Re: Multi Mini-Review Begins Today

On Fri, 7 Aug 2026 at 14:01, Matt Borland via Boost
<[email protected]> wrote:
>
> Hello,
>
> Today begins the mini-review of Multi, authored by Alfredo Correa. Multi is a modern C++ library that provides manipulation and access of data in multidimensional arrays for both CPU and GPU memory. The review will run through Wednesday 12 August.
>
> You can find the library and documentation links below:
> - https://github.com/correaa/boost-multi
> - https://correaa.github.io/boost-multi/multi/intro.html
>
> The review earlier this year raised a number of issues that primarily revolved around the state of the documentation, not the library itself. I have labeled all of the issues related to conditions found in last review, and many have notes or resolution from Alfredo:



Hi all,

This is my re-review of Boost.Multi. I'll be checking
the points that I raised during my previous review [1]
and a couple more new findings.

* The discussion has improved since last review.
  The page on concepts [2] is worth reading.
  Although it is called "reference", but it is not.
* The reference section [3] needs a lot more work.
  The front page needs a table organizing the entities
  (see how Boost.Url does it, for example [4]).
  There must be no entries with blank documentation.
  All adapters are also missing there.
* I like seeing std::mdspan compatibility through multi::array::to_mdspan(),
  but the function is absent from the documentation. There is an example in
  Godbolt about it that doesn't compile.
* Examples have no comments yet, and are still under examples/ (rather
than example/),
  and aren't built by CI.
* The operator& overloads are still there, and subarrays still don't work
  with fmtlib (the example [5] doesn't built in my machine).
* strided() with a stride not divisible by the corresponding dimension
  causes an infinite loop in my machine. I was trying to see if Asan complained
  about the invalid pointer formation issue [6]. It doesn't, but the code
  misbehaves. The following causes an infinite loop:

#include <boost/multi/array.hpp>

namespace multi = boost::multi;

int main() {
multi::array<double, 2> arr = {
{1.0, 2.0, 3.0},
{3.0, 4.0, 5.0},
{6.0, 7.0, 8.0},
};

for(auto&& elm : arr.strided(2)) {
std::cout << elm.size() << std::endl;
}
}

* strided() with negative indices and dimensionality greater than 1
  causes an infinite loop, too:

#include <boost/multi/array.hpp>

#include <iostream>

namespace multi = boost::multi;

int main() {
multi::array<double, 2> arr = {
{1.0, 2.0, 3.0,  4.0},
{3.0, 4.0, 5.0,  8.0},
{6.0, 7.0, 8.0, 10.0},
};

for(auto&& subarr : arr.strided(-1))
std::cout << subarr.size() << std::endl;
}

* Uninitialized elements still happen by default.
  The library has gained a trait to treat std::complex
  as trivial:

template<> inline constexpr
bool multi::force_element_trivial_default_construction<std::complex<double>>
= true;  // should be defined as early as possible

This is only well-defined in C++20 (and UB in C++17). It is also
prone to ODR violations, especially since the specializations are controlled
by a macro definition.

* The include/ directory still has lots of files that don't
  belong there.
* There are still many #if 0 preprocessor blocks.
* The NOEXCEPT_ASSIGNMENT macro is still there.
* The min and max functions are still unguarded.
* Many tests are still missing from Jamfile.
* The dynamic_array_cast leftover is still there.

I feel the library is at a similar point as last time I looked at it, so
I keep my original vote: REJECT with the encouragement of a re-review,
once the feedback is incorporated into it.

Regards,
Ruben.

[1] https://lists.boost.org/archives/list/[email protected]/message/Q4GLGQVJ26LVQE5INMF73PKBSJ67FUCH/
[2] https://correaa.github.io/boost-multi/multi/reference.html
[3] https://correaa.github.io/boost-multi/multi/reference/index.html
[4] https://www.boost.org/doc/libs/latest/doc/antora/url/reference.html
[5] https://correaa.github.io/boost-multi/multi/interop.html#interop_fmt
[6] https://correaa.github.io/boost-multi/multi/technical.html#technical_iteration
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