CmdPacket& msg)
    {
        bool bStatus = true;
        size_t write_length = 0;

        try
        {
            write_length = boost::asio::write( *m_pSocket, boost::asio::buffer(msg.Serialize(), (std::size_t)msg.SerializedLength()) );
        }
        catch (...)
        {
            /* error handled here */
            return false;
        }

        if (write_length != msg.SerializedLength())
        {
            /* error handled here */
            return false;
        }

        return true;
    }
    
    void StartAsyncRead()
    {
        m_pInMsg = new CmdPacket();
        boost::asio::async_read(*m_pSocket, boost::asio::buffer(m_pInMsg->CreateBuffer(MSG_HEADER_LENGTH), MSG_HEADER_LENGTH),
            boost::bind(&TcpConnect::handle_read_header, shared_from_this(),
            boost::asio::placeholders::error,
            boost::asio::placeholders::bytes_transferred));
    }
    void handle_read_header(const boost::system::error_code& error, size_t bytes_transferred)
    {
        if (!error && bytes_transferred == MSG_HEADER_LENGTH && m_pInMsg->ReadMessageLength())
        {
            boost::asio::async_read(*m_pSocket,
                boost::asio::buffer(m_pInMsg->CreateBuffer(m_pInMsg->SerializedLength()), m_pInMsg->SerializedLength()),
                boost::bind(&TcpConnect::handle_read_body, shared_from_this(),
                boost::asio::placeholders::error,
                boost::asio::placeholders::bytes_transferred));
        }
        else if (error)
        {
            /* errors handled */
        }
    }
    void handle_read_body(const boost::system::error_code& error, size_t bytes_transferred)
    {
        bool deserialized = false;
        if (!error && (deserialized = m_pInMsg->Deserialize()))
        {
            // if not yet connected, expecting handshake message, in which case acknowledge, otherwise error
            if (m_pInMsg->IsHandshake())
            {
                m_pInMsg->SetAcknowledge(true);
                std::size_t write_length = boost::asio::write(
                    *m_pSocket, boost::asio::buffer(m_pInMsg->Serialize(), (std::size_t)m_pInMsg->SerializedLength()));
                if (write_length == m_pInMsg->SerializedLength())
                {
                    /* we sent the acknowledgement, so we consider we're connected */
                }
                else
                {
                    /* handling error here */
                    return;
                }
                delete m_pInMsg;
                m_pInMsg = NULL;
            }
            // if graceful disconnect, notify the connection manager of new status, which will remove the connection from the map
            else if (m_pInMsg->IsDisconnect())
            {
                /* disconnect request handled here */
                return;
            }
            else
            {
                /* message received, passing it to the local process here */
            }

            // set up to handle the next read
            m_pInMsg = new CmdPacket;
            boost::asio::async_read(*m_pSocket, boost::asio::buffer(m_pInMsg->CreateBuffer(MSG_HEADER_LENGTH), MSG_HEADER_LENGTH),
                boost::bind(&TcpConnect::handle_read_header, shared_from_this(),
                    boost::asio::placeholders::error,
                    boost::asio::placeholders::bytes_transferred));
        }
        else if (error)
        {
            /* handle case where boost error */
        }
        else if (!deserialized)
        {
            /* handle case where message not correctly deserialized */
        }
    }

protected:
    ConnType eConnType;
    boost::asio::ip::tcp::socket* m_pSocket{ 0 };
    boost::asio::io_service* m_pio_service;
    boost::asio::ip::tcp::resolver::iterator m_itEndpoint;
    CmdPacket* m_pInMsg{ 0 };
    std::string m_strHostIp;
    int m_iHostPort;
};
typedef boost::shared_ptr<TcpConnect> ConnectionPtr;

TcpClient class:

class TcpClient : public TcpConnect
{
public:
    TcpClient(boost::asio::io_service& io_service, ConnType connType, const std::string& sIP, int iPort)
        : TcpConnect(&io_service, connType, sIP, iPort)
    {
    }
    ~TcpClient() { /* does what is needed - this works */ }

    //virtual ObjType Type() { return OT_CLIENT; }
    //virtual int sessionId() { return -1; } // client end does not have a session id

    //Use the following to initialize and to to reestablish the connection.
    bool Connect()
    {
        bool bStatus = true;

        //Convert the port to a string
        std::stringstream ss;
        ss << m_iHostPort;
        std::string strPort = ss.str();
        
        //Establish the connection
        try
        {
            boost::system::error_code ec;

            // create TCP resolver and query and resolve the endpoint
            boost::asio::ip::tcp::resolver resolver(*m_pio_service);
            boost::asio::ip::tcp::resolver::query query(m_strHostIp.c_str(), strPort.c_str());
            boost::asio::ip::tcp::resolver::iterator m_iterEndpoint = resolver.resolve(query, ec);

            if (ec)
            {
                /* error handled here */
                bStatus = false;
            }
            else
            {
                // close an old socket (shouldn't ever be the case)
                if (m_pSocket != NULL) CloseSocket();  /* NOTE: this is defined in TcpConnect, but not shown here */

                // create the socket on the io_service object and connect to the endpoint
                m_pSocket = new boost::asio::ip::tcp::socket(*m_pio_service);
                boost::asio::connect(*m_pSocket, m_iterEndpoint, ec);
                if (ec)
                {
                    /* error handled here */
                    bStatus = false;
                }
            }
        } //end try
        catch(...)
        {
            /* error handled here */
            bStatus = false;
        }
        
        return bStatus;
    }
};
typedef boost::shared_ptr<TcpClient> TcpClientPtr;

TcpServer class (run by TcpWorker and creates TcpSession objects):

class TcpServer;

class TcpSession : public TcpConnect
{
public:
    TcpSession(boost::asio::io_service& io_service)
        : TcpConnect(&io_service)
        , m_session_id(next_session_id())
        , m_pSocket(new tcp::socket(io_service))
    {
    }
    virtual ~TcpSession() { /* NOTE: m_pSocket is owned and deleted by TcpConnect */ }

private:
    int next_session_id()
    {
        static int id = 0;
        return (++id > 0) ? id : 1;
    }

private:
    int m_session_id;
};
typedef boost::shared_ptr<TcpSession> TcpSessionPtr;


class TcpServer
{
public:
    TcpServer(boost::asio::io_service& io_service, short port)
        : m_pio_service(&io_service)
        , m_acceptor(io_service, tcp::endpoint(tcp::v4(), port))
    {
        m_acceptor.listen();
    }
    ~TcpServer()
    {
        boost::system::error_code errorcode;
        m_acceptor.close(errorcode);
    }

    void start_accept()
    {
        TcpSessionPtr new_session(new TcpSession(*m_pio_service));

        // start listening for this session
        m_acceptor.async_accept(new_session->socket(),
            boost::bind(&TcpServer::handle_accept, this, new_session,
            boost::asio::placeholders::error));
        
        new_session.reset();
    }

private:
    void handle_accept(TcpSessionPtr new_session, const boost::system::error_code& error)
    {
        if (!error)
        {
            new_session->StartAsyncRead(); /* NOTE: there is code for aggregating session objects */
            /* NOTE: The result of an async_read() will be handled in TcpConnect::handle_read_header() */
        }
        else
        {
            /* error handled here */
        }
        // listen for the next connection
        start_accept();
    }

private:
    boost::asio::io_service* m_pio_service;
    tcp::acceptor m_acceptor;
};


class TcpWorker
{
public:
    TcpWorker(unsigned int port)
        : m_port(port)
    {}
    ~TcpWorker() {}
    
    void StopWorker()
    {
        if (!m_io_service.stopped())
        {
            m_io_service.stop();
            while (!m_io_service.stopped()) { boost::this_thread::sleep(boost::posix_time::milliseconds(1)); }
        }
    }

    void operator()()  // threaded run function started from Communicator::Listen()
    {
        TcpServer server(m_io_service, (short)m_port);

        server.start_accept(); // set up async_accept() for listening

        std::size_t inumhandlers = m_io_service.run();  // blocks here until StopWorker() is called
    }

private:
    unsigned int m_port;
    boost::asio::io_service m_io_service;
    bool m_running;
};

Communicator class:

class Communicator {
public:
    Communicator() = default;
    ~Communicator() { /* does what is needed - this works */ }
    
    bool Listen()
    {
        if (!m_pServerThread || !m_pServerWorker)
        {
            m_pServerWorker = new TcpWorker(m_myPort);
            m_pServerThread = new boost::thread(&TcpWorker::operator(), m_pServerWorker);
            return true;
        }
        return false;
    }
    bool Connect(int srcId, int destId, std::string ipaddr, int port)
    {
        bool ret = false;

        if (connected(destId))
        {
            ret = true;
        }
        else
        {
            // if io_service is not running, start it (happens if never started, or if no remaining client sockets running)
            if (!ClientThreadRunning())
            {
                if (m_pClientThread) // since going to create a new thread, make sure this one is deleted if exists
                    delete m_pClientThread;
                if (!m_pclient_io_service) // if no io_service has yet been created
                    m_pclient_io_service = new boost::asio::io_service;
                m_pClientServiceThread = new boost::thread(boost::bind(&boost::asio::io_service::run, m_pclient_io_service));
            }

            // create the connection. Wait for Ack before returning.
            TcpClientPtr client(new TcpClient(*m_pclient_io_service, destId, ip, port));

            // connect to the client and do a handshake
            if (client->Connect())
            {
                // if an initial handshake works, we're connected
                CmdPacket msg(CMD_NONE, srcId, destId, port, ipaddr), rsp;
                msg.SetHandshake(); // this starts the handshake protocol, which is completed on receiving the necessary response.
                if (!client->SendBlocked(msg, rsp) || rsp != msg)
                {
                    client.reset();
                    ret = false;
                }
                else
                {
                    // Connected, now set up for asynchronous reading
                    client->StartAsyncRead(m_pclient_io_service);
                    // save it in the class
                    connection = client;
                    ret = true;
                }
            }
            // decrement reference count, if not added to shared pointer map, this will set client object for deletion
            client.reset();
        }
        return ret;
    }
    bool sendMessage(CmdPacket& msg)
    {
        bool bret = false;
        if (connection != nullptr)
        {
            iter->second->Send(msg);
        }
        return bret;
    }
private:
    TcpConnect *connection{ 0 };
    TcpWorker* m_pServerWorker{ 0 };
    boost::thread *m_pServerThread{ 0 };
    boost::thread* m_pClientThread{ 0 };
    boost::asio::io_service* m_pclient_io_service{ 0 };
    ConnectionPtr connection{ 0 };
};

Answer 1

Your code is not self-contained. It's also out of synch (TcpConnect constructors do not match the initializer lists, AsyncStartRead is called with an argument). It looks like you might have duplicated m_pSocket members in the TcpSession and TcpConnect classes, namely:

is very contradictory.

There's a unhealthy reliance on pointers obscuring ownership and lifetime.

If you actually enable compiler warnings, the compiler will let you know that several member variables are is not initialized in the order you seem to expect (based on the ordering in the constructor's initializer lists).

There is the code smell of race conditions (sleep in a loop with a boolean check?). Unused variabled ( m_running ?).

Undeclared variables with very confused names (what is m_pClientServiceThread ? What makes it different from m_pServerThread and m_pClientThread ?

Why is connection declared twice, once as a raw pointer, once as a shared_ptr?

Many functions are left out (connected? ClientThreadRunning?), variables (mentioned above, ip , iter comes out of nowhere), etc.

All in all, I stopped trying to make the sample compile after an hour or two:

Broken on Coliru

#undef NDEBUG // make sure we assert
#include <boost/asio.hpp>
#include <boost/bind/bind.hpp>
#include <boost/make_shared.hpp>
#include <boost/enable_shared_from_this.hpp>
#include <boost/endian/arithmetic.hpp>
#include <boost/json.hpp>
#include <boost/json/src.hpp>
#include <boost/thread.hpp>

namespace json = boost::json;
using byte = char;
using boost::asio::ip::tcp;

enum class ConnType {
    Session
};

using Big32 = boost::endian::big_uint32_t;
static constexpr size_t MSG_HEADER_LENGTH = 2 * sizeof(Big32);

enum CmdType {
    CMD_NONE,
    CMD_HANDSHAKE,
    CMD_ACK,
    CMD_DISCONNECT,
};

struct CmdPacket {
    std::string _buf;
    json::value _data; // { "cmd_type": MsgType, "value": json::value }

    bool IsHandshake() const { return _data.at("cmd_type") == CMD_HANDSHAKE; }
    bool IsDisconnect() const{ return _data.at("cmd_type") == CMD_DISCONNECT; }
    void SetAcknowledge(json::value v) {
        auto& o       = _data.as_object();
        o["cmd_type"] = CMD_ACK;
        o["value"] = v;
    }
    void SetHandshake() { _data.as_object()["cmd_type"] = CMD_HANDSHAKE; }

    byte* CreateBuffer(uint32_t n) { _buf.assign(n, '\0'); return _buf.data(); }

    uint32_t ReadMessageLength() const {
        assert(_buf.size() >= sizeof(Big32));
        boost::endian::big_uint32_t decode;
        memcpy(&decode, _buf.data(), sizeof(decode));
        return decode;
    }

    uint32_t BodyLength() const {
        return ReadMessageLength();
    }

    std::string Serialize() const { return json::serialize(_data); }

    bool Deserialize() {
        json::error_code ec;
        _data = json::parse(_buf, ec);
        return !ec;
    }

    size_t SerializedLength() const { return Serialize().length(); }
};

class TcpConnect : public boost::enable_shared_from_this<TcpConnect> {
  public:
    virtual ~TcpConnect()
    { /* does what is needed - this works */
    }

    auto& socket() { assert(m_pSocket); return *m_pSocket; }
    auto& socket() const { assert(m_pSocket); return *m_pSocket; }

  protected:
    TcpConnect(boost::asio::io_service* pio_service, ConnType connType,
               std::string sHostIp, uint16_t iHostPort)
        : m_pio_service(pio_service)
        , eConnType(connType)
        , m_pSocket(new tcp::socket(*pio_service))
        , m_strHostIp(sHostIp)
        , m_iHostPort(iHostPort)
    {
    }

    void CloseSocket() {
        if (m_pSocket)
            m_pSocket->close();
    }

    bool SendBlocked(CmdPacket& msg, CmdPacket& rsp)
    {
        // Function used for handling connection handshake response
        std::size_t write_length = boost::asio::write(
            *m_pSocket,
            boost::asio::buffer(msg.Serialize(),
                                (std::size_t)msg.SerializedLength()));
        if (msg.SerializedLength() != write_length) {
            return false;
        }

        boost::asio::streambuf    sbuff;
        boost::system::error_code error;
        size_t                    reply_length(0);

        // read header for message body length
        byte* buffer = rsp.CreateBuffer(MSG_HEADER_LENGTH);
        reply_length = boost::asio::read(
            *m_pSocket, boost::asio::buffer(buffer, MSG_HEADER_LENGTH),
            boost::asio::transfer_exactly(MSG_HEADER_LENGTH), error);
        if (error || !rsp.ReadMessageLength()) {
            /* error handled here */
            return false;
        }

        // read message body
        int expectedlen = rsp.BodyLength();
        buffer          = rsp.CreateBuffer(expectedlen);
        reply_length    = boost::asio::read(
            *m_pSocket, boost::asio::buffer(buffer, expectedlen),
            boost::asio::transfer_exactly(expectedlen), error);
        if (error) {
            /* error handled here */
            return false;
        }

        if (reply_length != rsp.BodyLength() || !rsp.Deserialize()) {
            /* error handled here */
            return false;
        }

        return true;
    }
    bool Send(CmdPacket& msg)
    {
        //bool   bStatus      = true;
        size_t write_length = 0;

        try {
            write_length = boost::asio::write(
                *m_pSocket,
                boost::asio::buffer(msg.Serialize(),
                                    (std::size_t)msg.SerializedLength()));
        } catch (...) {
            /* error handled here */
            return false;
        }

        if (write_length != msg.SerializedLength()) {
            /* error handled here */
            return false;
        }

        return true;
    }

  public:
    void StartAsyncRead()
    {
        m_pInMsg = new CmdPacket();
        boost::asio::async_read(
            *m_pSocket,
            boost::asio::buffer(m_pInMsg->CreateBuffer(MSG_HEADER_LENGTH),
                                MSG_HEADER_LENGTH),
            boost::bind(&TcpConnect::handle_read_header, shared_from_this(),
                        boost::asio::placeholders::error,
                        boost::asio::placeholders::bytes_transferred));
    }

  private:
    void handle_read_header(const boost::system::error_code& error,
                            size_t                           bytes_transferred)
    {
        if (!error && bytes_transferred == MSG_HEADER_LENGTH &&
            m_pInMsg->ReadMessageLength()) {
            boost::asio::async_read(
                *m_pSocket,
                boost::asio::buffer(
                    m_pInMsg->CreateBuffer(m_pInMsg->SerializedLength()),
                    m_pInMsg->SerializedLength()),
                boost::bind(&TcpConnect::handle_read_body, shared_from_this(),
                            boost::asio::placeholders::error,
                            boost::asio::placeholders::bytes_transferred));
        } else if (error) {
            /* errors handled */
        }
    }
    void handle_read_body(const boost::system::error_code& error,
                          size_t                           bytes_transferred)
    {
        bool deserialized = false;
        if (!error && (deserialized = m_pInMsg->Deserialize())) {
            // if not yet connected, expecting handshake message, in which case
            // acknowledge, otherwise error
            if (m_pInMsg->IsHandshake()) {
                m_pInMsg->SetAcknowledge(true);
                std::size_t write_length = boost::asio::write(
                    *m_pSocket,
                    boost::asio::buffer(
                        m_pInMsg->Serialize(),
                        (std::size_t)m_pInMsg->SerializedLength()));
                if (write_length == m_pInMsg->SerializedLength()) {
                    /* we sent the acknowledgement, so we consider we're
                     * connected */
                } else {
                    /* handling error here */
                    return;
                }
                delete m_pInMsg;
                m_pInMsg = NULL;
            }
            // if graceful disconnect, notify the connection manager of new
            // status, which will remove the connection from the map
            else if (m_pInMsg->IsDisconnect()) {
                /* disconnect request handled here */
                return;
            } else {
                /* message received, passing it to the local process here */
            }

            // set up to handle the next read
            m_pInMsg = new CmdPacket;
            boost::asio::async_read(
                *m_pSocket,
                boost::asio::buffer(m_pInMsg->CreateBuffer(MSG_HEADER_LENGTH),
                                    MSG_HEADER_LENGTH),
                boost::bind(&TcpConnect::handle_read_header, shared_from_this(),
                            boost::asio::placeholders::error,
                            boost::asio::placeholders::bytes_transferred));
        } else if (error) {
            /* handle case where boost error */
        } else if (!deserialized) {
            /* handle case where message not correctly deserialized */
        }
    }

  protected:
    boost::asio::io_service*                 m_pio_service;
    ConnType                                 eConnType;
    boost::asio::ip::tcp::socket*            m_pSocket{0};
    boost::asio::ip::tcp::resolver::iterator m_itEndpoint;
    CmdPacket*                               m_pInMsg{0};
    std::string                              m_strHostIp;
    uint16_t                                 m_iHostPort;
};
typedef boost::shared_ptr<TcpConnect> ConnectionPtr;

// TcpClient class:

class TcpClient : public TcpConnect {
  public:
    TcpClient(boost::asio::io_service& io_service, ConnType connType,
              const std::string& sIP, uint16_t iPort)
        : TcpConnect(&io_service, connType, sIP, iPort)
    {
    }
    ~TcpClient()
    { /* does what is needed - this works */
    }

    // virtual ObjType Type() { return OT_CLIENT; }
    // virtual int sessionId() { return -1; } // client end does not have a
    // session id

    // Use the following to initialize and to to reestablish the connection.
    bool Connect()
    {
        bool bStatus = true;

        // Convert the port to a string
        std::stringstream ss;
        ss << m_iHostPort;
        std::string strPort = ss.str();

        // Establish the connection
        try {
            boost::system::error_code ec;

            // create TCP resolver and query and resolve the endpoint
            boost::asio::ip::tcp::resolver           resolver(*m_pio_service);
            boost::asio::ip::tcp::resolver::query    query(m_strHostIp.c_str(),
                                                        strPort.c_str());
            boost::asio::ip::tcp::resolver::iterator m_iterEndpoint =
                resolver.resolve(query, ec);

            if (ec) {
                /* error handled here */
                bStatus = false;
            } else {
                // close an old socket (shouldn't ever be the case)
                if (m_pSocket != NULL)
                    CloseSocket(); /* NOTE: this is defined in TcpConnect, but
                                      not shown here */

                // create the socket on the io_service object and connect to the
                // endpoint
                m_pSocket = new boost::asio::ip::tcp::socket(*m_pio_service);
                boost::asio::connect(*m_pSocket, m_iterEndpoint, ec);
                if (ec) {
                    /* error handled here */
                    bStatus = false;
                }
            }
        } // end try
        catch (...) {
            /* error handled here */
            bStatus = false;
        }

        return bStatus;
    }
};
typedef boost::shared_ptr<TcpClient> TcpClientPtr;

// TcpServer class (run by TcpWorker and creates TcpSession objects):

class TcpServer;

class TcpSession : public TcpConnect {
  public:
    TcpSession(boost::asio::io_service& io_service)
        : TcpConnect(&io_service, ConnType::Session, "127.0.0.1", 8787)
        , m_session_id(next_session_id())
    {
    }
    virtual ~TcpSession()
    { /* NOTE: m_pSocket is owned and deleted by TcpConnect */
    }

  private:
    int next_session_id()
    {
        static int id = 0;
        return (++id > 0) ? id : 1;
    }

  private:
    int m_session_id;
};
typedef boost::shared_ptr<TcpSession> TcpSessionPtr;

class TcpServer {
  public:
    TcpServer(boost::asio::io_service& io_service, uint16_t port)
        : m_pio_service(&io_service)
        , m_acceptor(io_service, tcp::endpoint(tcp::v4(), port))
    {
        m_acceptor.listen();
    }
    ~TcpServer()
    {
        boost::system::error_code errorcode;
        m_acceptor.close(errorcode);
    }

    void start_accept()
    {
        TcpSessionPtr new_session(new TcpSession(*m_pio_service));

        // start listening for this session
        m_acceptor.async_accept(new_session->socket(),
                                boost::bind(&TcpServer::handle_accept, this,
                                            new_session,
                                            boost::asio::placeholders::error));

        new_session.reset();
    }

  private:
    void handle_accept(TcpSessionPtr                    new_session,
                       const boost::system::error_code& error)
    {
        if (!error) {
            new_session->StartAsyncRead(); /* NOTE: there is code for
                                              aggregating session objects */
            /* NOTE: The result of an async_read() will be handled in
             * TcpConnect::handle_read_header() */
        } else {
            /* error handled here */
        }
        // listen for the next connection
        start_accept();
    }

  private:
    boost::asio::io_service* m_pio_service;
    tcp::acceptor            m_acceptor;
};

class TcpWorker {
  public:
    TcpWorker(uint16_t port) : m_port(port) {}
    ~TcpWorker() {}

    void StopWorker()
    {
        if (!m_io_service.stopped()) {
            m_io_service.stop();
            while (!m_io_service.stopped()) {
                boost::this_thread::sleep(boost::posix_time::milliseconds(1));
            }
        }
    }

    void
    operator()() // threaded run function started from Communicator::Listen()
    {
        TcpServer server(m_io_service, m_port);

        server.start_accept(); // set up async_accept() for listening

        std::size_t inumhandlers =
            m_io_service.run(); // blocks here until StopWorker() is called
    }

  private:
    uint16_t                m_port;
    boost::asio::io_service m_io_service;
    bool                    m_running;
};

// Communicator class:

class Communicator {
  public:
    uint16_t m_myPort = 8787;

    Communicator() = default;
    ~Communicator()
    { /* does what is needed - this works */
    }

    bool Listen()
    {
        if (!m_pServerThread || !m_pServerWorker) {
            m_pServerWorker = new TcpWorker(m_myPort);
            m_pServerThread =
                new boost::thread(&TcpWorker::operator(), m_pServerWorker);
            return true;
        }
        return false;
    }
    bool Connect(int srcId, int destId, std::string ipaddr, uint16_t port)
    {
        bool ret = false;

        if (connected(destId)) {
            ret = true;
        } else {
            // if io_service is not running, start it (happens if never started,
            // or if no remaining client sockets running)
            if (!ClientThreadRunning()) {
                if (m_pClientThread) // since going to create a new thread, make
                                     // sure this one is deleted if exists
                    delete m_pClientThread;
                if (!m_pclient_io_service) // if no io_service has yet been
                                           // created
                    m_pclient_io_service = new boost::asio::io_service;
                m_pClientServiceThread = new boost::thread(boost::bind(
                    &boost::asio::io_service::run, m_pclient_io_service));
            }

            // create the connection. Wait for Ack before returning.
            TcpClientPtr client(
                new TcpClient(*m_pclient_io_service, destId, ip, port));

            // connect to the client and do a handshake
            if (client->Connect()) {
                // if an initial handshake works, we're connected
                CmdPacket msg(CMD_NONE, srcId, destId, port, ipaddr), rsp;
                msg.SetHandshake(); // this starts the handshake protocol, which
                                    // is completed on receiving the necessary
                                    // response.
                if (!client->SendBlocked(msg, rsp) || rsp != msg) {
                    client.reset();
                    ret = false;
                } else {
                    // Connected, now set up for asynchronous reading
                    client->StartAsyncRead(m_pclient_io_service);
                    // save it in the class
                    connection = client;
                    ret        = true;
                }
            }
            // decrement reference count, if not added to shared pointer map,
            // this will delete client object
            client.reset();
        }
        return ret;
    }
    bool sendMessage(CmdPacket& msg)
    {
        bool bret = false;
        if (connection != nullptr) {
            iter->second->Send(msg);
        }
        return bret;
    }

  private:
    ConnectionPtr            connection;
    TcpWorker*               m_pServerWorker{0};
    boost::thread*           m_pServerThread{0};
    boost::thread*           m_pClientThread{0};
    boost::asio::io_service* m_pclient_io_service{0};
};

int main() {}

Out Of The Box

All of this reminds me very much of this answer: How do I make this HTTPS connection persistent in Beast? in that answer I show how to to keep a pool of connections for different destinations. It uses a different style, so hopefully you can use that as inspiration.