std::ranges::find, std::ranges::find_if, std::ranges::find_if_not

定义于头文件 `<algorithm>`
调用签名
template< std::input_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 I find( I first, S last, const T& value, Proj proj = {} );	(1)	(C++20 起) (C++26 前)
template< std::input_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 I find( I first, S last, const T& value, Proj proj = {} );	(1)		(C++26 起)
template< ranges::input_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_iterator_t<R> find( R&& r, const T& value, Proj proj = {} );	(2)	(C++20 起) (C++26 前)
template< ranges::input_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_iterator_t<R> find( R&& r, const T& value, Proj proj = {} );	(2)		(C++26 起)
template< std::input_iterator I, std::sentinel_for<I> S, class Proj = std::identity, std::indirect_unary_predicate<std::projected<I, Proj>> Pred > constexpr I find_if( I first, S last, Pred pred, Proj proj = {} );	(3)	(C++20 起)
template< ranges::input_range R, class Proj = std::identity, std::indirect_unary_predicate <std::projected<ranges::iterator_t<R>, Proj>> Pred > constexpr ranges::borrowed_iterator_t<R> find_if( R&& r, Pred pred, Proj proj = {} );	(4)	(C++20 起)
template< std::input_iterator I, std::sentinel_for<I> S, class Proj = std::identity, std::indirect_unary_predicate<std::projected<I, Proj>> Pred > constexpr I find_if_not( I first, S last, Pred pred, Proj proj = {} );	(5)	(C++20 起)
template< ranges::input_range R, class Proj = std::identity, std::indirect_unary_predicate <std::projected<ranges::iterator_t<R>, Proj>> Pred > constexpr ranges::borrowed_iterator_t<R> find_if_not( R&& r, Pred pred, Proj proj = {} );	(6)	(C++20 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*> I find( 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_iterator_t<R> find( 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 > I find_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_iterator_t<R> find_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 > I find_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_iterator_t<R> find_if_not( Ep&& policy, R&& r, Pred pred, Proj proj = {} );	(12)	(C++26 起)

关于 /*execution-policy*/ 的定义，参见此页面；关于 /*sized-random-access-range*/ 的定义，参见此页面。

在源范围 proj (经投影 [first, last) 或 r) 中搜索第一个满足特定准则的元素：

1,2) find 搜索第一个等于目标值 value 的元素。

3,4) find_if 搜索第一个使得谓词 pred 返回 true 的元素。

5,6) find_if_not 搜索第一个使得谓词 pred 返回 false 的元素。

7-12) 同 (1-6)，但根据 policy 执行。

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

调用它们中的任何一个时都不能指定显式模板参数列表。
它们对实参依赖查找不可见。
当它们中的任何一个通过正常无限定查找作为函数调用运算符左侧的名称被发现时，实参依赖查找会被禁止。

参数

first, last	-	定义源范围
的迭代器-哨位对	r	-
源范围	value	-
目标值	pred	-
应用到（投影后）元素的谓词	proj	-
应用到元素的投影	policy	- 使用的执行策略

返回值

指向首个满足条件的元素的迭代器，若未找到此类元素则返回 last。

复杂度

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

1,2) 至多

N

次比较和 proj 的应用。

3-6) 至多

N

次 pred 和 proj 的应用。

7,8)

𝓞(N)

次比较和 proj 的应用。

9-12)

𝓞(N)

次 pred 和 proj 的应用。

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，则抛出 std::bad_alloc。
如果通过算法参数访问对象时抛出未被捕获的异常，则行为由执行策略决定（对于标准策略，会调用 std::terminate）。

注释

功能测试宏	值	标准	功能
`__cpp_lib_algorithm_default_value_type`	`202403`	(C++26)	列表初始化用于算法 ( 1,2 )

可能的实现

find
struct find_fn { template<std::input_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 I operator()(I first, S last, const T& value, Proj proj = {}) const { for (; first != last; ++first) if (std::invoke(proj, first) == value) return first; return first; } template<ranges::input_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_iterator_t<R> operator()(R&& r, const T& value, Proj proj = {}) const { return (this)(ranges::begin(r), ranges::end(r), value, std::ref(proj)); } 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_iterator_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_fn find;
find_if
struct find_if_fn { template<std::input_iterator I, std::sentinel_for<I> S, class Proj = std::identity, std::indirect_unary_predicate<std::projected<I, Proj>> Pred> constexpr I operator()(I first, S last, Pred pred, Proj proj = {}) const { for (; first != last; ++first) if (std::invoke(pred, std::invoke(proj, first))) return first; return first; } template<ranges::input_range R, class Proj = std::identity, std::indirect_unary_predicate <std::projected<ranges::iterator_t<R>, Proj>> Pred> constexpr ranges::borrowed_iterator_t<R> operator()(R&& r, Pred pred, Proj proj = {}) const { return (this)(ranges::begin(r), ranges::end(r), std::ref(pred), std::ref(proj)); } template<ranges::forward_range R, class Proj = std::identity, std::indirect_unary_predicate <std::projected<ranges::iterator_t<R>, Proj>> Pred> constexpr ranges::borrowed_iterator_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_if_fn find_if;
find_if_not
struct find_if_not_fn { template<std::input_iterator I, std::sentinel_for<I> S, class Proj = std::identity, std::indirect_unary_predicate<std::projected<I, Proj>> Pred> constexpr I operator()(I first, S last, Pred pred, Proj proj = {}) const { for (; first != last; ++first) if (!std::invoke(pred, std::invoke(proj, first))) return first; return first; } template<ranges::input_range R, class Proj = std::identity, std::indirect_unary_predicate <std::projected<ranges::iterator_t<R>, Proj>> Pred> constexpr ranges::borrowed_iterator_t<R> operator()(R&& r, Pred pred, Proj proj = {}) const { return (this)(ranges::begin(r), ranges::end(r), std::ref(pred), std::ref(proj)); } template<ranges::forward_range R, class Proj = std::identity, std::indirect_unary_predicate <std::projected<ranges::iterator_t<R>, Proj>> Pred> constexpr ranges::borrowed_iterator_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_if_not_fn find_if_not;

示例

运行此代码

#include <algorithm>
#include <cassert>
#include <complex>
#include <format>
#include <iostream>
#include <iterator>
#include <string>
#include <vector>

void projector_example()
{
    struct folk_info
    {
        unsigned uid;
        std::string name, position;
    };
    
    std::vector<folk_info> folks
    {
        {0, "Ana", "dev"},
        {1, "Bob", "devops"},
        {2, "Eve", "ops"}
    };
    
    const auto who{"Eve"};
    if (auto it = std::ranges::find(folks, who, &folk_info::name); it != folks.end())
        std::cout << std::format("Profile:\n"
                                 "    UID: {}\n"
                                 "    Name: {}\n"
                                 "    Position: {}\n\n",
                                 it->uid, it->name, it->position);
}

int main()
{
    namespace ranges = std::ranges;
    
    projector_example();
    
    const int n1 = 3;
    const int n2 = 5;
    const auto v = {4, 1, 3, 2};
    
    if (ranges::find(v, n1) != v.end())
        std::cout << "v contains: " << n1 << '\n';
    else
        std::cout << "v does not contain: " << n1 << '\n';
    
    if (ranges::find(v.begin(), v.end(), n2) != v.end())
        std::cout << "v contains: " << n2 << '\n';
    else
        std::cout << "v does not contain: " << n2 << '\n';
    
    auto is_even = [](int x) { return x % 2 == 0; };
    
    if (auto result = ranges::find_if(v.begin(), v.end(), is_even); result != v.end())
        std::cout << "First even element in v: " << *result << '\n';
    else
        std::cout << "No even elements in v\n";
    
    if (auto result = ranges::find_if_not(v, is_even); result != v.end())
        std::cout << "First odd element in v: " << *result << '\n';
    else
        std::cout << "No odd elements in v\n";
    
    auto divides_13 = [](int x) { return x % 13 == 0; };
    
    if (auto result = ranges::find_if(v, divides_13); result != v.end())
        std::cout << "First element divisible by 13 in v: " << *result << '\n';
    else
        std::cout << "No elements in v are divisible by 13\n";
    
    if (auto result = ranges::find_if_not(v.begin(), v.end(), divides_13);
        result != v.end())
        std::cout << "First element indivisible by 13 in v: " << *result << '\n';
    else
        std::cout << "All elements in v are divisible by 13\n";
    
    std::vector<std::complex<double>> nums{{4, 2}};
    #ifdef __cpp_lib_algorithm_default_value_type
        // T gets deduced in (2) making list-initialization possible
        const auto it = ranges::find(nums, {4, 2});
    #else
        const auto it = ranges::find(nums, std::complex<double>{4, 2});
    #endif
    assert(it == nums.begin());
}

输出：

Profile:
    UID: 2
    Name: Eve
    Position: ops

v contains: 3
v does not contain: 5
First even element in v: 4
First odd element in v: 1
No elements in v are divisible by 13
First element indivisible by 13 in v: 4

参阅

findfind_iffind_if_not (C++11)	查找满足特定条件的首个元素 (函数模板)
ranges::adjacent_find (C++20)	查找第一对相邻的相等（或满足给定谓词的）元素 (算法函数对象)
ranges::find_end (C++20)	查找某个范围内元素的最后出现序列 (算法函数对象)
ranges::find_first_of (C++20)	搜索集合中的任意一个元素 (算法函数对象)
ranges::mismatch (C++20)	查找两个范围首次出现差异的位置 (算法函数对象)
ranges::search (C++20)	搜索某个元素范围的首次出现 (算法函数对象)

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Namespaces

Variants

std::ranges::find, std::ranges::find_if, std::ranges::find_if_not

内容

参数

返回值

复杂度

异常

注释

可能的实现

示例

参阅