4. starts_with

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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

参照