26 Ranges library [ranges]

26.4 Range requirements [range.req]

26.4.2 Ranges [range.range]

The range concept defines the requirements of a type that allows iteration over its elements by providing an iterator and sentinel that denote the elements of the range.

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template<class T>
  concept range =
    requires(T& t) {
      ranges::begin(t);         // sometimes equality-preserving (see below)
      ranges::end(t);
    };

The required expressions ranges::begin(t) and ranges::end(t) of the range concept do not require implicit expression variations ([concepts.equality]).

Given an expression t such that decltype((t)) is T&, T models range only if

(3.1)
[ranges::begin(t), ranges::end(t)) denotes a range ([iterator.requirements.general]),
(3.2)
both ranges::begin(t) and ranges::end(t) are amortized constant time and non-modifying, and
(3.3)
if the type of ranges::begin(t) models forward_iterator, ranges::begin(t) is equality-preserving.

[Note 1:

Equality preservation of both ranges::begin and ranges::end enables passing a range whose iterator type models forward_iterator to multiple algorithms and making multiple passes over the range by repeated calls to ranges::begin and ranges::end.

Since ranges::begin is not required to be equality-preserving when the return type does not model forward_iterator, it is possible for repeated calls to not return equal values or to not be well-defined.

— end note]

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template<class T>
  concept borrowed_range =
    range<T> && (is_lvalue_reference_v<T> || enable_borrowed_range<remove_cvref_t<T>>);

Let U be remove_reference_t<T> if T is an rvalue reference type, and T otherwise.

Given a variable u of type U, T models borrowed_range only if the validity of iterators obtained from u is not tied to the lifetime of that variable.

[Note 2:

Since the validity of iterators is not tied to the lifetime of a variable whose type models borrowed_range, a function with a parameter of such a type can return iterators obtained from it without danger of dangling.

— end note]

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template<class>
  constexpr bool enable_borrowed_range = false;

Remarks: Pursuant to [namespace.std], users may specialize enable_borrowed_range for cv-unqualified program-defined types.

Such specializations shall be usable in constant expressions ([expr.const]) and have type const bool.

[Example 1:

Each specialization S of class template subrange ([range.subrange]) models borrowed_range because

(8.1)
enable_borrowed_range<S> is specialized to have the value true, and
(8.2)
S's iterators do not have validity tied to the lifetime of an S object because they are “borrowed” from some other range.

— end example]