[range.req]

26.4.1 General [range.req.general]

Ranges are an abstraction that allows a C++ program to operate on elements of data structures uniformly.

Calling ranges::begin on a range returns an object whose type models input_or_output_iterator ([iterator.concept.iterator]).

Calling ranges::end on a range returns an object whose type S, together with the type I of the object returned by ranges::begin, models sentinel_for<S, I>.

The library formalizes the interfaces, semantics, and complexity of ranges to enable algorithms and range adaptors that work efficiently on different types of sequences.

2

#

The range concept requires that ranges::begin and ranges::end return an iterator and a sentinel, respectively.

The sized_range concept refines range with the requirement that ranges::size be amortized

O (1)

.

The view concept specifies requirements on a range type to provide operations with predictable complexity.

3

#

Several refinements of range group requirements that arise frequently in concepts and algorithms.

Common ranges are ranges for which ranges::begin and ranges::end return objects of the same type.

Random access ranges are ranges for which ranges::begin returns a type that models random_access_iterator ([iterator.concept.random.access]).

(Contiguous, bidirectional, forward, input, and output ranges are defined similarly.)

Viewable ranges can be converted to views.

26.4.2 Ranges [range.range]

1

#

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 =
    requires(T& t) {
      ranges::begin(t);         // sometimes equality-preserving (see below)
      ranges::end(t);
    };

2

#

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

(2.1)
[ranges::begin(t), ranges::end(t)) denotes a range ([iterator.requirements.general]),
(2.2)
both ranges::begin(t) and ranges::end(t) are amortized constant time and non-modifying, and
(2.3)
if the type of ranges::begin(t) models forward_iterator, ranges::begin(t) is equality-preserving.

3

#

[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]

🔗

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

4

#

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.

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#

[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;

6

#

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.

7

#

[Example 1:

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

(7.1)
enable_borrowed_range<S> is specialized to have the value true, and
(7.2)
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]

26.4.3 Sized ranges [range.sized]

1

#

The sized_range concept refines range with the requirement that the number of elements in the range can be determined in amortized constant time using ranges::size.

🔗

template<class T>
  concept sized_range =
    range<T> && requires(T& t) { ranges::size(t); };

2

#

Given an lvalue t of type remove_reference_t<T>, T models sized_range only if

(2.1)
ranges::size(t) is amortized $O (1)$ , does not modify t, and is equal to ranges::distance(ranges::begin(t), ranges::end(t)), and
(2.2)
if iterator_t<T> models forward_iterator, ranges::size(t) is well-defined regardless of the evaluation of ranges::begin(t).
[Note 1:
ranges::size(t) is otherwise not required to be well-defined after evaluating ranges::begin(t).

For example, it is possible for ranges::size(t) to be well-defined for a sized_range whose iterator type does not model forward_iterator only if evaluated before the first call to ranges::begin(t).
— end note]

🔗

template<class>
  constexpr bool disable_sized_range = false;

3

#

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

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

4

#

[Note 2:

disable_sized_range allows use of range types with the library that satisfy but do not in fact model sized_range .

— end note]

26.4.4 Views [range.view]

1

#

The view concept specifies the requirements of a range type that has the semantic properties below, which make it suitable for use in constructing range adaptor pipelines ([range.adaptors]).

🔗

template<class T>
  concept view =
    range<T> && movable<T> && enable_view<T>;

2

#

T models view only if:

(2.1)
T has $O (1)$ move construction; and
(2.2)
move assignment of an object of type T is no more complex than destruction followed by move construction; and
(2.3)
if N copies and/or moves are made from an object of type T that contained M elements, then those N objects have $O (N + M)$ destruction; and
(2.4)
copy_constructible<T> is false, or T has $O (1)$ copy construction; and
(2.5)
copyable<T> is false, or copy assignment of an object of type T is no more complex than destruction followed by copy construction.

3

#

[Note 1:

The constraints on copying and moving imply that a moved-from object of type T has

O (1)

destruction.

— end note]

4

#

[Example 1:

Examples of views are:

(4.1)
A range type that wraps a pair of iterators.
(4.2)
A range type that holds its elements by shared_ptr and shares ownership with all its copies.
(4.3)
A range type that generates its elements on demand.

A container such as vector<string> does not meet the semantic requirements of view since copying the container copies all of the elements, which cannot be done in constant time.

— end example]

5

#

Since the difference between range and view is largely semantic, the two are differentiated with the help of enable_view.

🔗

template<class T>
  constexpr bool is-derived-from-view-interface = see below;            // exposition only
template<class T>
  constexpr bool enable_view =
    derived_from<T, view_base> || is-derived-from-view-interface<T>;

6

#

For a type T, is-derived-from-view-interface<T> is true if and only if T has exactly one public base class view_interface<U> for some type U and T has no base classes of type view_interface<V> for any other type V.

7

#

Remarks: Pursuant to [namespace.std], users may specialize enable_view to true for cv-unqualified program-defined types which model view, and false for types which do not.

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

26.4.5 Other range refinements [range.refinements]

🔗

template<class R, class T>
  concept output_range =
    range<R> && output_iterator<iterator_t<R>, T>;

template<class T>
  concept input_range =
    range<T> && input_iterator<iterator_t<T>>;

template<class T>
  concept forward_range =
    input_range<T> && forward_iterator<iterator_t<T>>;

template<class T>
  concept bidirectional_range =
    forward_range<T> && bidirectional_iterator<iterator_t<T>>;

template<class T>
  concept random_access_range =
    bidirectional_range<T> && random_access_iterator<iterator_t<T>>;

🔗

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>>>;
    };

🔗

template<class T>
  concept common_range =
    range<T> && same_as<iterator_t<T>, sentinel_t<T>>;

🔗

template<class R>
  constexpr bool is-initializer-list = see below;               // exposition only

🔗

template<class T>
  concept viewable_range =
    range<T> &&
    ((view<remove_cvref_t<T>> && constructible_from<remove_cvref_t<T>, T>) ||
     (!view<remove_cvref_t<T>> &&
      (is_lvalue_reference_v<T> || (movable<remove_reference_t<T>> && !is-initializer-list<T>))));

🔗

template<class T>
  concept constant_range =
    input_range<T> && constant-iterator<iterator_t<T>>;

26 Ranges library [ranges]

26.4 Range requirements [range.req]

26.4.1 General [range.req.general]

26.4.2 Ranges [range.range]

26.4.3 Sized ranges [range.sized]

26.4.4 Views [range.view]