C++对Rust中Result的简单模仿

📝 注意！
以下代码需要支持 C++23 的编译器

这里利用 std::expected 和 std::unexpected 进行模仿：

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#include <print>
#include <string>
#include <expected>

template <typename E = std::string>
using Err = std::unexpected<E>;

template <typename T, typename E = std::string>
using Result = std::expected<T, E>;

struct Inner {
    int a, b;
};

struct X {
    Inner* i_;

    X(Inner* i) {
        std::println("X()");
        i_ = i;
    }

    ~X() {
        std::println("~X() delete inner obj");
        delete i_;
    }
};

auto foo(bool flag) -> Result<X> {
    if (!flag) {
        return Err<>("Flag is false!");
    }
    
    Inner* i = new Inner();
    i->a = 1;
    i->b = 2;

    return Result<X>{i};
}

int main() {
    std::println("main--1");

    {
        std::println("main--2");
        auto x = foo(true);
        std::println("main--3");
    }

    {
        std::println("main--4");
        auto x = foo(false);
        std::println("Error message is: {}", x.error());
        std::println("main--5");
    }

    std::println("main--6");
}

输出为：

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main--1
main--2
X()
main--3
~X() delete inner obj
main--4
Error message is: Flag is false!
main--5
main--6