std::ranges::find_first_of
从 zh.cppreference.net
| 定义于头文件 <algorithm>
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| 调用签名 |
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template< std::input_iterator I1, std::sentinel_for<I1> S1,
std::forward_iterator I2, std::sentinel_for<I2> S2,
class Pred = ranges::equal_to,
class Proj1 = std::identity, class Proj2 = std::identity >
requires std::indirectly_comparable<I1, I2, Pred, Proj1, Proj2>
constexpr I1
find_first_of( I1 first1, S1 last1, I2 first2, S2 last2, Pred pred = {},
Proj1 proj1 = {}, Proj2 proj2 = {} );
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(1) | (从 C++20 起) |
template< ranges::input_range R1, ranges::forward_range R2,
class Pred = ranges::equal_to,
class Proj1 = std::identity, class Proj2 = std::identity >
requires std::indirectly_comparable<ranges::iterator_t<R1>,
ranges::iterator_t<R2>,
Pred, Proj1, Proj2>
constexpr ranges::borrowed_iterator_t<R1>
find_first_of( R1&& r1, R2&& r2, Pred pred = {},
Proj1 proj1 = {}, Proj2 proj2 = {} );
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(2) | (从 C++20 起) |
template< /*execution-policy*/ Ep,
std::random_access_iterator I1, std::sized_sentinel_for<I1> S1,
std::random_access_iterator I2, std::sized_sentinel_for<I2> S2,
class Pred = ranges::equal_to,
class Proj1 = std::identity, class Proj2 = std::identity >
requires std::indirectly_comparable<I1, I2, Pred, Proj1, Proj2>
I1 find_first_of( Ep&& policy, I1 first1, S1 last1, I2 first2, S2 last2,
Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {} );
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(3) | (从 C++26 起) |
template< /*execution-policy*/ Ep,
/*sized-random-access-range*/ R1,
/*sized-random-access-range*/ R2,
class Pred = ranges::equal_to,
class Proj1 = std::identity, class Proj2 = std::identity >
requires std::indirectly_comparable<ranges::iterator_t<R1>,
ranges::iterator_t<R2>,
Pred, Proj1, Proj2>
ranges::borrowed_iterator_t<R1>
find_first_of( Ep&& policy, R1&& r1, R2&& r2, Pred pred = {},
Proj1 proj1 = {}, Proj2 proj2 = {} );
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(4) | (从 C++26 起) |
关于 /*execution-policy*/ 的定义，请参见 此页面；关于 /*sized-random-access-range*/ 的定义，请参见 此页面。
在源范围中查找目标范围中的任何一个元素。元素（分别通过 proj1 和 proj2 投影）使用二元谓词 pred 进行比较。
1) 源范围是
[first1, last1)，目标范围是 [first2, last2)。2) 源范围是
r1，目标范围是 r2。3,4) 与 (1,2) 相同，但根据
policy 执行。本页面描述的函数式实体是 算法函数对象（非正式地称为 niebloids），也就是说：
参数
| first1, last1 | - | 定义源范围的迭代器‑哨兵对 范围 |
| first2, last2 | - | 定义目标范围的迭代器‑哨兵对 范围 |
| r1 | - | 源范围 |
| r2 | - | 目标范围 |
| pred | - | 应用于（投影后的）元素的谓词 |
| proj1 | - | 应用于源范围中元素的投影 |
| proj2 | - | 应用于目标范围中元素的投影 |
| policy | - | 要使用的 执行策略 |
返回值
指向源范围内与目标范围内元素匹配的第一个元素的迭代器。
如果目标范围为空或未找到这样的元素，则返回：
1,3)
last12,4)
ranges::next(ranges::begin(r1), ranges::end(r1))复杂度
给定 N1 作为 ranges::distance(first1, last1) 或 ranges::distance(r1)，且 N2 作为 ranges::distance(first2, last2) 或 ranges::distance(r2)：
1,2) 最多 N1⋅N2 次应用
pred 和 proj。3,4) 𝓞(N1⋅N2) 次应用
pred 和 proj。异常
3,4) 在执行过程中：
- 如果并行化所需的临时内存资源不可用，std::bad_alloc 将被抛出。
- 如果通过算法参数访问对象时抛出了未捕获的异常，其行为由执行策略决定（对于标准策略， std::terminate 将被调用）。
可能的实现
struct find_first_of_fn
{
template<std::input_iterator I1, std::sentinel_for<I1> S1,
std::forward_iterator I2, std::sentinel_for<I2> S2,
class Pred = ranges::equal_to,
class Proj1 = std::identity, class Proj2 = std::identity>
requires std::indirectly_comparable<I1, I2, Pred, Proj1, Proj2>
constexpr I1 operator()(I1 first1, S1 last1, I2 first2, S2 last2, Pred pred = {},
Proj1 proj1 = {}, Proj2 proj2 = {}) const
{
for (; first1 != last1; ++first1)
for (auto i = first2; i != last2; ++i)
if (std::invoke(pred, std::invoke(proj1, *first1), std::invoke(proj2, *i)))
return first1;
return first1;
}
template<ranges::input_range R1, ranges::forward_range R2,
class Pred = ranges::equal_to,
class Proj1 = std::identity, class Proj2 = std::identity>
requires std::indirectly_comparable<ranges::iterator_t<R1>,
ranges::iterator_t<R2>,
Pred, Proj1, Proj2>
constexpr ranges::borrowed_iterator_t<R1>
operator()(R1&& r1, R2&& r2, Pred pred = {},
Proj1 proj1 = {}, Proj2 proj2 = {}) const
{
return (*this)(ranges::begin(r1), ranges::end(r1),
ranges::begin(r2),
ranges::next(ranges::begin(r2), ranges::end(r2)),
std::move(pred), std::move(proj1), std::move(proj2));
}
template<ranges::forward_range R1, ranges::forward_range R2,
class Pred = ranges::equal_to,
class Proj1 = std::identity, class Proj2 = std::identity>
requires std::indirectly_comparable<ranges::iterator_t<R1>,
ranges::iterator_t<R2>,
Pred, Proj1, Proj2>
constexpr ranges::borrowed_iterator_t<R1>
operator()(R1&& r1, R2&& r2, Pred pred = {},
Proj1 proj1 = {}, Proj2 proj2 = {}) const
{
return (*this)(ranges::begin(r1),
ranges::next(ranges::begin(r1), ranges::end(r1)),
ranges::begin(r2),
ranges::next(ranges::begin(r2), ranges::end(r2)),
std::move(pred), std::move(proj1), std::move(proj2));
}
};
inline constexpr find_first_of_fn find_first_of{};
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示例
运行此代码
#include <algorithm>
#include <iostream>
#include <iterator>
int main()
{
using std::ranges::find_first_of;
constexpr static auto haystack = {1, 2, 3, 4};
constexpr static auto needles = {0, 3, 4, 3};
constexpr auto found1 = find_first_of(haystack.begin(), haystack.end(),
needles.begin(), needles.end());
static_assert(std::distance(haystack.begin(), found1) == 2);
constexpr auto found2 = find_first_of(haystack, needles);
static_assert(std::distance(haystack.begin(), found2) == 2);
constexpr static auto negatives = {-6, -3, -4, -3};
constexpr auto not_found = find_first_of(haystack, negatives);
static_assert(not_found == haystack.end());
constexpr auto found3 = find_first_of(haystack, negatives,
[](int x, int y) { return x == -y; }); // uses a binary comparator
static_assert(std::distance(haystack.begin(), found3) == 2);
struct P { int x, y; };
constexpr static auto p1 = {P{1, -1}, P{2, -2}, P{3, -3}, P{4, -4}};
constexpr static auto p2 = {P{5, -5}, P{6, -3}, P{7, -5}, P{8, -3}};
// Compare only P::y data members by projecting them:
const auto found4 = find_first_of(p1, p2, {}, &P::y, &P::y);
std::cout << "First equivalent element {" << found4->x << ", " << found4->y
<< "} was found at position " << std::distance(p1.begin(), found4)
<< ".\n";
}
输出：
First equivalent element {3, -3} was found at position 2.
参见
|
搜索一组元素中的任意一个
(函数模板) |
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(C++20)
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查找首个相邻的相等元素（或满足给定谓词的元素）
(算法函数对象) |
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(C++20)
(C++20)
(C++20)
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查找首个满足特定条件的元素
(算法函数对象) |
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(C++20)
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在特定范围内查找元素的最后序列
(算法函数对象) |
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(C++20)
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搜索元素范围的首次出现
(算法函数对象) |
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(C++20)
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在范围内搜索连续多次出现的元素的首次出现
(算法函数对象) |