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algorithm/count : C++26波カッコ初期化に対応 #1311 
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‎reference/algorithm/count.md‎

Lines changed: 68 additions & 6 deletions
Original file line numberDiff line numberDiff line change
@@ -5,24 +5,41 @@
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```cpp
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namespace std {
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template <class InputIterator, class T>
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template <class InputIterator,
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class T>
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typename iterator_traits<InputIterator>::difference_type
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count(InputIterator first,
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InputIterator last,
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const T& value); // (1) C++03
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template <class InputIterator, class T>
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template <class InputIterator,
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class T>
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constexpr typename iterator_traits<InputIterator>::difference_type
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count(InputIterator first,
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InputIterator last,
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const T& value); // (1) C++20
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template <class InputIterator,
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class T = typename iterator_traits<InputIterator>::value_type>
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constexpr typename iterator_traits<InputIterator>::difference_type
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count(InputIterator first,
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InputIterator last,
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const T& value); // (1) C++26
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20-
template<class ExecutionPolicy, class ForwardIterator, class T>
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template <class ExecutionPolicy,
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class ForwardIterator,
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class T>
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typename iterator_traits<ForwardIterator>::difference_type
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count(ExecutionPolicy&& exec,
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ForwardIterator first,
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ForwardIterator last,
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const T& value); // (2) C++17
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template <class ExecutionPolicy,
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class ForwardIterator,
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class T = typename iterator_traits<ForwardIterator>::value_type>
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typename iterator_traits<ForwardIterator>::difference_type
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count(ExecutionPolicy&& exec,
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ForwardIterator first,
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ForwardIterator last,
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const T& value); // (2) C++16
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}
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```
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* iterator_traits[link /reference/iterator/iterator_traits.md]
@@ -39,7 +56,16 @@ namespace std {
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正確に `last - first` 回の比較を行う
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## 備考
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- (1), (2) :
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- C++26 : 引数として波カッコ初期化`{}`を受け付ける
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```cpp
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std::vector<T> v;
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int n = std::count(v.begin(), v.begin(), {a, b});
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```
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## 例
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### 基本的な使い方
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```cpp example
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#include <algorithm>
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#include <iostream>
@@ -49,16 +75,50 @@ int main() {
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std::vector<int> v = { 1,4,3,3,1,2,2,1 };
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// 値が 1 の要素がいくつあるかを数える
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std::cout << "count of 1: " << std::count(v.begin(), v.end(), 1) << std::endl;
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int n = std::count(v.begin(), v.end(), 1);
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std::cout << "count of 1: " << n << std::endl;
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}
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```
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* std::count[color ff0000]
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### 出力
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#### 出力
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```
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count of 1: 3
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```
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### 波カッコ初期化を入力として使用する (C++26)
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```cpp example
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#include <algorithm>
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#include <iostream>
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#include <vector>
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struct Point {
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int x;
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int y;
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bool operator==(const Point& other) const = default;
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};
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int main() {
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std::vector<Point> v = {
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{1, 2},
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{3, 4},
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{5, 6},
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{1, 2},
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};
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// 値が {1, 2} の要素がいくつあるかを数える
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int n = std::count(v.begin(), v.end(), {1, 2});
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std::cout << "count of {1,2}: " << n << std::endl;
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}
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```
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* std::count[color ff0000]
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#### 出力
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```
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count of {1,2}: 2
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```
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## 実装例
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```cpp
@@ -85,3 +145,5 @@ typename iterator_traits<InputIterator>::difference_type
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## 参照
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- [P0202R3 Add Constexpr Modifiers to Functions in `<algorithm>` and `<utility>` Headers](http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0202r3.html)
87147
- [P0467R2 Iterator Concerns for Parallel Algorithms](http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0467r2.html)
148+
- [P2248R8 Enabling list-initialization for algorithms](https://open-std.org/jtc1/sc22/wg21/docs/papers/2024/p2248r8.html)
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- C++26で波カッコ初期化 (リスト初期化) に対応した

‎reference/algorithm/ranges_count.md‎

Lines changed: 73 additions & 4 deletions
Original file line numberDiff line numberDiff line change
@@ -12,14 +12,37 @@ namespace std::ranges {
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class Proj = identity>
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requires indirect_binary_predicate<ranges::equal_to, projected<I, Proj>, const T*>
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constexpr iter_difference_t<I>
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count(I first, S last, const T& value, Proj proj = {}); // (1) C++20
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count(I first,
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S last,
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const T& value,
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Proj proj = {}); // (1) C++20
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template <input_iterator I,
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sentinel_for<I> S,
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class Proj = identity,
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class T = projected_value_t<I, Proj>>
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requires indirect_binary_predicate<ranges::equal_to, projected<I, Proj>, const T*>
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constexpr iter_difference_t<I>
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count(I first,
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S last,
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const T& value,
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Proj proj = {}); // (1) C++26
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template <input_range R,
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class T,
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class Proj = identity>
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requires indirect_binary_predicate<ranges::equal_to, projected<iterator_t<R>, Proj>, const T*>
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constexpr range_difference_t<R>
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count(R&& r, const T& value, Proj proj = {}); // (2) C++20
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count(R&& r,
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const T& value,
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Proj proj = {}); // (2) C++20
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template <input_range R,
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class Proj = identity,
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class T = projected_value_t<iterator_t<R>, Proj>>
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requires indirect_binary_predicate<ranges::equal_to, projected<iterator_t<R>, Proj>, const T*>
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constexpr range_difference_t<R>
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count(R&& r,
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const T& value,
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Proj proj = {}); // (2) C++26
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}
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```
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* input_iterator[link /reference/iterator/input_iterator.md]
@@ -55,7 +78,17 @@ namespace std::ranges {
5578
正確に `last - first` 回の比較を行う
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## 備考
82+
- (1), (2) :
83+
- C++26 : 引数として波カッコ初期化`{}`を受け付ける
84+
```cpp
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std::vector<T> v;
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int n = std::ranges::count(v, {a, b});
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```
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## 例
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### 基本的な使い方
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```cpp example
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#include <algorithm>
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#include <iostream>
@@ -65,16 +98,50 @@ int main() {
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constexpr std::array v = { 1,4,3,3,1,2,2,1 };
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67100
// 値が 1 の要素がいくつあるかを数える
68-
std::cout << "count of 1: " << std::ranges::count(v, 1) << std::endl;
101+
int n = std::ranges::count(v, 1);
102+
std::cout << "count of 1: " << n << std::endl;
69103
}
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```
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* std::ranges::count[color ff0000]
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73-
### 出力
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#### 出力
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```
75109
count of 1: 3
76110
```
77111

112+
### 波カッコ初期化を入力として使用する (C++26)
113+
```cpp example
114+
#include <algorithm>
115+
#include <iostream>
116+
#include <vector>
117+
118+
struct Point {
119+
int x;
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int y;
121+
122+
bool operator==(const Point& other) const = default;
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};
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125+
int main() {
126+
std::vector<Point> v = {
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{1, 2},
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{3, 4},
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{5, 6},
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{1, 2},
131+
};
132+
133+
// 値が {1, 2} の要素がいくつあるかを数える
134+
int n = std::ranges::count(v, {1, 2});
135+
std::cout << "count of {1,2}: " << n << std::endl;
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}
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```
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* std::ranges::count[color ff0000]
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#### 出力
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```
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count of {1,2}: 2
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```
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## 実装例
80147
```cpp
@@ -124,3 +191,5 @@ inline constexpr count_impl count;
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125192
## 参照
126193
- [N4861 25 Algorithms library](https://timsong-cpp.github.io/cppwp/n4861/algorithms)
194+
- [P2248R8 Enabling list-initialization for algorithms](https://open-std.org/jtc1/sc22/wg21/docs/papers/2024/p2248r8.html)
195+
- C++26で波カッコ初期化 (リスト初期化) に対応した

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