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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) |
(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) |
(C++20 起) |
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1) 在范围 [first1, last1) 中搜索任何范围
[first2, last2) 中的元素，分别以 proj1 与 proj2 投影范围。用二元谓词
pred 比较投影后的元素。
2) 同 (1) ，但以 r1
为第一源范围并以 r2 为第二源范围，如同以 ranges::begin(r1) 为 first1 ，以 ranges::end(r1) 为 last1 ，以 ranges::begin(r2) 为 first2 ，并以 ranges::end(r2) 为 last2 。
此页面上描述的仿函数实体是 niebloid，即：
实际上，它们能以函数对象，或者某些特殊编译器扩展实现。
参数
| first1, last1
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要检验的元素范围（又称草堆）
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| first2, last2
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要搜索的元素范围（又称针）
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| r1
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要检验的元素范围（又称草堆）
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| r2
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-
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要搜索的元素范围（又称针）
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| pred
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比较元素的二元谓词
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| proj1
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应用到第一范围中元素的投影
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| proj2
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-
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应用到第二范围中元素的投影
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返回值
指向范围 [first1, last1) 中首个在投影后等于来自范围 [first2, last2)
中元素的迭代器。若找不到这种元素，则返回等于 last1 的迭代器。
复杂度
至多应用 (S*N) 次比较和各自的投影，其中
(1) S = ranges::distance(first2, last2) 而 N = ranges::distance(first1, last1) ；
(2) S = ranges::size(r2) 而 N = ranges::size(r1) 。
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::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
#include <algorithm>
#include <iostream>
#include <iterator>
int main()
{
namespace rng = std::ranges;
constexpr static auto haystack = {1, 2, 3, 4};
constexpr static auto needles = {0, 3, 4, 3};
constexpr auto found1 = rng::find_first_of(haystack.begin(), haystack.end(),
needles.begin(), needles.end());
static_assert(std::distance(haystack.begin(), found1) == 2);
constexpr auto found2 = rng::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 = rng::find_first_of(haystack, negatives);
static_assert(not_found == haystack.end());
constexpr auto found3 = rng::find_first_of(haystack, negatives,
[](int x, int y) { return x == -y; }); // 使用二元比较器
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} };
// 仅比较 P::y 数据成员，通过投影它们：
const auto found4 = rng::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 equavalent element {3, -3} was found at position 2.
参阅
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搜索元素集合中的任意元素
(函数模板) |
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查找首对相邻的相同（或满足给定谓词的）元素
(niebloid) |
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查找满足特定条件的的第一个元素
(niebloid) |
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查找特定范围中最后出现的元素序列
(niebloid) |
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搜索一个元素范围
(niebloid) |
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在范围中搜索一定量的某个元素的连续副本
(niebloid) |