Express stride in bytes in boost::multi_array

Question

I have a two-dimensional native C array which was read as shorts, and I wish to map a boost::multi_array_ref onto part of it, but express this as floats. Is there a way to set the stride in bytes (rather than multiples of the data size)?

What do you want to achieve? Type punning like that is undefined behaviour unless you employ unions and adhere to the laws governing them. I suppose you really want to just convert the data from short to floats? — sehe, May 01 '12 at 20:48
Indeed - the data is stored on disk as shorts, but we are deprecating that and operating on floats, so we have a conversion from short to float. The data is not even homogenous, and it would have been nice to start with a byte array, then create a reference to part of it as a view ref of floats, and another part as a view ref of shorts. — DAmann, May 10 '12 at 20:12

score 0 · Answer 1 · answered Feb 27 '23 at 07:05

It is really too much to ask for an array library to support strides in bytes. Even if the alignment problems are somehow solved, there are still a lot of problems with manipulating subviews and pointer arithmetic.

If you really need to manipulate byte strides, I recommend having an array of chars (or std::byte) and only leaving the reinterpretation of the bytes for the end.

This can be done with Boost.MultiArray, but it is easier if you use a library that does the reinterpret dirty work for you. For example Multi

#include <multi/array.hpp>  // from https://gitlab.com/correaa/boost-multi/-/blob/master/test/element_transformed.cpp

namespace multi = boost::multi;

int main() { 

    auto n = 4, m = 3;

    std::vector<float> fake_data(n*m);
    fake_data[0] = 1.0F;
    fake_data[1] = 2.0F;

    auto custom_stride = sizeof(float);  // can be different for disk-encoded data

    multi::array_ref<char, 2> A({n, m*custom_stride}, reinterpret_cast<char*>(fake_data.data()));

    auto const& B = A.rotated().strided(custom_stride).unrotated();  // create a strided by on CHAR!

    auto [Bn, Bm] = sizes(B);
    assert( Bn == 4 and Bm == 3 );

    {  // you can do this with any library. Boost.MultiArray for example
        assert(  reinterpret_cast<float const&>(B[0][1]) ==  fake_data[1]  );

        assert( &reinterpret_cast<float const&>(B[0][1]) == &fake_data[1]  );
    }
    {  // with this library you can make it a bit more automatic
        auto const& C = B.element_transformed([](auto const& c) -> decltype(auto) {return reinterpret_cast<float const&>(c);});
    
        auto [Cn, Cm] = sizes(C);
        assert( Cn == 4 and Cm == 3 );

        assert(  C[0][1] ==  fake_data[1]  );

        assert( &C[0][1] == &fake_data[1]  );
    }
}

https://godbolt.org/z/hnjxor19T

Express stride in bytes in boost::multi_array

1 Answers1