C++ named requirements: LegacyForwardIterator

 

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C++ named requirements

Basic
DefaultConstructible CopyAssignable CopyConstructible	MoveAssignable (C++11) MoveConstructible (C++11) Destructible
Type properties
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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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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 exactly

• T& if It satisfies LegacyOutputIterator (It is mutable)
• 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	Notes
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 I> concept __LegacyForwardIterator =   __LegacyInputIterator<I> && std::constructible_from<I> &&   std::is_lvalue_reference_v<std::iter_reference_t<I>> &&   std::same_as<     std::remove_cvref_t<std::iter_reference_t<I>>,     typename std::indirectly_readable_traits<I>::value_type> &&   requires(I i) {     {  i++ } -> std::convertible_to<const I&>;     { *i++ } -> std::same_as<std::iter_reference_t<I>>;   }; where the exposition-only concept __LegacyInputIterator<T> is described in LegacyInputIterator#Concept.

(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