Can I get a const slice_array from a const valarray?

Question

I have a function that accepts a const reference to a valarray, and I want to be able to slice the array and pass the slice into another function that expects a const slice_array. I know that I can just use operator[] and a slice to get a new, copied valarray out of the original valarray, but I want to avoid the copy. Everything is const, so I feel like that should be okay. However, the documentation for the subscript operator for valarray ONLY returns a slice_array objects when applied to non-const valarrays. This feels kind of a major deficiency in the valarray class. Am I missing something? How do I get a slice of a const valarray without incurring a copy?

Here is an example of what I'm talking about:

void foo( const valarray<double> &big_stuff )
{
    const slice_array<double> small_stuff = big_stuff[slice(start, size, stride)];
    bar( small_stuff );
}

Thanks!

Answer 1

How do I get a slice of a const valarray without incurring a copy?

There is no way to do this, because of std::valarray not contain std::slice_array for any request, so it cat not give you just link(pointer,reference) to std::slice_array that you request.

But std::slice_array can contains only three machine words, hard to imagine situation where not copy of std::slice_array can optimize something. For example from gcc/libstdc++ (only data, functions stripped):

 template<typename _Tp>
  class slice_array {
    const size_t      _M_sz;
    const size_t      _M_stride;
    const _Array<_Tp> _M_array;
  };

where _Array:

template<typename _Tp>
  struct _Array { _Tp* const __restrict__ _M_data; };