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. if operator= may be a template. There is no notion of
value_typeas 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; |
|||
| *r++ = o | (not used) | *r = o; ++r; |
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 (since C++20)).
Standard library
The following standard library iterators are output iterators that are not forward iterators:
- std::ostream_iterator
- std::ostreambuf_iterator
- std::insert_iterator
- std::back_insert_iterator
- std::front_insert_iterator
See also
| (C++20) |
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 (concept) |
| Iterator library | provides definitions for iterators, iterator traits, adaptors, and utility functions |