Defined in header <iterator>
template <class I, class T>
  concept output_iterator =
    std::input_or_output_iterator<I> &&
    std::indirectly_writable<I, T> &&
    requires(I i, T&& t) {
      *i++ = std::forward<T>(t);        // not required to be equality-preserving
(since C++20)

The output_iterator concept is a refinement of input_or_output_iterator, adding the requirement that it can be used to write values of type and value category encoded by T (via indirectly_writable). equality_comparable is not required.

Semantic requirements

Let E be an expression such that decltype((E)) is T, and i be a dereferenceable object of type I. output_iterator<I, T> is modeled only if all the concepts it subsumes are modeled, and *i++ = E; has effects equivalent to *i = E; ++i;.

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.


Unlike the LegacyOutputIterator requirements, the output_iterator concept does not require that the iterator category tag be defined.

Algorithms on output iterators should be single pass.

See also

specifies that objects of a type can be incremented and dereferenced

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