C++ named requirements: LegacyForwardIterator

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

Technical specifications

Symbols index

External libraries

 

C++ named requirements

Basic
DefaultConstructible CopyAssignable CopyConstructible	MoveAssignable (C++11) MoveConstructible (C++11) Destructible
Type properties
ScalarType TriviallyCopyable (C++11) StandardLayoutType (C++11)	PODType TrivialType (C++11) ImplicitLifetimeType
Library-Wide
EqualityComparable LessThanComparable Swappable ValueSwappable (C++11) NullablePointer (C++11) Hash (C++11)	Allocator FunctionObject Callable Predicate BinaryPredicate Compare
Container
Container SequenceContainer ContiguousContainer (C++17) ReversibleContainer	AssociativeContainer AllocatorAwareContainer UnorderedAssociativeContainer (C++11)
Container Elements
DefaultInsertable (C++11) CopyInsertable (C++11) MoveInsertable (C++11)	EmplaceConstructible (C++11) Erasable (C++11)
Iterator
LegacyIterator LegacyOutputIterator LegacyInputIterator LegacyForwardIterator	LegacyBidirectionalIterator LegacyRandomAccessIterator LegacyContiguousIterator (C++17) ConstexprIterator (C++20)
Stream I/O
FormattedInputFunction UnformattedInputFunction	FormattedOutputFunction UnformattedOutputFunction
Formatting
BasicFormatter (C++20)	Formatter (C++20)
Random Numbers
SeedSequence (C++11) RandomNumberEngine (C++11)	RandomNumberDistribution (C++11) UniformRandomBitGenerator (C++11) RandomNumberEngineAdaptor (C++11)
Concurrency
BasicLockable (C++11) Lockable (C++11) TimedLockable (C++11) SharedLockable (C++14) SharedTimedLockable (C++14)	Mutex (C++11) TimedMutex (C++11) SharedMutex (C++17) SharedTimedMutex (C++14)
Other
CharTraits RegexTraits (C++11) BitmaskType NumericType LiteralType (C++11)	UnaryTypeTrait (C++11) BinaryTypeTrait (C++11) TransformationTrait (C++11) Clock (C++11) TrivialClock (C++11)

 

A LegacyForwardIterator is a LegacyIterator that can read data from the pointed-to element.

Unlike LegacyInputIterator and LegacyOutputIterator, it can be used in multipass algorithms.

If a LegacyForwardIterator it originates from a Container, then it's value_type is the same as the container's, so dereferencing (*it) obtains the container's value_type.

Requirements

The type It satisfies LegacyForwardIterator if

• The type It satisfies LegacyInputIterator
• The type It satisfies DefaultConstructible
• Objects of the type It provide multipass guarantee described below
• The type std::iterator_traits<It>::reference must be either

• a reference to T if It satisfies LegacyOutputIterator (It is mutable), or
• a reference to const T otherwise (It is constant),

(where T is the type denoted by std::iterator_traits<It>::value_type)

• Equality and inequality comparison is defined over all iterators for the same underlying sequence and the value initialized-iterators (since C++14).

And, given

• i, dereferenceable iterator of type It
• reference, the type denoted by std::iterator_traits<It>::reference

The following expressions must be valid and have their specified effects

Expression	Return type	Equivalent expression
i++	It	It ip = i; ++i; return ip;
*i++	reference

A mutable LegacyForwardIterator is a LegacyForwardIterator that additionally satisfies the LegacyOutputIterator requirements.

Multipass guarantee

Given a and b, dereferenceable iterators of type It

• If a and b compare equal (a == b is contextually convertible to true) then either they are both non-dereferenceable or *a and *b are references bound to the same object.
• If *a and *b refer to the same object, then a == b.
• Assignment through a mutable ForwardIterator iterator cannot invalidate the iterator (implicit due to reference defined as a true reference).
• Incrementing a copy of a does not change the value read from a (formally, either It is a raw pointer type or the expression (void)++It(a), *a is equivalent to the expression *a).
• a == b implies ++a == ++b.

Singular iterators A value-initialized LegacyForwardIterator behaves like the past-the-end iterator of some unspecified empty container: it compares equal to all value-initialized LegacyForwardIterators of the same type.

(since C++14)

Concept For the definition of std::iterator_traits, the following exposition-only concept is defined. template<class It> concept __LegacyForwardIterator =   __LegacyInputIterator<It> && std::constructible_from<It> &&   std::is_lvalue_reference_v<std::iter_reference_t<It>> &&   std::same_as<     std::remove_cvref_t<std::iter_reference_t<It>>,     typename std::indirectly_readable_traits<It>::value_type> &&   requires(It it) {     {  it++ } -> std::convertible_to<const It&>;     { *it++ } -> std::same_as<std::iter_reference_t<It>>;   }; where the exposition-only concept __LegacyInputIterator<T> is described in LegacyInputIterator#Concept. Note that whilst LegacyForwardIterator allows for iterators to have their reference types being rvalue reference types, this concept does not. And so it is important for such iterators (e.g. std::move_iterator) to explicitly specify their It::iterator_category to be std::forward_iterator_tag (or stronger iterator category tags), as std::iterator_traits<It>::iterator_category would be incorrectly deduced as std::input_iterator_tag otherwise.

(since C++20)

See also

forward_iterator (C++20)	specifies that an input_iterator is a forward iterator, supporting equality comparison and multi-pass (concept)
Iterator library	provides definitions for iterators, iterator traits, adaptors, and utility functions