Send a c++ std::vector<bool> via mpi

Question

I know that the storage of a std::vector<bool> is not necessarily an array of bools .

If I want to send receive int data stored in a std::vector<int> , I would use MPI_Send(vect.data(),num_of_ints,MPI_INT,dest_rk,tag,comm) .

How should I use MPI_Send to send a std::vector<bool> ? In particular :

• Can / should I use vect.data() as the pointer to buffer ?
• What MPI type should I give ? Somehow, I feel like MPI_CXX_BOOL does not apply (see this question )
• What number of elements should I give ? (related to the previous point)

Answer 1

std::vector<bool> specialization does not have the data() member function. The underlying storage scheme is not specified by the standard:

There is no requirement that the data be stored as a contiguous allocation of bool values. A space-optimized representation of bits is recommended instead.

The only reasonable option to send std::vector<bool> is to copy it into a vector of char (or some similar type like std::int8_t ), and then send that vector. A better option might be to avoid std::vector<bool> in your code from the very beginning.