c++ - What is the purpose of boost mpi request's m_handler -

i trying test mpi request if done or not. however, there problem not figure out. if use test_all method below, see request not done.

string msg; boost::mpi::request req = world->irecv(some_rank, 0, msg);  vector<boost::mpi::request> waitingrequests; waitingrequests.push_back(req); if(boost::mpi::test_all(waitingrequests.begin(), waitingrequests.end()))     cout << "test_all done" << endl; 

when try code, see request done:

string msg; boost::mpi::request req = world->irecv(some_rank, 0, msg);  if(req.test())     cout << "test done" << endl; 

so, looked @ code in test_all function , realized returns false because of condition "first->m_handler" (line 5 in below code).

template<typename forwarditerator> bool test_all(forwarditerator first, forwarditerator last) {     std::vector<mpi_request> requests;     (; first != last; ++first) {     // if have non-trivial request, no requests can completed.         if (first->m_handler || first->m_requests[1] != mpi_request_null)             return false;          requests.push_back(first->m_requests[0]);     }      int flag = 0;     int n = requests.size();     boost_mpi_check_result(mpi_testall,                       (n, &requests[0], &flag, mpi_statuses_ignore));     return flag != 0; } 

now, wonder m_handler for.

mpi not support intrinsicly complex c++ objects such std::string. that's why boost.mpi serialises , correspondingly deserialises such objects when passing them around in form of mpi messages. semantic point of view, non-blocking operation started irecv() should complete once data has been received , std::string object has been filled in appropriately. additional step of processing received message , deserialising performed special handler method, pointer stored in m_handler variable:

... if (m_handler) {   // request receive serialized type. use   // handler test completion.   return m_handler(this, ra_test); } else ... 

no such handling needed simple datatypes.

the same applies isend() when operates on c++ objects. in case handler not attached, class data sent in form of 2 separate messages , special care taken both sends complete. that's second boolean expression (m_requests[1] != mpi_request_null) for.