如何就地构造一个可选的聚合？

Question

如何就地构造一个可选的聚合？看来我只能构建一个可选的单一事物，而不能构建一个可选的事物集合。

#include <optional>
#include <iostream>

struct Unmovable
{
    Unmovable(const Unmovable&) = delete;
    Unmovable(Unmovable&&) = delete;
    Unmovable& operator=(const Unmovable&) = delete;
    Unmovable& operator=(Unmovable&&) = delete;

    explicit Unmovable(const char* msg) {
        std::cout << msg << '\n';
    }
};

struct Things
{
    Unmovable one;
    Unmovable two;
};

int main(int argc, char* argv[]) {
    const bool y = argc > 1 && argv[1][0] == 'y';

    std::optional<Unmovable> optionalThing = y
        ? std::optional<Unmovable>{"works"}
        : std::nullopt;
    
    std::optional<Things> optionalThings = y
        ? std::optional<Things>{
#if ATTEMPT == 1
            "jadda", "neida"
#elif ATTEMPT == 2
            {"jadda", "neida"}
#elif ATTEMPT == 3
            Things{"jadda", "neida"}
#elif ATTEMPT == 4
            Unmovable{"jadda"}, Unmovable{"neida"}
#elif ATTEMPT == 5
            {Unmovable{"jadda"}, Unmovable{"neida"}}
#elif ATTEMPT == 6
            Things{Unmovable{"jadda"}, Unmovable{"neida"}}
#elif ATTEMPT == 7
            std::in_place_t{}, "jadda", "neida"
#elif ATTEMPT == 8
            std::in_place_t{}, {"jadda", "neida"}
#elif ATTEMPT == 9
            std::in_place_t{}, Things{"jadda", "neida"}
#elif ATTEMPT == 10
            std::in_place_t{}, Unmovable{"jadda"}, Unmovable{"neida"}
#elif ATTEMPT == 11
            std::in_place_t{}, {Unmovable{"jadda"}, Unmovable{"neida"}}
#elif ATTEMPT == 12
            std::in_place_t{}, Things{Unmovable{"jadda"}, Unmovable{"neida"}}
#endif
        } : std::nullopt;
}

Answer 1

如果你可以使用 C++20，那么你想要的是

std::optional<Things>{std::in_place, "jadda", "neida"};

正如在这个活生生的例子中看到的那样。你需要 C++20 的原因是std::in_place_t构造函数使用的形式

T(std::forward<Args>(args)...)

初始化对象，但()仅适用于具有构造函数的类，而Things没有构造函数。C++ 已更新以解决此问题，并且该更改使其成为 C++20。

在 C++17 中，您可以通过提供用于Things初始化成员的构造函数来使此代码工作。那看起来像

struct Things
{
    Things(const char* msg1, const char* msg2) : one(msg1), two(msg2) {}
    Unmovable one;
    Unmovable two;
};

int main()
{
    std::optional<Things>{std::in_place, "jadda", "neida"};
}

你可以看到在这个活生生的例子中工作

如果您好奇，可以在[dcl.init.general]/15.6.2.2 中找到在 C++20 中添加的用于处理此问题的新语言