Richard Biener <richard.guent...@gmail.com> writes:
> On Wed, Aug 10, 2022 at 6:04 PM Martin Jambor <mjam...@suse.cz> wrote:
>>
>> 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.
>
> We could alternatively add length in addition to size (and maybe size to
> vec<> if std::vector has size but not length) with a comment deprecating
> the "non-standard" variant?
Yeah, I'd prefer to do the latter: add vec::size as a synonym of
vec::length, and deprecate length. Doing anything else seems like
it's going to increase the inconsistency rather than decrease it.
E.g. we already have uses of (hopefully) uncontroversial standard
containers like std::array (my fault).
(FWIW, I keep tripping up in the opposite direction: expecting
size to be available in vec, like for standard containers.)
Thanks,
Richard
