Hello,
I have one more question/comment about array_slice. Ever since I
started to use it...
On Fri, Nov 13 2020, Richard Sandiford via Gcc-patches wrote:
> A later patch wants to be able to pass around subarray views of an
> existing array. The standard class to do that is std::span, but it's
> a C++20 thing. This patch just adds a cut-down version of it.
>
> The intention is just to provide what's currently needed.
>
> gcc/
> * vec.h (array_slice): New class.
> ---
> gcc/vec.h | 120 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 120 insertions(+)
>
> diff --git a/gcc/vec.h b/gcc/vec.h
> index f02beddc975..7768de9f518 100644
> --- a/gcc/vec.h
> +++ b/gcc/vec.h
> @@ -2128,6 +2128,126 @@ release_vec_vec (vec<vec<T> > &vec)
> vec.release ();
> }
>
> +// Provide a subset of the std::span functionality. (We can't use std::span
> +// itself because it's a C++20 feature.)
> +//
> +// In addition, provide an invalid value that is distinct from all valid
> +// sequences (including the empty sequence). This can be used to return
> +// failure without having to use std::optional.
> +//
> +// There is no operator bool because it would be ambiguous whether it is
> +// testing for a valid value or an empty sequence.
> +template<typename T>
> +class array_slice
> +{
> + template<typename OtherT> friend class array_slice;
> +
> +public:
> + using value_type = T;
> + using iterator = T *;
> + using const_iterator = const T *;
> +
> + array_slice () : m_base (nullptr), m_size (0) {}
> +
> + template<typename OtherT>
> + array_slice (array_slice<OtherT> other)
> + : m_base (other.m_base), m_size (other.m_size) {}
> +
> + array_slice (iterator base, unsigned int size)
> + : m_base (base), m_size (size) {}
> +
> + template<size_t N>
> + array_slice (T (&array)[N]) : m_base (array), m_size (N) {}
> +
> + template<typename OtherT>
> + array_slice (const vec<OtherT> &v)
> + : m_base (v.address ()), m_size (v.length ()) {}
> +
> + iterator begin () { return m_base; }
> + iterator end () { return m_base + m_size; }
> +
> + const_iterator begin () const { return m_base; }
> + const_iterator end () const { return m_base + m_size; }
> +
> + value_type &front ();
> + value_type &back ();
> + value_type &operator[] (unsigned int i);
> +
> + const value_type &front () const;
> + const value_type &back () const;
> + const value_type &operator[] (unsigned int i) const;
> +
> + size_t size () const { return m_size; }
...this has been a constant source of compile errors, because vectors
have length () and this is size ().
I understand that the motivation was consistency with std::span, but do
we really want to add another inconsistency with ourselves?
Given that array_slice is not that much used yet, I believe we can still
change to be consistent with vectors. I personally think we should but
at the very least, if we keep it as it is, I'd like us to do so
deliberately.
Thanks,
Martin
