TCPView like example. It only displays IP4 TCP connections but you can extend it easily.

tcpview.cpp

#include <Windows.h>
#include <iphlpapi.h>
#include <cstdio>

#pragma comment(lib, "iphlpapi.lib")

static void Ip4ToString(DWORD ip4, char ip4_string[16])
{
  if (ip4 == 0)
  {
    std::snprintf(ip4_string, 16, "0.0.0.0");
  }
  else
  {
    const int n0 = (ip4) & 0xff;
    const int n1 = (ip4 >> 8) & 0xff;
    const int n2 = (ip4 >> 16) & 0xff;
    const int n3 = (ip4 >> 24) & 0xff;

    std::snprintf(ip4_string, 16, "%d.%d.%d.%d", n0, n1, n2, n3);
  }
}

static const char* TcpStatusToString(DWORD status)
{
  switch (status)
  {
  case MIB_TCP_STATE_CLOSED:
    return "CLOSED";
  case MIB_TCP_STATE_LISTEN:
    return "LISTEN";
  case MIB_TCP_STATE_SYN_SENT:
    return "SYN_SENT";
  case MIB_TCP_STATE_SYN_RCVD:
    return "SYN_RECEIVED";
  case MIB_TCP_STATE_ESTAB:
    return "ESTABLISHED";
  case MIB_TCP_STATE_FIN_WAIT1:
  case MIB_TCP_STATE_FIN_WAIT2:
    return "FIN_WAIT1";
  case MIB_TCP_STATE_CLOSE_WAIT:
    return "CLOSE_WAIT";
  case MIB_TCP_STATE_CLOSING:
    return "CLOSING";
  case MIB_TCP_STATE_LAST_ACK:
    return "LAST_ACK";
  case MIB_TCP_STATE_TIME_WAIT:
    return "TIME_WAIT";
  case MIB_TCP_STATE_DELETE_TCB:
    return "DELETE_TCB";
  default:
    break;
  }

  return "";
}

int main()
{
  const auto heap = GetProcessHeap();

  DWORD buffer_size = sizeof(MIB_TCPTABLE_OWNER_PID);
  auto buffer = HeapAlloc(heap, HEAP_GENERATE_EXCEPTIONS, buffer_size);
  DWORD result;

  while ((result = GetExtendedTcpTable(buffer, &buffer_size, TRUE, AF_INET, TCP_TABLE_OWNER_PID_ALL, 0)) ==
    ERROR_INSUFFICIENT_BUFFER)
  {
    buffer = HeapReAlloc(heap, 0, buffer, buffer_size);
  }

  if (result == NO_ERROR)
  {
    const auto mib_tcp_table = reinterpret_cast<const MIB_TCPTABLE_OWNER_PID *>(buffer);

    for (DWORD i = 0; i < mib_tcp_table->dwNumEntries; ++i)
    {
      const auto mib_tcp_table_row = &mib_tcp_table->table[i];

      char ip4_address_string[16]{0};
      Ip4ToString(mib_tcp_table_row->dwLocalAddr, &ip4_address_string[0]);

      std::printf("%-10lu TCP %12s %16s:%-5d",
                  mib_tcp_table_row->dwOwningPid,
                  TcpStatusToString(mib_tcp_table_row->dwState),
                  ip4_address_string,
                  mib_tcp_table_row->dwLocalPort);


      if (mib_tcp_table_row->dwState != MIB_TCP_STATE_LISTEN)
      {
        Ip4ToString(mib_tcp_table_row->dwRemoteAddr, &ip4_address_string[0]);
        std::printf(" <-> %s:%-5d", ip4_address_string, mib_tcp_table_row->dwRemotePort);
      }

      std::printf("\n");
    }
  }

  HeapFree(heap, 0, buffer);
}

_byteswap_ushort(mib_tcp_table_row->dwLocalPort);
_byteswap_ushort(mib_tcp_table_row->dwRemotePort);