std::string  lv1 = "string,";       // lv1 is a lvalue
    // std::string&& r1 = s1;           // illegal, rvalue can't ref to lvalue
    std::string&& rv1 = std::move(lv1); // legal, std::move can convert lvalue to rvalue
    std::cout << rv1 << std::endl;      // string,

    const std::string& lv2 = lv1 + lv1; // legal, const lvalue reference can extend temp variable's lifecycle
    // lv2 += "Test";                   // illegal, const ref can't be modified
    std::cout << lv2 << std::endl;      // string,string

    std::string lv1 = "string,"; // lv1 是一个左值
    // std::string&& r1 = lv1; // 非法, 右值引用不能引用左值
    std::string&& rv1 = std::move(lv1); // 合法, std::move可以将左值转移为右值
    std::cout << rv1 << std::endl; // string,

    const std::string& lv2 = lv1 + lv1; // 合法, 常量左值引用能够延长临时变量的生命周期
    // lv2 += "Test"; // 非法, 常量引用无法被修改
    std::cout << lv2 << std::endl; // string,string

    std::string  lv1 = "string,";       // lv1 is a lvalue
    // std::string&& r1 = s1;           // illegal, rvalue can't ref to lvalue
    std::string&& rv1 = std::move(lv1); // legal, std::move can convert lvalue to rvalue
    std::cout << rv1 << std::endl;      // string,

    const std::string& lv2 = lv1 + lv1; // legal, const lvalue reference can extend temp variable's lifecycle
    // lv2 += "Test";                   // illegal, const ref can't be modified
    std::cout << lv2 << std::endl;      // string,string,

    std::string lv1 = "string,"; // lv1 是一个左值
    // std::string&& r1 = lv1; // 非法, 右值引用不能引用左值
    std::string&& rv1 = std::move(lv1); // 合法, std::move可以将左值转移为右值
    std::cout << rv1 << std::endl; // string,

    const std::string& lv2 = lv1 + lv1; // 合法, 常量左值引用能够延长临时变量的生命周期
    // lv2 += "Test"; // 非法, 常量引用无法被修改
    std::cout << lv2 << std::endl; // string,string,