C++ named requirements: LegacyOutputIterator

A LegacyOutputIterator is a LegacyIterator

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_type
• 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
++r	X&		r is incrementable	r and ++r designate the same iterator object, r	After this operation r is not required to be incrementable and any copies of the previous value of r
r++	convertible to const X&	X temp = r; ++r; return temp;
*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=

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:

ostream_iterator	output iterator that writes to std::basic_ostream (class template)
ostreambuf_iterator	output iterator that writes to std::basic_streambuf (class template)
insert_iterator	iterator adaptor for insertion into a container (class template)
back_insert_iterator	iterator adaptor for insertion at the end of a container (class template)
front_insert_iterator	iterator adaptor for insertion at the front of a container (class template)

See also

output_iterator (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