Support for C++20 ranges

[rabauke]

Support for C++20 ranges

[rabauke]

@mlund: Thanks for suggestion. I would like to point out that allowing begin and end iterators of different types will not suffice to support ranges. Ranges support is probably not so easy to implement.

The ranges library implements lazy evaluation. The underlying MPI, however, is not compatible with lazy evaluation. In particular, when views::transform is involved the range must be evaluated, e.g., by ranges::to_vector, before the data can be sent. In some situations, evaluating the range might be a unnecessary and lead to avoidable performance degradations. Furthermore, the usage of the ranges library involves typically anonymous temporary objects, which imposes further challenges for non-blocking communication.

Also note that the MPL documentation warns that sending STL containers via iterators "is a convenience method, which may have non-optimal performance characteristics. Use alternative overloads in performance critical code sections."

Ranges support has no high priority for me. Nevertheless, it should be considered when C++21 compilers have become more widespread. Ranges support will certainly not rely on an iterator interface. It should be possible to pass a range directly as, for example, in std::ranges::sort.

[mlund]

Thanks for the feedback. Yes, I understand the woes of sending non-contiguous objects and hopefully the explicit conversion to STL containers will make it into C++23.