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Showing content from https://en.cppreference.com/w/cpp/algorithm/../named_req/Iterator.html below:

C++ named requirements: LegacyIterator - cppreference.com

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.

[edit] 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) [edit] Notes

Note on terminology: The following table shows the names used on this site and corresponding C++ Standard names (with the same meaning). The "Legacy" (and âCpp17â) prefix emphasizes compatibility with pre-C++20 standards and is used to distinguish these requirements from the new iterator concepts introduced with C++20.

â LegacyContiguousIterator category was only formally specified in C++17, but the iterators of std::vector, std::basic_string, std::array, and std::valarray, as well as pointers into C arrays are often treated as a separate category in pre-C++17 code.

[edit] 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 [edit] See also

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