C++ named requirements: LegacyOutputIterator

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C++ named requirements

A LegacyOutputIterator is a LegacyIterator that can write to the pointed-to element.

An example of a type that implements LegacyOutputIterator is std::ostream_iterator.

When LegacyForwardIterator, LegacyBidirectionalIterator, or LegacyRandomAccessIterator satisfies the LegacyOutputIterator requirements in addition to its own requirements, it is described as mutable.


The type X satisfies LegacyOutputIterator if

  • The type X satisfies LegacyIterator
  • X is a class type or a pointer type

And, given

  • o, a value of some type that is writable to the output iterator (there may be multiple types that are writable, e.g. if operator= may be a template. There is no notion of value_type as for the input iterators)
  • r, an lvalue of type X,

The following expressions must be valid and have their specified effects

Expression Return Equivalent expression Pre-condition Post-conditions Notes
*r = o (not used) r is dereferencable r is incrementable After this operation r is not required to be dereferenceable and any copies of the previous value of r are no longer required to be dereferenceable or incrementable.
++r X& r is incrementable r and ++r designate the same iterator object, r is dereferencable or past-the-end After this operation r is not required to be incrementable and any copies of the previous value of r are no longer required to be dereferenceable or incrementable.
r++ convertible to const X& X temp = r;

return temp;

*r++ = o (not used) *r = o;



The only valid use of operator* with an output iterator is on the left of an assignment: operator* may return a proxy object, which defines a member operator= (which may be a template)

Equality and inequality may not be defined for output iterators. Even if an operator== is defined, x == y need not imply ++x == ++y.

Assignment through the same value of an output iterator happens only once: algorithms on output iterators must be single-pass algorithms.

Assignment through an output iterator is expected to alternate with incrementing. Double-increment is undefined behavior (C++ standard currently claims that double increment is supported, contrary to the STL documentation; this is LWG #2035)

Pure output-only iterator is allowed to declare its iterator_traits<X>::value_type, iterator_traits<X>::difference_type, iterator_traits<X>::pointer, and iterator_traits<X>::reference to be void (and iterators such as std::back_insert_iterator do just that).

Standard library

The following standard library iterators are output iterators that are not forward iterators:

See also

specifies that a type is an output iterator for a given value type, that is, values of that type can be written to it and it can be both pre- and post-incremented