C++ named requirements: LegacyOutputIterator
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.
Requirements
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. ifoperator=
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 dereferenceable | 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 dereferenceable 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; | |||
*r++ = o | (not used) | *r = o; |
Notes
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 issue 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 except for difference_type,
which is now defined to satisfy std::output_iterator
(od C++20)).
Standard library
The following standard library iterators are output iterators that are not forward iterators: