C++ ranges find and return std::optional like Java stream

Question

在Java、Stream.findFirst() returns Optional<T>. I'd like to have the similar behavior for std::ranges::find()。如果找不到该值，则使用 returns last 迭代器。如果 T 是一个结构并且我试图从中获取一个成员，这会很不方便。这是一个演示代码：

    struct Person {
        int age;
        int height;
    };

    std::vector<Person> people;
    people.emplace_back(20, 100);
    people.emplace_back(23, 170);

    // find the height of a 23yo person
    std::optional<int> height1 = std::ranges::find(people, 23, &Person::age)->height;

    // find the height of a 26yo person
    std::optional<int> height2 = std::ranges::find(people, 26, &Person::age)->height;  // error

当然，我可以在每个 find 周围放置一些包装代码来转换迭代器，但这会使代码变得冗长和样板化。我想知道 C++20 中是否有一些更惯用的方法来做到这一点？

    std::optional<int> height2;
    auto iter = std::ranges::find(people, 26, &Person::age);
    if (iter == people.end()) {
        height2 = std::nullopt;
    } else {
        height2 = iter->height;
    }

Answer 1

template <class ...ArgsT>
auto OpRangesFind(auto &&Range, ArgsT &&...Args)
{
    auto Iter = std::ranges::find(Range, std::forward<ArgsT>(Args)...);
    return Iter != Range.end() ? Iter : std::optional<decltype(Iter)>{std::nullopt};
}

int main()
{
    /* ... */

    auto OpIter = OpRangesFind(people, 26, &Person::age);
    if (!OpIter.has_value()) {
        std::cout << "not found\n";
    } else {
        std::cout << "found\n";
        std::cout << "height: " << OpIter.value()->height << "\n";
        //                               ^^^^^^^^
    }
}

Answer 2

这是我自己编写通用包装器的解决方案。与Sprite的版本相比，我的版本returns成员变量而不是迭代器。

template<std::ranges::input_range R, class T, class ProjFind = std::identity, class ProjOut = std::identity>
requires std::indirect_binary_predicate<std::ranges::equal_to,
                                        std::projected<std::ranges::iterator_t<R>, ProjFind>,
                                        const T *>
auto optionalFind( R&& r, const T& value, ProjFind projFind = {}, ProjOut projOut = {} ) {
    const auto iter = std::ranges::find(r, value, projFind);
    return iter == r.end() ? std::nullopt : std::optional(std::invoke(projOut, *iter));
}

并与

一起使用

std::optional<int> height2 = optionalFind(people, 26, &Person::age, &Person::height);

或

std::optional<Person> person = optionalFind(people, 26, &Person::age);

C++ ranges find and return std::optional like Java stream

C++ ranges find and return std::optional like Java stream

c++

c++20

stdoptional