std::ranges::find_last, std::ranges::find_last_if, std::ranges::find_last_if_not - cppreference.com

在标头 `<algorithm>` 定义
调用签名
template< std::forward_iterator I, std::sentinel_for<I> S, class T, class Proj = std::identity > requires std::indirect_binary_predicate <ranges::equal_to, std::projected<I, Proj>, const T*> constexpr ranges::subrange<I> find_last( I first, S last, const T& value, Proj proj = {} );	(1)	(C++23 起) (C++26 前)
template< std::forward_iterator I, std::sentinel_for<I> S, class Proj = std::identity, class T = std::projected_value_t<I, Proj> > requires std::indirect_binary_predicate <ranges::equal_to, std::projected<I, Proj>, const T*> constexpr ranges::subrange<I> find_last( I first, S last, const T& value, Proj proj = {} );	(1)		(C++26 起)
template< ranges::forward_range R, class T, class Proj = std::identity > requires std::indirect_binary_predicate <ranges::equal_to, std::projected<ranges::iterator_t<R>, Proj>, const T*> constexpr ranges::borrowed_subrange_t<R> find_last( R&& r, const T& value, Proj proj = {} );	(2)	(C++23 起) (C++26 前)
template< ranges::forward_range R, class Proj = std::identity, class T = std::projected_value_t<ranges::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> find_last( R&& r, const T& value, Proj proj = {} );	(2)		(C++26 起)
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> find_last_if( I first, S last, Pred pred, Proj proj = {} );	(3)	(C++23 起)
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> find_last_if( R&& r, Pred pred, Proj proj = {} );	(4)	(C++23 起)
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> find_last_if_not( I first, S last, Pred pred, Proj proj = {} );	(5)	(C++23 起)
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> find_last_if_not( R&& r, Pred pred, Proj proj = {} );	(6)	(C++23 起)
template< /execution-policy/ Ep, std::random_access_iterator I, std::sized_sentinel_for<I> S, class Proj = std::identity, class T = std::projected_value_t<I, Proj> > requires std::indirect_binary_predicate <ranges::equal_to, std::projected<I, Proj>, const T*> ranges::subrange<I> find_last( Ep&& policy, I first, S last, const T& value, Proj proj = {} );	(7)	(C++26 起)
template< /execution-policy/ Ep, /sized-random-access-range/ R, class Proj = std::identity, class T = std::projected_value_t<ranges::iterator_t<R>, Proj> > requires std::indirect_binary_predicate <ranges::equal_to, std::projected<ranges::iterator_t<R>, Proj>, const T*> ranges::borrowed_subrange_t<R> find_last( Ep&& policy, R&& r, const T& value, Proj proj = {} );	(8)	(C++26 起)
template< /execution-policy/ Ep, std::random_access_iterator I, std::sized_sentinel_for<I> S, class Proj = std::identity, std::indirect_unary_predicate<std::projected<I, Proj>> Pred > ranges::subrange<I> find_last_if( Ep&& policy, I first, S last, Pred pred, Proj proj = {} );	(9)	(C++26 起)
template< /execution-policy/ Ep, /sized-random-access-range/ R, class Proj = std::identity, std::indirect_unary_predicate <std::projected<ranges::iterator_t<R>, Proj>> Pred > ranges::borrowed_subrange_t<R> find_last_if( Ep&& policy, R&& r, Pred pred, Proj proj = {} );	(10)	(C++26 起)
template< /execution-policy/ Ep, std::random_access_iterator I, std::sized_sentinel_for<I> S, class Proj = std::identity, std::indirect_unary_predicate<std::projected<I, Proj>> Pred > ranges::subrange<I> find_last_if_not( Ep&& policy, I first, S last, Pred pred, Proj proj = {} );	(11)	(C++26 起)
template< /execution-policy/ Ep, /sized-random-access-range/ R, class Proj = std::identity, std::indirect_unary_predicate <std::projected<ranges::iterator_t<R>, Proj>> Pred > ranges::borrowed_subrange_t<R> find_last_if_not( Ep&& policy, R&& r, Pred pred, Proj proj = {} );	(12)	(C++26 起)

/*execution-policy*/ 的定义见此页；/*sized-random-access-range*/ 的定义见此页。

搜索源范围 [first, last) 或 r 中最后一个满足特定判别标准的（以 proj 投影后的）元素：

1,2) find_last 搜索等于目标值 value 的最后一个元素。

3,4) find_last_if 搜索谓词 pred 对其返回 true 的最后一个元素。

5,6) find_last_if_not 搜索谓词 pred 对其返回 false 的最后一个元素。

7-12) 同 (1-6)，但按照 policy 执行。

此页面上描述的函数式实体是算法函数对象（非正式地称为 niebloid），即：

调用它们中的任何一个时不能显式指定模板实参。
它们都不对实参依赖查找可见。
在通过函数调用运算符左侧的名字进行无限定查找，找到它们之一时，禁止实参依赖查找。

参数

first, last	-	表示源范围的迭代器-哨位对
r	-	源范围
value	-	目标值
pred	-	会应用到（投影后的）元素的谓词
proj	-	会应用到元素的投影
policy	-	所用的执行策略

返回值

从满足条件的最后一个元素到源范围末尾的子范围，或在找不到这种元素时返回空范围。

复杂度

给定 $\scriptsize N$ $N$ 为 ranges::distance(first, last) 或 ranges::distance(r)：

1,2) 最多进行 $\scriptsize N$

N

次比较和应用 proj。

3-6) 最多应用 $\scriptsize N$

N

次 pred 和 proj。

7,8) 进行 $\scriptsize \mathcal{O}(N)$

𝓞(N)

次比较和应用 proj。

9-12) 应用 $\scriptsize \mathcal{O}(N)$

𝓞(N)

次 pred 和 proj。

异常

7-12) 在执行过程中：

如果并行化所需的临时内存资源不可用，那么就会抛出 std::bad_alloc。
如果在通过算法实参访问对象时抛出了未捕获的异常，那么行为由执行策略决定（标准策略会调用 std::terminate）。

注解

如果 ranges::find_last、ranges::find_last_if、ranges::find_last_if_not 的 I 是 bidirectional_iterator 或（更好的）random_access_iterator，那么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`
`__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
struct find_last_fn { template<std::forward_iterator I, std::sentinel_for<I> S, class Proj = std::identity, class T = std::projected_value_t<ranges::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, 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)(ranges::begin(r), ranges::next(ranges::begin(r), ranges::end(r)), value, std::ref(proj)); } }; inline constexpr find_last_fn find_last;
find_last_if
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, 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)(ranges::begin(r), ranges::next(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
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, 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)(ranges::begin(r), ranges::next(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

运行此代码

#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

ranges::find_end (C++20)	查找元素序列在特定范围中最后一次出现 (算法函数对象) [编辑]
ranges::findranges::find_ifranges::find_if_not (C++20)(C++20)(C++20)	查找首个满足特定条件的元素 (算法函数对象) [编辑]
ranges::search (C++20)	搜索元素范围的首次出现 (算法函数对象) [编辑]
ranges::includes (C++20)	判断一个序列是否为另一个序列的子序列 (算法函数对象) [编辑]
ranges::binary_search (C++20)	使用二分搜索判断元素是否在范围中 (算法函数对象) [编辑]
ranges::containsranges::contains_subrange (C++23)(C++23)	检查范围是否包含给定元素或子范围 (算法函数对象) [编辑]

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