⚠️ ALERT This library is under acitive Boost review until March 15, 2026.
If you are interested in reviewing the library, please send an email to the review manager, Matt Borland (matt AT mattborland DOT com).

[Boost.] Multi

Disclosure: This is not an official or accepted Boost library and is unrelated to the std::mdspan proposal. It is in the process of being proposed for inclusion in Boost and it doesn't depend on Boost libraries.

© Alfredo A. Correa, 2018-2026

Multi is a modern C++ library that provides manipulation and access of data in multidimensional arrays for both CPU and GPU memory.

#include <cassert>          // for assert
#include <multi/array.hpp>  // from https://gitlab.com/correaa/boost-multi or https://gitlab.com/correaa/boost-multi

namespace multi = boost::multi;

int main() {
    multi::array<int, 2> A = {  // 2D array of integers
        {1, 2, 3},
        {4, 5, 6}
    };

    assert(A.size() == 2);                    // the array has 2 rows
    assert(A.size() == A.end() - A.begin());  // iterators to rows

    assert(A[1][1] == 5);  // element access through indexing
    A[1][1] = 55;          // element modified

    assert(A.elements().size() == 2 * 3);  // array has 6 elements
    assert(A.elements()[4] == 55);          // elements gives "flat" sequences

    using multi::_;  // wildcard

    auto&& col1 = A(_, 1);  // second column
    assert( col1.size() == 2 );

    auto&& row1 = A(1, _);  // second row
    assert( row1.size() == 3 );

    auto&& block = A({0, 2}, {0, 2});  // a 2x2 block
    assert( block.elements().size() == 4 );  
}

(online)

• Online documentation

Before installing the library, you can try it online through the Godbolt's Compiler Explorer.

Alternatively, the core of the library can be downloaded from https://correaa.gitlab.io/boost-multi/boost-multi.hpp as a single (amalgamated) header, and used locally with #include <DIR/boost-multi.hpp>,

Multi has no external dependencies and can be used immediately after downloading.

git clone https://gitlab.com/correaa/boost-multi.git

Multi doesn't require installation since it is a header-only library, and including the main header brings the code library:

// c++ -I multi_root_dir/include main.cpp
#include <boost/multi.hpp>

int main() { ... }

The library can be also installed with CMake. The header (and CMake) files will be installed in the chosen prefix location (by default, /usr/local/include/multi and /usr/local/share/multi).

cd boost-multi
mkdir -p build && cd build
cmake . -B ./build  # --install-prefix=$HOME/.local
cmake --install ./build  # or sudo ...

Testing the library requires Boost.Core (headers), installed for example, via sudo apt install cmake git g++ libboost-test-dev make or sudo dnf install boost-devel cmake gcc-c++ git. A CMake build system is provided to compile and run basic tests.

ctest -C ./build

Once installed, other CMake projects (targets) can depend on Multi by adding a simple add_subdirectory(my_multi_path) or by find_package:

find_package(multi)  # see https://gitlab.com/correaa/boost-multi

Alternatively, the library can be fetched on demand:

include(FetchContent)
FetchContent_Declare(multi GIT_REPOSITORY https://gitlab.com/correaa/boost-multi.git)
FetchContent_MakeAvailable(multi)
...
target_link_libraries(my_target PUBLIC multi)

• File a Gitlab issue or Github issue.
• Join the #boost-multi discussion group at cpplang.slack.com