指定派生类的析构函数时,std::move不起作用

我试图将一个实例移动到另一个实例,如下所示:

#include <iostream>

class student {
public:
    student() = default;
    student(const student& student) {
        std::cout << "copy student" << std::endl;
    }
    student(student&& student) {
        std::cout << "move student" << std::endl;
    }
    virtual ~student() = default;
};

class undergraduate: public student {
public:
    undergraduate(): student() {}
    ~undergraduate() override = default; // removing this line will make std::move work
};

int main() {
    undergraduate student;
    undergraduate student1 = std::move(student);
}

以下是输出:

copy student

如你所见, std::move student 被复制而不是移动,但是,如果我删除 undergraduate

~undergraduate() override = default; // removing this line will make std::move work

输出将变为 move student ,这意味着 作品为什么?为什么不呢 移动