然后构造函数打印出一条警告消息指明没有回调函数，因为列表是不可变的，所以我们没有机会用一个新的值来替代现有的回调函数列表。

The constructor then prints a warning message that there are no callbacks, since lists are immutable and there is no way to replace the callback list val with a new one.

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virtio_driver 结构定义上层设备驱动程序、驱动程序支持的设备 ID 的列表、一个特性表单（取决于设备类型）和一个回调函数列表。

The virtio_driver structure defines the upper-level device driver, list of device IDs that the driver supports, a features table (dependent upon the device type), and a list of callback functions.

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主构造函数，类的主题，有一个参数列表来接受标签字符串和回调函数的列表。

The primary constructor, which is the body of the entire class, has a parameter list that takes a label string and a list of callback functions.

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