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### 538. 241 again: Does unique_copy() require CopyConstructible and Assignable?

Section: 25.7.9 [alg.unique] Status: CD1 Submitter: Howard Hinnant Opened: 2006-02-09 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

I believe I botched the resolution of 241 "Does unique_copy() require CopyConstructible and Assignable?" which now has WP status.

This talks about `unique_copy` requirements and currently reads:

> -5- Requires: The ranges `[first, last)` and `[result, result+(last-first))` shall not overlap. The expression `*result = *first` shall be valid. If neither `InputIterator` nor `OutputIterator` meets the requirements of forward iterator then the value type of `InputIterator` must be CopyConstructible (20.1.3). Otherwise CopyConstructible is not required.

The problem (which Paolo discovered) is that when the iterators are at their most restrictive (`InputIterator`, `OutputIterator`), then we want `InputIterator::value_type` to be both `CopyConstructible` and `CopyAssignable` (for the most efficient implementation). However this proposed resolution only makes it clear that it is `CopyConstructible`, and that one can assign from `*first` to `*result`. This latter requirement does not necessarily imply that you can:

>     *first = *first;

Proposed resolution:

> -5- Requires: The ranges `[first, last)` and `[result, result+(last-first))` shall not overlap. The expression `*result = *first` shall be valid. If neither `InputIterator` nor `OutputIterator` meets the requirements of forward iterator then the value type `value_type` of `InputIterator` must be CopyConstructible (20.1.3) and Assignable. Otherwise CopyConstructible is not required.