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3381. begin and data must agree for contiguous_range

Section: 25.4.5 [range.refinements] Status: C++20 Submitter: Casey Carter Opened: 2020-01-25 Last modified: 2021-02-25

Priority: 0

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Discussion:

The definition of the contiguous_range concept in 25.4.5 [range.refinements]/2 requires that ranges::data(r) be valid for a contiguous_range r, but fails to impose the obvious semantic requirement that to_address(ranges::begin(r)) == ranges::data(r). In other words, data and begin must agree so that [begin(r), end(r)) and the counted range data(r) + [0, size(r)) (this is the new "counted range" specification syntax per working draft issue 2932) denote the same sequence of elements.

[2020-02 Prioritized as IMMEDIATE Monday morning in Prague]

Proposed resolution:

This wording is relative to N4849.

1. Modify 25.4.5 [range.refinements] as indicated:

   -2- contiguous_range additionally requires that the ranges::data customization point (25.3.13 [range.prim.data]) is usable with the range.

   template<class T>
     concept contiguous_range =
       random_access_range<T> && contiguous_iterator<iterator_t<T>> &&
       requires(T& t) {
         { ranges::data(t) } -> same_as<add_pointer_t<range_reference_t<T>>>;
       };
   

   -?- Given an expression t such that decltype((t)) is T&, T models contiguous_range only if (to_address(ranges::begin(t)) == ranges::data(t)).

   -3- The common_range concept […]