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output_iteratorSection: 27.7.5 [alg.replace], 27.7.6 [alg.fill] Status: LEWG Submitter: Hewill Kang Opened: 2023-01-29 Last modified: 2023-02-06
Priority: 4
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Discussion:
In C++20, output_iterator is required to support *o++ = t mainly for backward compatibility,
that is to say, in addition to avoiding needless breakage, this semantic is actually not very useful, as it can
be equivalently replaced by *o = t; ++o;.
output_iterator, there is no code of the form *o++ = t in practice,
and the latter of a more generic form is used instead.
It seems to me that constrained algorithms should never require output_iterator, since there really isn't
any desirable reason to use *o++ = t in the new iterator system.
It would be more appropriate to relax output_iterator to weakly_incrementable
(or input_or_output_iterator) and indirectly_writable,
given that many constrained algorithms already do that.
[2023-02-06; Reflector poll]
Set priority to 4 after reflector poll. Send to LEWG. Several votes for NAD.
Proposed resolution:
This wording is relative to N4928.
Modify 27.4 [algorithm.syn], header <algorithm> synopsis, as indicated:
#include <initializer_list> // see 17.10.2 [initializer.list.syn] namespace std { […] namespace ranges { template<class I, class O> using replace_copy_result = in_out_result<I, O>; template<input_iterator I, sentinel_for<I> S, class T1, class T2, weakly_incrementableoutput_iterator<const T2&>O, class Proj = identity> requires indirectly_writable<O, const T2&> && indirectly_copyable<I, O> && indirect_binary_predicate<ranges::equal_to, projected<I, Proj>, const T1*> constexpr replace_copy_result<I, O> replace_copy(I first, S last, O result, const T1& old_value, const T2& new_value, Proj proj = {}); template<input_range R, class T1, class T2, weakly_incrementableoutput_iterator<const T2&>O, class Proj = identity> requires indirectly_writable<O, const T2&> && indirectly_copyable<iterator_t<R>, O> && indirect_binary_predicate<ranges::equal_to, projected<iterator_t<R>, Proj>, const T1*> constexpr replace_copy_result<borrowed_iterator_t<R>, O> replace_copy(R&& r, O result, const T1& old_value, const T2& new_value, Proj proj = {}); template<class I, class O> using replace_copy_if_result = in_out_result<I, O>; template<input_iterator I, sentinel_for<I> S, class T, weakly_incrementableoutput_iterator<const T&>O, class Proj = identity, indirect_unary_predicate<projected<I, Proj>> Pred> requires indirectly_writable<O, const T&> && indirectly_copyable<I, O> constexpr replace_copy_if_result<I, O> replace_copy_if(I first, S last, O result, Pred pred, const T& new_value, Proj proj = {}); template<input_range R, class T, weakly_incrementableoutput_iterator<const T&>O, class Proj = identity, indirect_unary_predicate<projected<iterator_t<R>, Proj>> Pred> requires indirectly_writable<O, const T&> && indirectly_copyable<iterator_t<R>, O> constexpr replace_copy_if_result<borrowed_iterator_t<R>, O> replace_copy_if(R&& r, O result, Pred pred, const T& new_value, Proj proj = {}); } […] namespace ranges { template<class T, input_or_output_iteratoroutput_iterator<const T&>O, sentinel_for<O> S> requires indirectly_writable<O, const T&> constexpr O fill(O first, S last, const T& value); template<class T, output_range<const T&> R> constexpr borrowed_iterator_t<R> fill(R&& r, const T& value); template<class T, input_or_output_iteratoroutput_iterator<const T&>O> requires indirectly_writable<O, const T&> constexpr O fill_n(O first, iter_difference_t<O> n, const T& value); } […] }
Modify 27.7.5 [alg.replace] as indicated:
[…] template<input_iterator I, sentinel_for<I> S, class T1, class T2, weakly_incrementableoutput_iterator<const T2&>O, class Proj = identity> requires indirectly_writable<O, const T2&> && indirectly_copyable<I, O> && indirect_binary_predicate<ranges::equal_to, projected<I, Proj>, const T1*> constexpr ranges::replace_copy_result<I, O> ranges::replace_copy(I first, S last, O result, const T1& old_value, const T2& new_value, Proj proj = {}); template<input_range R, class T1, class T2, weakly_incrementableoutput_iterator<const T2&>O, class Proj = identity> requires indirectly_writable<O, const T2&> && indirectly_copyable<iterator_t<R>, O> && indirect_binary_predicate<ranges::equal_to, projected<iterator_t<R>, Proj>, const T1*> constexpr ranges::replace_copy_result<borrowed_iterator_t<R>, O> ranges::replace_copy(R&& r, O result, const T1& old_value, const T2& new_value, Proj proj = {}); template<input_iterator I, sentinel_for<I> S, class T, weakly_incrementableoutput_iterator<const T&>O, class Proj = identity, indirect_unary_predicate<projected<I, Proj>> Pred> requires indirectly_writable<O, const T&> && indirectly_copyable<I, O> constexpr ranges::replace_copy_if_result<I, O> ranges::replace_copy_if(I first, S last, O result, Pred pred, const T& new_value, Proj proj = {}); template<input_range R, class T, weakly_incrementableoutput_iterator<const T&>O, class Proj = identity, indirect_unary_predicate<projected<iterator_t<R>, Proj>> Pred> requires indirectly_writable<O, const T&> && indirectly_copyable<iterator_t<R>, O> constexpr ranges::replace_copy_if_result<borrowed_iterator_t<R>, O> ranges::replace_copy_if(R&& r, O result, Pred pred, const T& new_value, Proj proj = {});-6- Let E be
[…]
Modify 27.7.6 [alg.fill] as indicated:
[…] template<class T, input_or_output_iteratoroutput_iterator<const T&>O, sentinel_for<O> S> requires indirectly_writable<O, const T&> constexpr O ranges::fill(O first, S last, const T& value); template<class T, output_range<const T&> R> constexpr borrowed_iterator_t<R> ranges::fill(R&& r, const T& value); template<class T, input_or_output_iteratoroutput_iterator<const T&>O> requires indirectly_writable<O, const T&> constexpr O ranges::fill_n(O first, iter_difference_t<O> n, const T& value);-1- Let N be max(0,
[…]n) for thefill_nalgorithms, andlast - firstfor thefillalgorithms.