Difference between revisions of "cpp/iterator/sentinel for"
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{{ddcl|header=iterator|since=c++20|1= | {{ddcl|header=iterator|since=c++20|1= | ||
| − | template<class S, class I> | + | template< class S, class I > |
| − | + | concept sentinel_for = | |
| − | + | std::semiregular<S> && | |
| − | + | std::input_or_output_iterator<I> && | |
| − | + | __WeaklyEqualityComparableWith<S, I>; | |
}} | }} | ||
| Line 12: | Line 12: | ||
===Semantic requirements=== | ===Semantic requirements=== | ||
| − | Let {{tt|s}} and {{tt|i}} be values of type {{tt|S}} and {{tt|I}}, respectively, such that {{ | + | Let {{tt|s}} and {{tt|i}} be values of type {{tt|S}} and {{tt|I}}, respectively, such that {{range|i|s}} denotes a [[cpp/iterator#Ranges|range]]. {{tt|sentinel_for<S, I>}} is modeled only if: |
* {{c|1=i == s}} is well-defined. | * {{c|1=i == s}} is well-defined. | ||
| − | * If {{c|1=bool(i != s)}} then {{tt|i}} is dereferenceable and {{ | + | * If {{c|1=bool(i != s)}} then {{tt|i}} is dereferenceable and {{range|++i|s}} denotes a range. |
* {{c|std::assignable_from<I&, S>}} is either modeled or not satisfied. | * {{c|std::assignable_from<I&, S>}} is either modeled or not satisfied. | ||
| − | The domain of {{tt|1===}} can change over time. Given an iterator {{tt|i}} and sentinel {{tt|s}} such that {{ | + | The domain of {{tt|1===}} can change over time. Given an iterator {{tt|i}} and sentinel {{tt|s}} such that {{range|i|s}} denotes a range and {{c|1=i != s}}, {{range|i|s}} is not required to continue to denote a range after incrementing any iterator equal to {{tt|i}} (and so {{c|1=i == s}} is no longer required to be well-defined after such an increment). |
===Notes=== | ===Notes=== | ||
| − | A sentinel type and its corresponding iterator type are not required to model {{lconcept|equality_comparable_with}}, because the sentinel type may | + | A sentinel type and its corresponding iterator type are not required to model {{lconcept|equality_comparable_with}}, because the sentinel type may not be comparable with itself, and they are not required to have a common reference type. |
It has been permitted to use a sentinel type different from the iterator type in the [[cpp/language/range-for|range-based {{tt|for}} loop]] since C++17. | It has been permitted to use a sentinel type different from the iterator type in the [[cpp/language/range-for|range-based {{tt|for}} loop]] since C++17. | ||
Latest revision as of 05:23, 12 May 2024
| Defined in header <iterator>
|
||
| template< class S, class I > concept sentinel_for = |
(since C++20) | |
The sentinel_for concept specifies the relationship between an input_or_output_iterator type and a semiregular type whose values denote a range. The exposition-only concept __WeaklyEqualityComparableWith is described in equality_comparable.
[edit] Semantic requirements
Let s and i be values of type S and I, respectively, such that [i, s) denotes a range. sentinel_for<S, I> is modeled only if:
- i == s is well-defined.
- If bool(i != s) then
iis dereferenceable and[++i,s)denotes a range. - std::assignable_from<I&, S> is either modeled or not satisfied.
The domain of == can change over time. Given an iterator i and sentinel s such that [i, s) denotes a range and i != s, [i, s) is not required to continue to denote a range after incrementing any iterator equal to i (and so i == s is no longer required to be well-defined after such an increment).
[edit] Notes
A sentinel type and its corresponding iterator type are not required to model equality_comparable_with, because the sentinel type may not be comparable with itself, and they are not required to have a common reference type.
It has been permitted to use a sentinel type different from the iterator type in the range-based for loop since C++17.
[edit] Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
| DR | Applied to | Behavior as published | Correct behavior |
|---|---|---|---|
| LWG 3453 | C++20 | semantic requirements for sentinel_for were too loose for ranges::advance
|
strengthened |
