关于golang中的结构体

这些天看aws的golang sdk中结构体中有

_ struct{} `type:"structure"`

不知道 下划线 _ 是什么意思，注意不是tag

// Please also see https://docs.aws.amazon.com/goto/WebAPI/s3-2006-03-01/MultipartUpload
type MultipartUpload struct {
    _ struct{} `type:"structure"`

    // Date and time at which the multipart upload was initiated.
    Initiated *time.Time `type:"timestamp" timestampFormat:"iso8601"`

    // Identifies who initiated the multipart upload.
    Initiator *Initiator `type:"structure"`

    // Key of the object for which the multipart upload was initiated.
    Key *string `min:"1" type:"string"`

    Owner *Owner `type:"structure"`

    // The class of storage used to store the object.
    StorageClass *string `type:"string" enum:"StorageClass"`

    // Upload ID that identifies the multipart upload.
    UploadId *string `type:"string"`
}