C++ named requirements: LegacyForwardIterator

 

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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
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EqualityComparable LessThanComparable Swappable ValueSwappable (C++11)	NullablePointer (C++11) Hash (C++11) Allocator FunctionObject	Callable Predicate BinaryPredicate Compare
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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)
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FormattedInputFunction UnformattedInputFunction	FormattedOutputFunction	UnformattedOutputFunction
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RangeAdaptorObject (C++20)	RangeAdaptorClosureObject (C++20)
Other
CharTraits RegexTraits (C++11) BitmaskType LiteralType (C++11)	NumericType 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
• Let T be the value type of It. The type std::iterator_traits<It>::reference must be either

• T& or T&& (since C++11) if It satisfies LegacyOutputIterator (It is mutable), or
• const T& or const T&& (since C++11) 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 lvalue 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_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.

(since C++20)

Notes

Unlike the std::forward_iterator concept, the LegacyForwardIterator requirements requires dereference to return a reference.

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 1212 (N3066)	C++98	the return type of *i++ did not match the return type of *i-- required by LegacyBidirectionalIterator	changed the return type to reference
LWG 1311 (N3066)	C++98	'a == b implies ++a == ++b' alone did not offer multipass guarantee	also requires 'a == b implies ++a != b'
LWG 3798	C++20	__LegacyForwardIterator required std::iter_reference_t<It> to be an lvalue reference type	also allows rvalue reference types

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