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.


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.


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

specifies that objects of a type can be incremented and dereferenced
Iterator library provides definitions for iterators, iterator traits, adaptors, and utility functions

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