I have got the reply to that question here: http://stackoverflow.com/questions/8482445/how-to-specify-degenerate-dimension-of-boost-multi-array-at-runtime

On Tue, Dec 13, 2011 at 12:24 AM, Anton Daneyko <self@mezhaka.com> wrote:
Greetings to Boosters,

I have a 3D multi_array and I would like to make 2D slices using dimensions specified at runtime. I know the index of degenerate dimension and an index of slice, that I want to extract in that degenerate dimension. Currently the ugly workaround looks like that:
    if (0 == degenerate_dimension)
        Slice slice = input_array[boost::indices[slice_index][range()][range()]];
    else if (1 == degenerate_dimension)
        Slice slice = input_array[boost::indices[range()][slice_index][range()]];
    else if (2 == degenerate_dimension)
        Slice slice = input_array[boost::indices[range()][range()][slice_index]];
Is there a more beautiful way to construct index_gen object?
Something like that:
    var slicer;
    for(int i = 0; i < 3; ++i) {
        if (degenerate_dimension == i)
            slicer = boost::indices[slice_index];
            slicer = boost::indices[range()];
    Slice slice = input_array[slicer];

It seems each subsequent call to boost::indices::operator[] returns a different type depending on the dimensionality, so there's no way to use a single variable, that can hold the temporary index_gen object.