ranges_starts_with.md

starts_with

• algorithm[meta header]
• std::ranges[meta namespace]
• function template[meta id-type]
• cpp23[meta cpp]

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
}

• input_iterator[link /reference/iterator/input_iterator.md]
• sentinel_for[link /reference/iterator/sentinel_for.md]
• ranges::equal_to[link /reference/functional/ranges_equal_to.md]
• identity[link /reference/functional/identity.md]
• indirectly_comparable[link /reference/iterator/indirectly_comparable.md]
• input_range[link /reference/ranges/input_range.md]
• iterator_t[link /reference/ranges/iterator_t.md]

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

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

#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;
}

• std::ranges::starts_with[color ff0000]

出力

1
0

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;

• input_iterator[link /reference/iterator/input_iterator.md]
• sentinel_for[link /reference/iterator/sentinel_for.md]
• ranges::equal_to[link /reference/functional/ranges_equal_to.md]
• identity[link /reference/functional/identity.md]
• indirectly_comparable[link /reference/iterator/indirectly_comparable.md]
• input_range[link /reference/ranges/input_range.md]
• iterator_t[link /reference/ranges/iterator_t.md]
• move[link /reference/utility/move.md]
• mismatch[link ranges_mismatch.md]
• begin[link /reference/ranges/begin.md]
• end[link /reference/ranges/end.md]
• ref[link /reference/functional/ref.md]

バージョン

言語

• C++23

処理系

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

参照

• N4950 27 Algorithms library