[range.range] (original) (raw)

25 Ranges library [ranges]

25.4 Range requirements [range.req]

25.4.2 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.

template<class T> concept [range](#concept:range "25.4.2 Ranges [range.range]") = requires(T& t) { ranges::begin(t); // sometimes equality-preserving (see below) ranges::end(t);};

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

• [ranges​::​begin(t), ranges​::​end(t)) denotes a range ([iterator.requirements.general]),
• bothranges​::​begin(t)andranges​::​end(t)are amortized constant time and non-modifying, and
• if the type of ranges​::​begin(t) modelsforward_iterator, ranges​::​begin(t) is equality-preserving.

[Note 1:

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

template<class T> concept [borrowed_range](#concept:borrowed%5Frange "25.4.2 Ranges [range.range]") = [range](#concept:range "25.4.2 Ranges [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 uis 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_]

template<class> constexpr bool enable_borrowed_range = false;

Remarks: Pursuant to [namespace.std], users may specialize enable_borrowed_rangefor 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

• enable_borrowed_range<S> is specialized to have the value true, and
• 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_]