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:

template<class I>
concept __LegacyIterator =
    requires(I i)
    {
        {   *i } -> __Referenceable;
        {  ++i } -> std::same_as<I&>;
        { *i++ } -> __Referenceable;
    } && std::copyable<I>;

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 “Cpp17”) prefix emphasizes compatibility with pre-C++20 standards and is used to distinguish these requirements from the new iterator concepts introduced with C++20.

Defect reports

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

See also