最終更新日時: 2026年02月05日 13時53分18秒 (JST)
Akira Takahashi が更新

履歴 編集

function template
<algorithm>

# std::ranges::starts_with(C++23)

    namespace std::ranges {
      template <input_iterator I1,
                sentinel_for<I1> S1,
                input_iterator I2,
                sentinel_for<I2> S2,
                class Pred = ranges::equal_to,
                class Proj1 = identity,
                class Proj2 = identity>
        requires indirectly_comparable<I1, I2, Pred, Proj1, Proj2>
      constexpr bool
        starts_with(I1 first1,
                    S1 last1,
                    I2 first2,
                    S2 last2,
                    Pred pred = {},
                    Proj1 proj1 = {},
                    Proj2 proj2 = {}); // (1) C++23

      template <input_range R1,
                input_range R2,
                class Pred = ranges::equal_to,
                class Proj1 = identity,
                class Proj2 = identity>
        requires indirectly_comparable<iterator_t<R1>, iterator_t<R2>, Pred, Proj1, Proj2>
      constexpr bool
        starts_with(R1&& r1,
                    R2&& r2,
                    Pred pred = {},
                    Proj1 proj1 = {},
                    Proj2 proj2 = {}); // (2) C++23

      template <execution-policy Ep,
                random_access_iterator I1,
                sized_sentinel_for<I1> S1,
                random_access_iterator I2,
                sized_sentinel_for<I2> S2,
                class Pred = ranges::equal_to,
                class Proj1 = identity,
                class Proj2 = identity>
        requires indirectly_comparable<I1, I2, Pred, Proj1, Proj2>
      bool
        starts_with(Ep&& exec,
                    I1 first1,
                    S1 last1,
                    I2 first2,
                    S2 last2,
                    Pred pred = {},
                    Proj1 proj1 = {},
                    Proj2 proj2 = {}); // (3) C++26

      template <execution-policy Ep,
                sized-random-access-range R1,
                sized-random-access-range R2,
                class Pred = ranges::equal_to,
                class Proj1 = identity,
                class Proj2 = identity>
        requires indirectly_comparable<iterator_t<R1>, iterator_t<R2>, Pred, Proj1, Proj2>
      bool
        starts_with(Ep&& exec,
                    R1&& r1,
                    R2&& r2,
                    Pred pred = {},
                    Proj1 proj1 = {},
                    Proj2 proj2 = {}); // (4) C++26
    }

## 概要

シーケンスの先頭が指定されたシーケンスと一致するかを調べる

- (1): イテレータ範囲を指定する
- (2): Rangeを直接指定する
- (3): (1)の並列アルゴリズム版。実行ポリシーを指定する
- (4): (2)の並列アルゴリズム版。実行ポリシーを指定する

## 戻り値

`ranges::mismatch(std::move(first1), last1, std::move(first2), last2, pred, proj1, proj2).in2 == last2`

## 計算量

最大で `min(last1 - first1, last2 - first2)` 回の対応する述語が適用される。

## 例

### 基本的な使い方

    #include <algorithm>
    #include <iostream>
    #include <vector>

    int main() {
      const std::vector v  = { 1,2,3,4,5,6 };
      const std::vector v1 = { 1,2,3 };
      const std::vector v2 = { 2,3,4 };

      std::cout << std::ranges::starts_with(v, v1) << std::endl;
      std::cout << std::ranges::starts_with(v, v2) << std::endl;
    }

#### 出力

    1
    0

### 並列アルゴリズムの例 (C++26)

    #include <algorithm>
    #include <execution>
    #include <iostream>
    #include <vector>

    int main() {
      std::vector<int> v = {1, 2, 3, 4, 5, 6};
      std::vector<int> prefix = {1, 2, 3};

      std::cout << std::boolalpha;

      // 並列に先頭が一致するかを判定
      bool result = std::ranges::starts_with(std::execution::par, v, prefix);
      std::cout << result << std::endl;
    }

#### 出力

    true

## 実装例

    struct starts_with_impl {
      template<input_iterator I1, sentinel_for<I1> S1, input_iterator I2, sentinel_for<I2> S2, class Pred = ranges::equal_to, class Proj1 = identity, class Proj2 = identity>
        requires indirectly_comparable<I1, I2, Pred, Proj1, Proj2>
      constexpr bool operator()(I1 first1, S1 last1, I2 first2, S2 last2, Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {}) const {
        return mismatch(move(first1), last1, move(first2), last2, pred, proj1, proj2).in2 == last2;
      }

      template<input_range R1, input_range R2, class Pred = ranges::equal_to, class Proj1 = identity, class Proj2 = identity>
        requires indirectly_comparable<iterator_t<R1>, iterator_t<R2>, Pred, Proj1, Proj2>
      constexpr bool operator()(R1&& r1, R2&& r2, Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {}) const {
        return (*this)(begin(r1), end(r1), begin(r2), end(r2), ref(pred), ref(proj1), ref(proj2));
      }
    };

    inline constexpr starts_with_impl starts_with;

## バージョン

### 言語

- C++23

### 処理系

- Clang: ??
- GCC: ??
- ICC: ??
- Visual C++: ??

## 参照

- N4950 27 Algorithms library
- P3179R9 C++ parallel range algorithms