std::ranges::find() 算法

自 C++20 起

简化

// (1)
constexpr I
    find( I first, S last, const T& value, Proj proj = {} );

// (2)
constexpr ranges::borrowed_iterator_t<R>
    find( R&& r, const T& value, Proj proj = {} );

参数类型是泛型的，并具有以下约束

• I - std::input_iterator
• S - std::sentinel_for<I>
• T - （无）
• Proj - (无)
• (2) - R - std::ranges::input_range

对于所有重载，Proj 模板参数的默认类型为 std::identity。

此外，每个重载都有以下约束

• (1) - indirect_binary_predicate<ranges::equal_to, projected<I, Proj>, const T*>>
• (2) - indirect_binary_predicate<ranges::equal_to, projected<ranges::iterator_t<R>, Proj>, const T*>

（为方便阅读，此处省略了 std:: 命名空间）

详细

// (1)
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 = {} );

// (2)
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 = {} );

返回范围内满足特定条件的第一个元素的迭代器（如果没有这样的迭代器，则返回 last 迭代器）

• (1) 搜索等于 value 的元素（使用 operator==）

• (2) 与 (1) 相同，但使用 r 作为源范围，如同使用 ranges::begin(r) 作为 first 和 ranges::end(r) 作为 last。

本页描述的函数类实体是niebloids。

参数

first last	要应用函数的元素范围。
r	要应用函数的元素范围。
值	用于比较元素的数值。
proj	应用于元素的投影。

返回值

指向满足条件的第一个元素的迭代器，如果未找到此类元素，则为等于 last 的迭代器。

复杂度

精确执行 last - first 次谓词和投影应用。

异常

(无)

可能的实现

find(1)

struct find_fn
{
    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 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));
    }
};

inline constexpr find_fn find;

示例

Main.cpp

#include <algorithm>
#include <iostream>
#include <iterator>

int main()
{
    namespace ranges = std::ranges;

    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";
}

输出

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