为了管理具有指针成员的类，必须定义三个复制控制成员：复制构造函数、赋值操作符和析构函数。
To manage a class with pointer members, we must define all three copy-control members: the copy constructor, assignment operator, and the destructor.
具有指针成员的对象一般需要定义复制控制成员。
Objects with pointer members often need to define the copy-control members.
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