C++ named requirements: LegacyIterator

 

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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 (C++20)
Library-Wide
EqualityComparable LessThanComparable Swappable (C++11) 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)

 

The LegacyIterator requirements describe types that can be used to identify and traverse the elements of a container.

LegacyIterator is the base set of requirements used by other iterator types: LegacyInputIterator, LegacyOutputIterator, LegacyForwardIterator, LegacyBidirectionalIterator, and LegacyRandomAccessIterator. Iterators can be thought of as an abstraction of pointers.

Requirements

The type It satisfies LegacyIterator if

• The type It satisfies CopyConstructible, and
• The type It satisfies CopyAssignable, and
• The type It satisfies Destructible, and
• lvalues of type It satisfy Swappable, and
• std::iterator_traits<It> has member typedefs value_type (until C++20), difference_type, reference, pointer, and iterator_category , and
• Given r, an lvalue of type It, the following expressions must be valid and have their specified effects:

Expression	Return Type	Precondition
*r	unspecified	r is dereferenceable (see below)
++r	It&	r is incrementable (the behavior of the expression ++r is defined)

Dereferenceable iterators

Iterators for which the behavior of the expression *i is defined are called dereferenceable.

Iterators are not dereferenceable if

• they are past-the-end iterators (including pointers past the end of an array) or before-begin iterators. Such iterators may be dereferenceable in a particular implementation, but the library never assumes that they are.
• they are singular iterators, that is, iterators that are not associated with any sequence. A null pointer, as well as a default-constructed pointer (holding an indeterminate value) is singular
• they were invalidated by one of the iterator-invalidating operations on the sequence to which they refer.

Concept For the definition of std::iterator_traits, the following exposition-only concept is defined. template<class I> concept __LegacyIterator =   requires(I i) {     {   *i } -> __Referenceable;     {  ++i } -> std::same_as<I&>;     { *i++ } -> __Referenceable;   } && std::copyable<I>; where the exposition-only concept __Referenceable<T> is satisfied if and only if T& is a valid type (in particular, T must not be void).

(since C++20)

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 3420	C++20	the exposition-only concept checks copyable first	copyable is checked only if the requires-expression yields true

See also

input_or_output_iterator (C++20)	specifies that objects of a type can be incremented and dereferenced (concept)
Iterator library	provides definitions for iterators, iterator traits, adaptors, and utility functions