mdspan_formatter

Make std::mdspan (multi-dimensional span; currently Kokkos::mdspan) formattable using std::format or fmt::format.

Usage

#include <print>

#include <mdspan_formatter.hpp>

int main(int, char**) {
    std::array nums { 1.f, 2.f, 3.f, 4.f };
    std::println("{}", std::mdspan { nums.data(), 4, 1 });
    /**
     * Output:
     * 
     * [[1],
     *  [2],
     *  [3],
     *  [4]]
    */

    std::println("{:: >4.1f}", std::mdspan<float, std::extents<std::size_t, 2, 2>> { nums.data() });
    /**
     * Output:
     * 
     * [[ 1.0,  2.0],
     *  [ 3.0,  4.0]]
    */

    std::string magic_square =
        "SATOR"
        "AREPO"
        "TENET"
        "OPERA"
        "ROTAS";
    std::println("{:s}", std::mdspan { magic_square.data(), 5, 5 });
    /**
     * Output:
     * 
     * [SATOR,
     *  AREPO,
     *  TENET,
     *  OPERA,
     *  ROTAS]
    */

    return 0;
}

Or you can use the library with fmt by header file mdspan_fmt_formatter.hpp.

#include <mdspan_fmt_formatter.hpp>

int main(int, char**) {
    std::array ints { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12 };
    std::mdspan<int, std::dextents<std::size_t, 3>, std::layout_left> int2x2x3 { ints.data(), 2, 2, 3 };
    
    fmt::println("{}", int2x2x3);
    /**
     * Output:
     * 
     * [[[1, 2],
     *   [3, 4]],
     *  [[5, 6],
     *   [7, 8]],
     *  [[9, 10],
     *   [11, 12]]]
     */
    
    return 0;
}

It supports C++23's range formatting specifier and left spacing for rank ≥ 2 span. For fixed size mdspan (whose extent is known at compile time), formatting is optimized using constexpr declared variables.

It is tested with μt.

Note that it only supports mdspan with default_accessor accessor.

How to use

include(FetchContent)
FetchContent_Declare(
    mdspan_formatter
    GIT_REPOSITORY https://github.com/stripe2933/mdspan_formatter.git
    GIT_TAG main
)
FetchContent_MakeAvailable(mdspan_formatter)

# Now you can link the library like:
# target_link_libraries(${PROJECT_NAME} PRIVATE mdspan_formatter)

Since it depends on range formatter, you need at least C++23 compatible compiler. if you want to use the library with std::format. Currently only Clang with libc++ ≥ 16 supports it (experimental support for libc++16, stable support for libc++ ≥ 17). GCC and MSVC are not tested yet.

For use library with fmt, Clang ≥ 11.0.0 and GCC ≥ 10.2 with std ≥ c++20 condition are tested. MSVC is not tested yet. Try on Compiler Explorer.

---

First, you need Kokkos's mdspan, which is reference implementation of std::mdspan because none of compiler implemented submdspan at now.

If you want to build the test, download the repository and build CMake project.