Defined in header
template <class I>
concept input_or_output_iterator =
input_or_output_iterator concept forms the basis of the iterator concept taxonomy; every iterator type satisfies the
The exposition-only concept
__Referenceable is satisfied if and only if the expression *std::declval<I&>() is valid and has a referenceable type (in particular, not void).
 Equality preservation
An expression is equality preserving if it results in equal outputs given equal inputs.
- The inputs to an expression consist of its operands.
- The outputs of an expression consist of its result and all operands modified by the expression (if any).
In specification of standard concepts, operands are defined as the largest subexpressions that include only:
The cv-qualification and value category of each operand is determined by assuming that each template type parameter denotes a cv-unqualified complete non-array object type.
Every expression required to be equality preserving is further required to be stable: two evaluations of such an expression with the same input objects must have equal outputs absent any explicit intervening modification of those input objects.
Unless noted otherwise, every expression used in a requires-expression is required to be equality preserving and stable, and the evaluation of the expression may only modify its non-constant operands. Operands that are constant must not be modified.
input_or_output_iterator itself only specifies operations for dereferencing and incrementing an iterator. Most algorithms will require additional operations, for example:
- comparing iterators with sentinels (see sentinel_for);
- reading values from an iterator (see indirectly_readable and input_iterator)
- writing values to an iterator (see indirectly_writable and output_iterator)
- a richer set of iterator movements (see forward_iterator, bidirectional_iterator, random_access_iterator)
Unlike the LegacyIterator requirements, the
input_or_output_iterator concept does not require copyability.