[iterator.concepts] (original) (raw)

24 Iterators library [iterators]

24.3 Iterator requirements [iterator.requirements]

24.3.4 Iterator concepts [iterator.concepts]

24.3.4.1 General [iterator.concepts.general]

For a type I, let ITER_TRAITS(I) denote the type I if iterator_traits names a specialization generated from the primary template.

Otherwise, ITER_TRAITS(I) denotesiterator_traits.

If the qualified-id ITER_TRAITS(I)::iterator_concept is valid and names a type, then ITER_CONCEPT(I) denotes that type.
Otherwise, if the qualified-id ITER_TRAITS(I)::iterator_category is valid and names a type, then ITER_CONCEPT(I) denotes that type.
Otherwise, if iterator_traits names a specialization generated from the primary template, then ITER_CONCEPT(I) denotes random_access_iterator_tag.
Otherwise, ITER_CONCEPT(I) does not denote a type.

[Note 1:

ITER_TRAITS enables independent syntactic determination of an iterator's category and concept.

— _end note_]

[Example 1:

struct I { using value_type = int;using difference_type = int;int operator*() const; I& operator++(); I operator++(int); I& operator--(); I operator--(int);bool operator==(I) const;}; iterator_traits::iterator_category denotes input_iterator_tag, and ITER_CONCEPT(I) denotes random_access_iterator_tag.

— _end example_]

24.3.4.2 Concept indirectly_readable [iterator.concept.readable]

Types that are indirectly readable by applying operator*model the indirectly_readable concept, including pointers, smart pointers, and iterators.

template<class In> concept indirectly-readable-impl = requires(const In in) { typename iter_value_t<In>;typename iter_reference_t<In>;typename iter_rvalue_reference_t<In>;{ *in } -> same_as<iter_reference_t<In>>;{ ranges::iter_move(in) } -> same_as<iter_rvalue_reference_t<In>>;} && common_reference_with<iter_reference_t<In>&&, iter_value_t<In>&> && common_reference_with<iter_reference_t<In>&&, iter_rvalue_reference_t<In>&&> && common_reference_with<iter_rvalue_reference_t<In>&&, const iter_value_t<In>&>;

template<class In> concept indirectly_readable = indirectly-readable-impl<remove_cvref_t<In>>;

Given a value i of type I, I models indirectly_readableonly if the expression *i is equality-preserving.

24.3.4.3 Concept indirectly_writable [iterator.concept.writable]

The indirectly_writable concept specifies the requirements for writing a value into an iterator's referenced object.

template<class Out, class T> concept indirectly_writable = requires(Out&& o, T&& t) { *o = std::forward<T>(t); *std::forward<Out>(o) = std::forward<T>(t); const_cast<const iter_reference_t<Out>&&>(*o) = std::forward<T>(t); const_cast<const iter_reference_t<Out>&&>(*std::forward<Out>(o)) = std::forward<T>(t); };

Let E be an expression such that decltype((E)) is T, and let o be a dereferenceable object of type Out.

Out and T model indirectly_writable<Out, T> only if:

If Out and T modelindirectly_readable<Out> && same_as<iter_value_t<Out>, decay_t<T>>, then *o after any above assignment is equal to the value of E before the assignment.

After evaluating any above assignment expression, o is not required to be dereferenceable.

[Note 1:

The only valid use of an operator* is on the left side of the assignment statement.

Assignment through the same value of the indirectly writable type happens only once.

— _end note_]

[Note 2:

indirectly_writable has the awkward const_cast expressions to reject iterators with prvalue non-proxy reference types that permit rvalue assignment but do not also permit const rvalue assignment.

Consequently, an iterator type I that returns std::stringby value does not model indirectly_writable<I, std::string>.

— _end note_]

24.3.4.4 Concept weakly_incrementable [iterator.concept.winc]

The weakly_incrementable concept specifies the requirements on types that can be incremented with the pre- and post-increment operators.

The increment operations are not required to be equality-preserving, nor is the type required to be equality_comparable .

template<class T> constexpr bool is-integer-like = see below; template<class T> constexpr bool is-signed-integer-like = see below; template<class I> concept weakly_incrementable = movable && requires(I i) { typename iter_difference_t;requires is-signed-integer-like<iter_difference_t>;{ ++i } -> same_as<I&>; i++; };

A type I is an integer-class typeif it is in a set of implementation-defined types that behave as integer types do, as defined below.

[Note 1:

An integer-class type is not necessarily a class type.

— _end note_]

The range of representable values of an integer-class type is the continuous set of values over which it is defined.

For any integer-class type, its range of representable values is either to (inclusive) for some integer N, in which case it is a signed-integer-class type, or0 to (inclusive) for some integer N, in which case it is an unsigned-integer-class type.

In both cases, N is called the width of the integer-class type.

The width of an integer-class type is greater than that of every integral type of the same signedness.

A type I other than cv bool is integer-likeif it models integral or if it is an integer-class type.

An integer-like type I is signed-integer-likeif it models signed_integral or if it is a signed-integer-class type.

An integer-like type I is unsigned-integer-likeif it models unsigned_integral or if it is an unsigned-integer-class type.

For every integer-class type I, let B(I) be a unique hypothetical extended integer type of the same signedness with the same width ([basic.fundamental]) as I.

[Note 2:

The corresponding hypothetical specialization numeric_limits<B(I)>meets the requirements on numeric_limits specializations for integral types ([numeric.limits]).

— _end note_]

For every integral type J, let B(J) be the same type as J.

Expressions of integer-class type are explicitly convertible to any integer-like type, and implicitly convertible to any integer-class type of equal or greater width and the same signedness.

Expressions of integral type are both implicitly and explicitly convertible to any integer-class type.

Conversions between integral and integer-class types and between two integer-class types do not exit via an exception.

The result of such a conversion is the unique value of the destination type that is congruent to the source modulo , where N is the width of the destination type.

Let a be an object of integer-class type I, let b be an object of integer-like type I2such that the expression b is implicitly convertible to I, let x and y be, respectively, objects of type B(I) and B(I2) as described above that represent the same values as a and b, and let c be an lvalue of any integral type.

The expressions a++ and a-- shall be prvalues of type Iwhose values are equal to that of a prior to the evaluation of the expressions.
The expression a++ shall modify the value of aby adding 1 to it.
The expression a-- shall modify the value of aby subtracting 1 from it.
The expressions ++a, --a, and &a shall be expression-equivalent toa += 1, a -= 1, and addressof(a), respectively.
For every unary-operator @ other than & for which the expression @x is well-formed, @a shall also be well-formed and have the same value, effects, and value category as @x.
If @x has type bool, so too does @a; if @x has type B(I), then @a has type I.
For every assignment operator @= for which c @= x is well-formed,c @= a shall also be well-formed and shall have the same value and effects as c @= x.
The expression c @= a shall be an lvalue referring to c.
For every assignment operator @=for which x @= y is well-formed,a @= b shall also be well-formed and shall have the same effects as x @= y, except that the value that would be stored into xis stored into a.
The expression a @= b shall be an lvalue referring to a.
For every non-assignment binary operator @ for which x @ y and y @ x are well-formed,a @ b and b @ a shall also be well-formed and shall have the same value, effects, and value category asx @ y and y @ x, respectively.
If x @ y or y @ x has type B(I), then a @ b or b @ a, respectively, has type I; if x @ y or y @ x has type B(I2), then a @ b or b @ a, respectively, has type I2; if x @ y or y @ x has any other type, then a @ b or b @ a, respectively, has that type.

An expression E of integer-class type I is contextually convertible to boolas if by bool(E != I(0)).

A value-initialized object of integer-class type has value 0.

For every (possibly cv-qualified) integer-class type I,numeric_limits is specialized such that each static data member mhas the same value as numeric_limits<B(I)>::m, and each static member function freturns I(numeric_limits<B(I)>::f()).

For any two integer-like types I1 and I2, at least one of which is an integer-class type,common_type_t<I1, I2> denotes an integer-class type whose width is not less than that of I1 or I2.

If both I1 and I2 are signed-integer-like types, then common_type_t<I1, I2> is also a signed-integer-like type.

is-integer-like is trueif and only if I is an integer-like type.

is-signed-integer-like is trueif and only if I is a signed-integer-like type.

Let i be an object of type I.

When i is in the domain of both pre- and post-increment, i is said to be incrementable.

I models weakly_incrementable only if:

The expressions ++i and i++ have the same domain.
If i is incrementable, then both ++i and i++ advance i to the next element.
If i is incrementable, thenaddressof(++i) is equal toaddressof(i).

Recommended practice: The implementation of an algorithm on a weakly incrementable type should never attempt to pass through the same incrementable value twice; such an algorithm should be a single-pass algorithm.

[Note 3:

For weakly_incrementable types, a equals b does not imply that ++aequals ++b.

(Equality does not guarantee the substitution property or referential transparency.)

Such algorithms can be used with istreams as the source of the input data through the istream_iterator class template.

— _end note_]

24.3.4.5 Concept incrementable [iterator.concept.inc]

The incrementable concept specifies requirements on types that can be incremented with the pre- and post-increment operators.

The increment operations are required to be equality-preserving, and the type is required to be equality_comparable .

[Note 1:

This supersedes the annotations on the increment expressions in the definition of weakly_incrementable .

— _end note_]

template<class I> concept incrementable = regular && weakly_incrementable && requires(I i) { { i++ } -> same_as;};

Let a and b be incrementable objects of type I.

I models incrementable only if:

If bool(a == b) then bool(a++ == b).
If bool(a == b) then bool(((void)a++, a) == ++b).

[Note 2:

The requirement thata equals bimplies++a equals ++b(which is not true for weakly incrementable types) allows the use of multi-pass one-directional algorithms with types that model incrementable .

— _end note_]

24.3.4.6 Concept input_or_output_iterator [iterator.concept.iterator]

The input_or_output_iterator concept forms the basis of the iterator concept taxonomy; every iterator models input_or_output_iterator .

This concept specifies operations for dereferencing and incrementing an iterator.

Most algorithms will require additional operations to compare iterators with sentinels ([iterator.concept.sentinel]), to read ([iterator.concept.input]) or write ([iterator.concept.output]) values, or to provide a richer set of iterator movements ([iterator.concept.forward], [iterator.concept.bidir], [iterator.concept.random.access]).

template<class I> concept input_or_output_iterator = requires(I i) { { *i } -> can-reference;} && weakly_incrementable;

[Note 1:

Unlike the Cpp17Iterator requirements, the input_or_output_iterator concept does not require copyability.

— _end note_]

24.3.4.7 Concept sentinel_for [iterator.concept.sentinel]

The sentinel_for concept specifies the relationship between an input_or_output_iterator type and a semiregular type whose values denote a range.

Let s and i be values of type S andI such that [i, s) denotes a range.

TypesS and I model sentinel_for<S, I> only if:

If bool(i != s) then i is dereferenceable and [++i, s) denotes a range.

The domain of == is not static.

Given an iterator i and sentinel s such that [i, s) denotes a range and i != s, i and s are not required to continue to denote a range after incrementing any other iterator equal to i.

Consequently, i == s is no longer required to be well-defined.

24.3.4.8 Concept sized_sentinel_for [iterator.concept.sizedsentinel]

The sized_sentinel_for concept specifies requirements on an input_or_output_iterator type I and a corresponding sentinel_forthat allow the use of the - operator to compute the distance between them in constant time.

template<class S, class I> concept [sized_sentinel_for](#concept:sized%5Fsentinel%5Ffor "24.3.4.8 Concept sized_sentinel_for [iterator.concept.sizedsentinel]") = [sentinel_for](#concept:sentinel%5Ffor "24.3.4.7 Concept sentinel_for [iterator.concept.sentinel]")<S, I> && !disable_sized_sentinel_for<remove_cv_t<S>, remove_cv_t> && requires(const I& i, const S& s) { { s - i } -> [same_as](concept.same#concept:same%5Fas "18.4.2 Concept same_as [concept.same]")<iter_difference_t>;{ i - s } -> [same_as](concept.same#concept:same%5Fas "18.4.2 Concept same_as [concept.same]")<iter_difference_t>;};

Let i be an iterator of type I, and sa sentinel of type S such that [i, s) denotes a range.

Let N be the smallest number of applications of ++inecessary to make bool(i == s) be true.

S and I model sized_sentinel_for<S, I> only if:

If N is representable by iter_difference_t, then s - i is well-defined and equals N.
If is representable by iter_difference_t, then i - s is well-defined and equals .

template<class S, class I> constexpr bool disable_sized_sentinel_for = false;

Remarks: Pursuant to [namespace.std], users may specialize disable_sized_sentinel_forfor cv-unqualified non-array object types S and Iif S and/or I is a program-defined type.

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

[Note 1:

disable_sized_sentinel_for allows use of sentinels and iterators with the library that satisfy but do not in fact model sized_sentinel_for .

— _end note_]

[Example 1:

The sized_sentinel_for concept is modeled by pairs ofrandom_access_iterators ([iterator.concept.random.access]) and by counted iterators and their sentinels ([counted.iterator]).

— _end example_]

24.3.4.9 Concept input_iterator [iterator.concept.input]

The input_iterator concept defines requirements for a type whose referenced values can be read (from the requirement forindirectly_readable ([iterator.concept.readable])) and which can be both pre- and post-incremented.

[Note 1:

Unlike the Cpp17InputIterator requirements ([input.iterators]), the input_iterator concept does not need equality comparison since iterators are typically compared to sentinels.

— _end note_]

template<class I> concept input_iterator = input_or_output_iterator && indirectly_readable && requires { typename ITER_CONCEPT(I); } && derived_from<_ITER_CONCEPT_(I), input_iterator_tag>;

24.3.4.10 Concept output_iterator [iterator.concept.output]

The output_iterator concept defines requirements for a type that can be used to write values (from the requirement forindirectly_writable ([iterator.concept.writable])) and which can be both pre- and post-incremented.

template<class I, class T> concept output_iterator = input_or_output_iterator && indirectly_writable<I, T> && requires(I i, T&& t) { *i++ = std::forward<T>(t); };

Let E be an expression such that decltype((E)) is T, and let i be a dereferenceable object of type I.

I and T model output_iterator<I, T> only if*i++ = E; has effects equivalent to:*i = E;++i;

Recommended practice: The implementation of an algorithm on output iterators should never attempt to pass through the same iterator twice; such an algorithm should be a single-pass algorithm.

24.3.4.11 Concept forward_iterator [iterator.concept.forward]

The forward_iterator concept adds copyability, equality comparison, and the multi-pass guarantee, specified below.

template<class I> concept forward_iterator = input_iterator && derived_from<_ITER_CONCEPT_(I), forward_iterator_tag> && incrementable && sentinel_for<I, I>;

The domain of == for forward iterators is that of iterators over the same underlying sequence.

However, value-initialized iterators of the same type may be compared and shall compare equal to other value-initialized iterators of the same type.

[Note 1:

Value-initialized iterators behave as if they refer past the end of the same empty sequence.

— _end note_]

Pointers and references obtained from a forward iterator into a range [i, s) shall remain valid while [i, s) continues to denote a range.

Two dereferenceable iterators a and b of type Xoffer the multi-pass guarantee if

a == b implies ++a == ++b and
the expression((void)[](X x){++x;}(a), *a) is equivalent to the expression *a.

[Note 2:

The requirement thata == bimplies++a == ++band the removal of the restrictions on the number of assignments through a mutable iterator (which applies to output iterators) allow the use of multi-pass one-directional algorithms with forward iterators.

— _end note_]

24.3.4.12 Concept bidirectional_iterator [iterator.concept.bidir]

The bidirectional_iterator concept adds the ability to move an iterator backward as well as forward.

template<class I> concept bidirectional_iterator = forward_iterator && derived_from<_ITER_CONCEPT_(I), bidirectional_iterator_tag> && requires(I i) { { --i } -> same_as<I&>;{ i-- } -> same_as;};

A bidirectional iterator r is decrementable if and only if there exists some q such that++q == r.

Decrementable iterators r shall be in the domain of the expressions--r and r--.

Let a and b be equal objects of type I.

I models bidirectional_iterator only if:

If a and b are decrementable, then all of the following are true:
- addressof(--a) == addressof(a)
- bool(a-- == b)
- after evaluating both a-- and --b,bool(a == b) is still true
- bool(++(--a) == b)
If a and b are incrementable, thenbool(--(++a) == b).

24.3.4.13 Concept random_access_iterator [iterator.concept.random.access]

The random_access_iterator concept adds support for constant-time advancement with +=, +, -=, and -, as well as the computation of distance in constant time with -.

Random access iterators also support array notation via subscripting.

template<class I> concept random_access_iterator = bidirectional_iterator && derived_from<_ITER_CONCEPT_(I), random_access_iterator_tag> && totally_ordered && sized_sentinel_for<I, I> && requires(I i, const I j, const iter_difference_t n) { { i += n } -> same_as<I&>;{ j + n } -> same_as;{ n + j } -> same_as;{ i -= n } -> same_as<I&>;{ j - n } -> same_as;{ j[n] } -> same_as<iter_reference_t>;};

Let a and b be valid iterators of type Isuch that b is reachable from aafter n applications of ++a, let D be iter_difference_t, and let n denote a value of type D.

I models random_access_iterator only if:

addressof(a += n) is equal to addressof(a).
(a + n) is equal to (a += n).
For any two positive valuesx and y of type D, if (a + D(x + y)) is valid, then(a + D(x + y)) is equal to ((a + x) + y).
(a + D(0)) is equal to a.
If (a + D(n - 1)) is valid, then(a + n) is equal to [](I c){ return ++c; }(a + D(n - 1)).
(b += D(-n)) is equal to a.
addressof(b -= n) is equal to addressof(b).
(b - n) is equal to (b -= n).
If b is dereferenceable, thena[n] is valid and is equal to *b.

24.3.4.14 Concept contiguous_iterator [iterator.concept.contiguous]

The contiguous_iterator concept provides a guarantee that the denoted elements are stored contiguously in memory.

template<class I> concept contiguous_iterator = random_access_iterator && derived_from<_ITER_CONCEPT_(I), contiguous_iterator_tag> && is_lvalue_reference_v<iter_reference_t> && same_as<iter_value_t, remove_cvref_t<iter_reference_t>> && requires(const I& i) { { to_address(i) } -> same_as<add_pointer_t<iter_reference_t>>;};

Let a and b be dereferenceable iterators andc be a non-dereferenceable iterator of type Isuch that b is reachable from a andc is reachable from b, and let D be iter_difference_t.

The type I models contiguous_iterator only if

to_address(a) == addressof(*a),
to_address(b) == to_address(a) + D(b - a),
to_address(c) == to_address(a) + D(c - a),
to_address(I{}) is well-defined,
ranges::iter_move(a) has the same type, value category, and effects as std::move(*a), and
if ranges::iter_swap(a, b) is well-formed, it has effects equivalent to ranges::swap(*a, *b).