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Section: 25.7.4 [alg.partitions] Status: Open Submitter: Jonathan Wakely Opened: 2016-07-06 Last modified: 2019-03-17
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Requires: InputIterator's value type shall be convertible to Predicate's argument type.
This seems to date from the days of adaptable function objects with an argument_type typedef, but in modern C++ the predicate might not have an argument type. It could have a function template that accepts various arguments, so it doesn't make sense to state requirements in terms of a type that isn't well defined.
[2016-07, Toronto Saturday afternoon issues processing]
The proposed resolution needs to be updated because the underlying wording has changed. Also, since the sequence is homogeneous, we shouldn't have to say that the expression is well-formed for all elements in the range; that implies that it need not be well-formed if the range is empty.
Marshall and JW to reword. Status to Open
Previous resolution [SUPERSEDED]:
This wording is relative to N4594.
Edit 25.7.4 [alg.partitions] as indicated:template <class InputIterator, class Predicate> bool is_partitioned(InputIterator first, InputIterator last, Predicate pred);
InputIterator's value type shall be convertible to Predicate's argument type.template <class InputIterator, class OutputIterator1, class OutputIterator2, class Predicate> pair<OutputIterator1, OutputIterator2> partition_copy(InputIterator first, InputIterator last, OutputIterator1 out_true, OutputIterator2 out_false, Predicate pred);
-12- Requires: InputIterator's value type shall be CopyAssignable, and shall be writable (23.3.1 [iterator.requirements.general]) to the out_true and out_false OutputIterators, and[…]
shall be convertible to Predicate's argument type. The input range shall not overlap with either of the output ranges.template<class ForwardIterator, class Predicate> ForwardIterator partition_point(ForwardIterator first, ForwardIterator last, Predicate pred);
ForwardIterator's value type shall be convertible to Predicate's argument type. [first, last) shall be partitioned by pred, i.e. all elements that satisfy pred shall appear before those that do not.
[2019-03-17; Daniel comments and removes previous wording]
In the recent working draft N4810 all the "shall be convertible to Predicate's argument type" are gone - they were cleaned up when "The One Range" proposal P0896R4 had been accepted in San Diego 2018.I also believe that we don't need the extra wording that was suggested in the previous P/R (effectively the need to say that the expression pred(*i) is well-formed), because this follows (in a more general way) by the expression invoke(pred, invoke(proj, e)) that is applied to the elements e of [first, last). Therefore I'm suggesting that the resolution for this issue should be: Resolved by P0896R4.