std::ranges::find_last, std::ranges::find_last_if, std::ranges::find_last_if_not
| 在标头 <algorithm> 定义
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| 调用签名 |
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| (1) | ||
template< std::forward_iterator I, std::sentinel_for<I> S, class T, |
(C++23 起) (C++26 前) |
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| template< std::forward_iterator I, std::sentinel_for<I> S, class Proj = std::identity, |
(C++26 起) | |
| (2) | ||
template< ranges::forward_range R, class T, |
(C++23 起) (C++26 前) |
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| template< ranges::forward_range R, class Proj = std::identity, |
(C++26 起) | |
template< std::forward_iterator I, std::sentinel_for<I> S, class Proj = std::identity, |
(3) | (C++23 起) |
| template< ranges::forward_range R, class Proj = std::identity, |
(4) | (C++23 起) |
template< std::forward_iterator I, std::sentinel_for<I> S, class Proj = std::identity, |
(5) | (C++23 起) |
| template< ranges::forward_range R, class Proj = std::identity, |
(6) | (C++23 起) |
返回范围 [first, last) 中符合特定条件的最后一个元素：
find_last 搜索等于 value 的元素。find_last_if 搜索范围 [first, last) 中谓词 pred 为之返回 true 的最后一个元素。find_last_if_not 搜索范围 [first, last) 中谓词 pred 为之返回 false 的最后一个元素。此页面上描述的函数式实体是 niebloid，即：
实践中，可以作为函数对象，或者用某些特殊编译器扩展实现它们。
参数
| first, last | - | 要检查的元素的范围 |
| r | - | 要检查的元素的范围 |
| value | - | 和元素进行比较的值 |
| pred | - | 应用到投影后元素的谓词 |
| proj | - | 应用到元素的投影 |
返回值
i 是 [first, last) 范围内使得 E 是 true 的最后一个迭代器。复杂度
最多应用 last - first 次谓词和投影。
注解
如果 ranges::find_last、ranges::find_last_if、ranges::find_last_if_not 的 I 为 bidirectional_iterator 或（更好的）random_access_iterator，则它们在通用实现上的效率更高。
| 功能特性测试宏 | 值 | 标准 | 功能特性 |
|---|---|---|---|
__cpp_lib_ranges_find_last |
202207L | (C++23) | ranges::find_last，ranges::find_last_if，ranges::find_last_if_not
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__cpp_lib_algorithm_default_value_type |
202403L | (C++26) | 算法的列表初始化 (1,2) |
可能的实现
这个实现只展示 I 为 forward_iterator 时较慢的算法。
| find_last (1,2) |
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struct find_last_fn { template<std::forward_iterator I, std::sentinel_for<I> S, class Proj = std::identity, class T = std::projected_value_t<iterator_t<R>, Proj>> requires std::indirect_binary_predicate <ranges::equal_to, std::projected<I, Proj>, const T*> constexpr ranges::subrange<I> operator()(I first, S last, const T &value, Proj proj = {}) const { // 注意：如果 I 只是 forward_iterator，那么只能从头走到尾。 std::optional<I> found; for (; first != last; ++first) if (std::invoke(proj, *first) == value) found = first; if (!found) return {first, first}; return {*found, std::ranges::next(*found, last)}; } template<ranges::forward_range R, class Proj = std::identity, class T = std::projected_value_t<iterator_t<R>, Proj>> requires std::indirect_binary_predicate <ranges::equal_to, std::projected<ranges::iterator_t<R>, Proj>, const T*> constexpr ranges::borrowed_subrange_t<R> operator()(R&& r, const T &value, Proj proj = {}) const { return this->operator()(ranges::begin(r), ranges::end(r), value, std::ref(proj)); } }; inline constexpr find_last_fn find_last; |
| find_last_if (3,4) |
struct find_last_if_fn { template<std::forward_iterator I, std::sentinel_for<I> S, class Proj = std::identity, std::indirect_unary_predicate<std::projected<I, Proj>> Pred> constexpr ranges::subrange<I> operator()(I first, S last, Pred pred, Proj proj = {}) const { // 注意：如果 I 只是 forward_iterator，那么只能从头走到尾。 std::optional<I> found; for (; first != last; ++first) if (std::invoke(pred, std::invoke(proj, *first))) found = first; if (!found) return {first, first}; return {*found, std::ranges::next(*found, last)}; } template<ranges::forward_range R, class Proj = std::identity, std::indirect_unary_predicate <std::projected<ranges::iterator_t<R>, Proj>> Pred> constexpr ranges::borrowed_subrange_t<R> operator()(R&& r, Pred pred, Proj proj = {}) const { return this->operator()(ranges::begin(r), ranges::end(r), std::ref(pred), std::ref(proj)); } }; inline constexpr find_last_if_fn find_last_if; |
| find_last_if_not (5,6) |
struct find_last_if_not_fn { template<std::forward_iterator I, std::sentinel_for<I> S, class Proj = std::identity, std::indirect_unary_predicate<std::projected<I, Proj>> Pred> constexpr ranges::subrange<I> operator()(I first, S last, Pred pred, Proj proj = {}) const { // 注意：如果 I 只是 forward_iterator，那么只能从头走到尾。 std::optional<I> found; for (; first != last; ++first) if (!std::invoke(pred, std::invoke(proj, *first))) found = first; if (!found) return {first, first}; return {*found, std::ranges::next(*found, last)}; } template<ranges::forward_range R, class Proj = std::identity, std::indirect_unary_predicate <std::projected<ranges::iterator_t<R>, Proj>> Pred> constexpr ranges::borrowed_subrange_t<R> operator()(R&& r, Pred pred, Proj proj = {}) const { return this->operator()(ranges::begin(r), ranges::end(r), std::ref(pred), std::ref(proj)); } }; inline constexpr find_last_if_not_fn find_last_if_not; |
示例
#include <algorithm> #include <cassert> #include <forward_list> #include <iomanip> #include <iostream> #include <string_view> int main() { namespace ranges = std::ranges; constexpr static auto v = {1, 2, 3, 1, 2, 3, 1, 2}; { constexpr auto i1 = ranges::find_last(v.begin(), v.end(), 3); constexpr auto i2 = ranges::find_last(v, 3); static_assert(ranges::distance(v.begin(), i1.begin()) == 5); static_assert(ranges::distance(v.begin(), i2.begin()) == 5); } { constexpr auto i1 = ranges::find_last(v.begin(), v.end(), -3); constexpr auto i2 = ranges::find_last(v, -3); static_assert(i1.begin() == v.end()); static_assert(i2.begin() == v.end()); } auto abs = [](int x) { return x < 0 ? -x : x; }; { auto pred = [](int x) { return x == 3; }; constexpr auto i1 = ranges::find_last_if(v.begin(), v.end(), pred, abs); constexpr auto i2 = ranges::find_last_if(v, pred, abs); static_assert(ranges::distance(v.begin(), i1.begin()) == 5); static_assert(ranges::distance(v.begin(), i2.begin()) == 5); } { auto pred = [](int x) { return x == -3; }; constexpr auto i1 = ranges::find_last_if(v.begin(), v.end(), pred, abs); constexpr auto i2 = ranges::find_last_if(v, pred, abs); static_assert(i1.begin() == v.end()); static_assert(i2.begin() == v.end()); } { auto pred = [](int x) { return x == 1 or x == 2; }; constexpr auto i1 = ranges::find_last_if_not(v.begin(), v.end(), pred, abs); constexpr auto i2 = ranges::find_last_if_not(v, pred, abs); static_assert(ranges::distance(v.begin(), i1.begin()) == 5); static_assert(ranges::distance(v.begin(), i2.begin()) == 5); } { auto pred = [](int x) { return x == 1 or x == 2 or x == 3; }; constexpr auto i1 = ranges::find_last_if_not(v.begin(), v.end(), pred, abs); constexpr auto i2 = ranges::find_last_if_not(v, pred, abs); static_assert(i1.begin() == v.end()); static_assert(i2.begin() == v.end()); } using P = std::pair<std::string_view, int>; std::forward_list<P> list { {"one", 1}, {"two", 2}, {"three", 3}, {"one", 4}, {"two", 5}, {"three", 6}, }; auto cmp_one = [](const std::string_view &s) { return s == "one"; }; // 寻找满足比较器的最后一个元素，其按 pair::first 投影 const auto subrange = ranges::find_last_if(list, cmp_one, &P::first); std::cout << "所找到的元素和其后的尾部为:\n"; for (P const& e : subrange) std::cout << '{' << std::quoted(e.first) << ", " << e.second << "} "; std::cout << '\n'; #if __cpp_lib_algorithm_default_value_type const auto i3 = ranges::find_last(list, {"three", 3}); // (2) C++26 #else const auto i3 = ranges::find_last(list, P{"three", 3}); // (2) C++23 #endif assert(i3.begin()->first == "three" && i3.begin()->second == 3); }
输出：
所找到的元素和其后的尾部为:
{"one", 4} {"two", 5} {"three", 6}参阅
| (C++20) |
查找特定范围中最后出现的元素序列 (niebloid) |
| (C++20)(C++20)(C++20) |
查找满足特定条件的的第一个元素 (niebloid) |
| (C++20) |
搜索一个元素范围的首次出现 (niebloid) |
| (C++20) |
若一个序列是另一个的子列则返回 true (niebloid) |
| (C++20) |
确定元素是否存在于某范围中 (niebloid) |
| (C++23)(C++23) |
检查范围是否包含给定的元素或子范围 (niebloid) |