Boost TCP client to connect to multiple servers

Question

I want my TCP client to connect to multiple servers(each server has a separate IP and port).
I am using async_connect. I can successfully connect to different servers but the read/write fails since the server's corresponding tcp::socket object is not available.
Can you please suggest how I could store each server's socket in some data structure? I tried saving the IP, socket to a std::map, but the first server's socket object is not available in memory and the app crashes.
I tried making the socket static, but it does not help either.

Please help me!!

Also, I hope I am logically correct in making a single TCP client connect to 2 different servers. I am sharing below the simplified header & cpp file.

class TCPClient: public Socket
{
public:
    TCPClient(boost::asio::io_service& io_service,
        boost::asio::ip::tcp::endpoint ep);
    virtual ~TCPClient();
    void Connect(boost::asio::ip::tcp::endpoint ep, boost::asio::io_service &ioService, void (Comm::*SaveClientDetails)(std::string,void*),
        void *pClassInstance);
    
    void TransmitData(const INT8 *pi8Buffer);
    void HandleWrite(const boost::system::error_code& err, 
    size_t szBytesTransferred);
    void HandleConnect(const boost::system::error_code &err, 
        void (Comm::*SaveClientDetails)(std::string,void*),
        void *pClassInstance, std::string sIPAddr);
    static tcp::socket* CreateSocket(boost::asio::io_service &ioService)    
        {   return new tcp::socket(ioService); }
    static tcp::socket *mSocket;
private:
    std::string sMsgRead;
    INT8 i8Data[MAX_BUFFER_LENGTH];
    std::string sMsg;
    boost::asio::deadline_timer mTimer;
};

tcp::socket* TCPClient::mSocket = NULL;

TCPClient::TCPClient(boost::asio::io_service &ioService,
        boost::asio::ip::tcp::endpoint ep) :
        mTimer(ioService)
{
}

void TCPClient::Connect(boost::asio::ip::tcp::endpoint ep, 
        boost::asio::io_service &ioService, 
        void (Comm::*SaveServerDetails)(std::string,void*),
        void *pClassInstance)
{
    mSocket = CreateSocket(ioService);
    std::string sIPAddr = ep.address().to_string();
    /* To send connection request to server*/
    mSocket->async_connect(ep,boost::bind(&TCPClient::HandleConnect, this,
            boost::asio::placeholders::error, SaveServerDetails,
            pClassInstance, sIPAddr));
}

void TCPClient::HandleConnect(const boost::system::error_code &err,
        void (Comm::*SaveServerDetails)(std::string,void*),
        void *pClassInstance, std::string sIPAddr)
{
    if (!err)
    {
        Comm* pInstance = (Comm*) pClassInstance;
        if (NULL == pInstance) 
        {
            break;
        }
        (pInstance->*SaveServerDetails)(sIPAddr,(void*)(mSocket));
    }
    else
    {
        break;
    }
}

void TCPClient::TransmitData(const INT8 *pi8Buffer)
{
    sMsg = pi8Buffer;
    if (sMsg.empty()) 
    {
        break;
    }
    mSocket->async_write_some(boost::asio::buffer(sMsg, MAX_BUFFER_LENGTH),
            boost::bind(&TCPClient::HandleWrite, this,
                    boost::asio::placeholders::error,
                    boost::asio::placeholders::bytes_transferred));
}

void TCPClient::HandleWrite(const boost::system::error_code &err,
        size_t szBytesTransferred) 
{
        if (!err) 
        {
            std::cout<< "Data written to TCP Client port! ";
        } 
        else 
        {
            break;
        }
}

Answer 1

You seem to know your problem: the socket object is unavailable. That's 100% by choice. You chose to make it static, of course there will be only one instance.

Also, I hope I am logically correct in making a single TCP client connect to 2 different servers.

It sounds wrong to me. You can redefine "client" to mean something having multiple TCP connections. In that case at the very minimum you expect a container of tcp::socket objects to hold those (or, you know, a Connection object that contains the tcp::socket.

BONUS: Demo

For fun and glory, here's what I think you should be looking for.

Notes:

• no more new, delete
• no more void*, reinterpret casts (!!!)
• less manual buffer sizing/handling
• no more bind
• buffer lifetimes are guaranteed for the corresponding async operations
• message queues per connection
• connections are on a strand for proper synchronized access to shared state in multi-threading environments
• I added in a connection max idle time timeout; it also limits the time taken for any async operation (connect/write). I assumed you wanted something like this because (a) it's common (b) there was an unused deadline_timer in your question code

Note the technique of using shared pointers to have Comm manage its own lifetime. Note also that _socket and _outbox are owned by the individual Comm instance.

Live On Coliru

#include <boost/asio.hpp>
#include <deque>
#include <iostream>

using INT8 = char;
using boost::asio::ip::tcp;
using boost::system::error_code;
//using SaveFunc = std::function<void(std::string, void*)>; // TODO abolish void*
using namespace std::chrono_literals;
using duration = std::chrono::high_resolution_clock::duration;

static inline constexpr size_t MAX_BUFFER_LENGTH = 1024;

using Handle  = std::weak_ptr<class Comm>;

class Comm : public std::enable_shared_from_this<Comm> {
  public:
    template <typename Executor>
    explicit Comm(Executor ex, tcp::endpoint ep, // ex assumed to be strand
                  duration max_idle)
        : _ep(ep)
        , _max_idle(max_idle)
        , _socket{ex}
        , _timer{_socket.get_executor()}
    {
    }

    ~Comm() { std::cerr << "Comm closed (" << _ep << ")\n"; }

    void Start() {
        post(_socket.get_executor(), [this, self = shared_from_this()] {
            _socket.async_connect(
                _ep, [this, self = shared_from_this()](error_code ec) {
                    std::cerr << "Connect: " << ec.message() << std::endl;
                    if (!ec)
                        DoIdle();
                    else
                        _timer.cancel();
                });
            DoIdle();
        });
    }

    void Stop() {
        post(_socket.get_executor(), [this, self = shared_from_this()] {
            if (not _outbox.empty())
                std::cerr << "Warning: some messages may be undelivered ("
                          << _ep << ")" << std::endl;
            _socket.cancel();
            _timer.cancel();
        });
    }

    void TransmitData(std::string_view msg) {
        post(_socket.get_executor(),
             [this, self = shared_from_this(), msg = std::string(msg.substr(0, MAX_BUFFER_LENGTH))] {
                 _outbox.emplace_back(std::move(msg));

                 if (_outbox.size() == 1) { // no send loop already active?
                     DoSendLoop();
                 }
             });
    }

  private:
    // The DoXXXX functions are assumed to be on the strand
    void DoSendLoop() {
        DoIdle(); // restart max_idle even after last successful send
        if (_outbox.empty())
            return;

        boost::asio::async_write(
            _socket, boost::asio::buffer(_outbox.front()),
            [this, self = shared_from_this()](error_code ec, size_t xfr) {
                std::cerr << "Write " << xfr << " bytes to " << _ep << " " << ec.message() << std::endl;
                if (!ec) {
                    _outbox.pop_front();
                    DoSendLoop();
                } else
                    _timer.cancel(); // causes Comm shutdown
            });
    }

    void DoIdle() {
        _timer.expires_from_now(_max_idle); // cancels any pending wait
        _timer.async_wait([this, self = shared_from_this()](error_code ec) {
            if (!ec) {
                std::cerr << "Timeout" << std::endl;
                _socket.cancel();
            }
        });
    }

    tcp::endpoint                      _ep;
    duration                           _max_idle;
    tcp::socket                        _socket;
    boost::asio::high_resolution_timer _timer;
    std::deque<std::string>            _outbox;
};

class TCPClient {
    boost::asio::any_io_executor _ex;
    std::deque<Handle>           _comms;

  public:
    TCPClient(boost::asio::any_io_executor ex) : _ex(ex) {}

    void Add(tcp::endpoint ep, duration max_idle = 3s)
    {
        auto pcomm = std::make_shared<Comm>(make_strand(_ex), ep, max_idle);
        pcomm->Start();
        _comms.push_back(pcomm);

        // optionally garbage collect expired handles:
        std::erase_if(_comms, std::mem_fn(&Handle::expired));
    }

    void TransmitData(std::string_view msg) {
        for (auto& handle : _comms)
            if (auto pcomm = handle.lock())
                pcomm->TransmitData(msg);
    }

    void Stop() {
        for (auto& handle : _comms)
            if (auto pcomm = handle.lock())
                pcomm->Stop();
    }
};

int main() {
    using std::this_thread::sleep_for;

    boost::asio::thread_pool ctx(1);
    TCPClient                c(ctx.get_executor());

    c.Add({{}, 8989});
    c.Add({{}, 8990}, 1s); // shorter timeout for demo

    c.TransmitData("Hello world\n");

    c.Add({{}, 8991});

    sleep_for(2s); // times out second connection

    c.TransmitData("Three is a crowd\n"); // only delivered to 8989 and 8991

    sleep_for(1s); // allow for delivery

    c.Stop();
    ctx.join();
}

Prints (on Coliru):

for p in {8989..8991}; do netcat -t -l -p $p& done
sleep .5; ./a.out
Hello world
Connect: Success
Connect: Success
Hello world
Connect: Success
Write 12 bytes to 0.0.0.0:8989 Success
Write 12 bytes to 0.0.0.0:8990 Success
Timeout
Comm closed (0.0.0.0:8990)
Write Three is a crowd
17Three is a crowd
 bytes to 0.0.0.0:8989 Success
Write 17 bytes to 0.0.0.0:8991 Success
Comm closed (0.0.0.0:8989)
Comm closed (0.0.0.0:8991)

The output is a little out of sequence there. Live local demo: