std::variant std::pair std::tuple std::any std::expected 使用注意事项

某些用法会破坏RVO造成不必要的MoveCopy

#include <format>
#include <exception>
#include <variant>
#include <iostream>

class LifeTime {
	int a_ = 0;
	public:
    LifeTime(int a):a_(a){
        std::cout << "LifeTime(): " << a_ << std::endl;
    }

    ~LifeTime() {
        std::cout << "~LifeTime(): " << a_ << std::endl;
    }

	LifeTime(const LifeTime&) {
        std::cout << "LifeTime(const LifeTime&): " << a_ << std::endl;
    }

	LifeTime(LifeTime&& other) noexcept {
		this->a_ = other.a_;
        std::cout << "LifeTime(LifeTime&&): " << a_ << std::endl;
    }

	LifeTime& operator=(const LifeTime&) {
        std::cout << "LifeTime& operator=(const LifeTime&): " << a_ << std::endl;
        return *this;
    }

	LifeTime& operator=(LifeTime&&) noexcept {
        std::cout << "LifeTime& operator=(LifeTime&&): " << a_ << std::endl;
        return *this;
    }
};

//break RVO
std::optional<LifeTime> create_v1() {
	LifeTime a{ 1 };
	return a;
}

//break RVO
std::optional<LifeTime> create_v2() {
	return LifeTime{2};
}

//break RVO
std::optional<LifeTime> create_v3() {
	std::optional<LifeTime> opt;
	opt = LifeTime{ 3 };
	return opt;
}

std::optional<LifeTime> create_v4() {
	std::optional<LifeTime> opt;
	opt.emplace(4);
	return opt;
}

std::optional<LifeTime> create_v5() {
	return std::optional<LifeTime>{5};
}

std::optional<LifeTime> create_v6() {
	return std::optional<LifeTime>{std::in_place, 6};
}


int main() {
	std::cout << "case1:" << std::endl;
	create_v1();
    // LifeTime(): 1
    // LifeTime(LifeTime&&): 1
    // ~LifeTime(): 1
    // ~LifeTime(): 1

	std::cout << "case2:" << std::endl;
    create_v2();
    // LifeTime(): 2
    // LifeTime(LifeTime&&): 2
    // ~LifeTime(): 2
    // ~LifeTime(): 2

	std::cout << "case3:" << std::endl;
    create_v3();
    // LifeTime(): 3
    // LifeTime(LifeTime&&): 3
    // ~LifeTime(): 3
    // ~LifeTime(): 3

	std::cout << "case4:" << std::endl;
	create_v4();
    // LifeTime(): 4
    // ~LifeTime(): 4

	std::cout << "case5:" << std::endl;
	create_v5();
    // LifeTime(): 5
    // ~LifeTime(): 5

	std::cout << "case6:" << std::endl;
	create_v6();
    // LifeTime(): 6
    // ~LifeTime(): 6
}