Linux C++ UDP multicast example, guidance here: https://www.tenouk.com/Module41c.html

multicast-rcv.cpp

// Guidance:  https://www.tenouk.com/Module41c.html
#include <arpa/inet.h>
#include <array>
#include <iostream>
#include <netinet/in.h>
#include "udpSock.hpp"
#include "zz_diagnose.hpp"

int main()
{
  UdpSock sock;
  diagnose(sock.isGood(), "Opening datagram socket for receive");

  {
    // enable SO_REUSEADDR to allow multiple instances of this application to
    //    receive copies of the multicast datagrams.
    int reuse = 1;
    diagnose(setsockopt(sock(), SOL_SOCKET, SO_REUSEADDR, (char*)&reuse,
                        sizeof(reuse)) >= 0, "Setting SO_REUSEADDR");
  }

  // Bind to the proper port number with the IP address specified as INADDR_ANY
  sockaddr_in localSock = {};    // initialize to all zeroes
  localSock.sin_family      = AF_INET;
  localSock.sin_port        = htons(4321);
  localSock.sin_addr.s_addr = INADDR_ANY;
  // Note from manpage that bind returns 0 on success
  diagnose(!bind(sock(), (sockaddr*)&localSock, sizeof(localSock)),
           "Binding datagram socket");

  // Join the multicast group on the local interface.  Note that this
  //    IP_ADD_MEMBERSHIP option must be called for each local interface over
  //    which the multicast datagrams are to be received.
  ip_mreq group = {};    // initialize to all zeroes
  group.imr_multiaddr.s_addr = inet_addr("226.1.1.1");
  group.imr_interface.s_addr = inet_addr("127.0.0.1");
  diagnose(setsockopt(sock(), IPPROTO_IP, IP_ADD_MEMBERSHIP, (char*)&group,
                      sizeof(group)) >= 0, "Adding multicast group");

  // Read from the socket
  std::array<char, 1024> arr;
  arr.fill(0);
  diagnose(read(sock(), arr.data(), arr.size()) >= 0,
           "Reading datagram message");
  std::cout << "Message from multicast sender: " << arr.data()
            << std::endl;
  return 0;
}

multicast-send.cpp

// Guidance:  www.tenouk.com/Module41c.html
#include <arpa/inet.h>
#include <netinet/in.h>
#include <sys/socket.h>
#include <sys/types.h>
#include "udpSock.hpp"
#include "zz_diagnose.hpp"

int main()
{
  UdpSock sock;
  diagnose(sock.isGood(), "Opening datagram socket for send");

  sockaddr_in groupSock = {};   // init to all zeroes
  groupSock.sin_family = AF_INET;
  groupSock.sin_addr.s_addr = inet_addr("226.1.1.1");
  groupSock.sin_port = htons(4321);

  in_addr localIface = {};   // init to all zeroes
  localIface.s_addr = inet_addr("127.0.0.1");
  diagnose(setsockopt(sock(), IPPROTO_IP, IP_MULTICAST_IF, (char*)&localIface,
                      sizeof(localIface)) >= 0, "Setting local interface");

  const std::string databuf = "Multicast from C++";
  diagnose(sendto(sock(), databuf.c_str(), databuf.length(), 0,
                  (sockaddr*)&groupSock, sizeof(groupSock)) >= 0,
           "Sending datagram message");

  return 0;
}

udpSock.hpp

#include <sys/socket.h>
#include <unistd.h>

class UdpSock {
  public:
    UdpSock() {
      m_sd = socket(AF_INET, SOCK_DGRAM, 0);
    }
    ~UdpSock() {
      // make sure the socket resource doesn't leak
      if (isGood())
        close(m_sd);
    }
    // Don't need the other default operations
    UdpSock(const UdpSock&) = delete;
    UdpSock& operator=(const UdpSock&) = delete;
    UdpSock(UdpSock&&) = delete;
    UdpSock& operator=(UdpSock&&) = delete;

    int operator()() const {
      return m_sd;
    }

    bool isGood() const {
      return m_sd >= 0;
    }
  private:
    int m_sd = -1;
};

zz_diagnose.hpp

// unusual file name is a hack to force GitHub Gist not to put this file at the top
#include <iostream>
#include <stdlib.h>
#include <string>

void diagnose(bool isGood, const std::string& action)
{
  if (isGood)
    std::cout << action.c_str() << " OK" << std::endl;
  else
  {
    std::cerr << action.c_str() << " error" << std::endl;
    exit(1);
  }
}