C++ named requirements: LegacyIterator

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.

All the categories of iterators require only those functions that are realizable for a given category in constant time (amortized). Therefore, requirement tables and concept definitions (since C++20) for the iterators do not specify complexity.

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
• The type It satisfies 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
++ r	It &	r is incrementable (the behavior of the expression ++ r is defined)

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 2437	C++98	* r is required to be reference	not required for output iterators
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