[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());  // interators to rows

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

    assert(A.elements().size() == 2 * 3);  // array has 6 elements
    assert(A.elements()[4] == 5);          // 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)

Learn about Multi

• Online documentation

Try Multi

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>,

Install Multi

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)

Support

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