golang从指针类型切片复制指针值

Question

type Order struct {
*Res
Status int
}
type Res struct {
ResID            int64
OtaBookID        string
StayDetail       []*ResElement
TotalCharge      float64
CustFName        string
CustLName        string
CreateTime       time.Time
}
type ResElement struct {
Res        *Res
OtaEleID   string
OtaRoomID  string
RoomID     int
Arrival    time.Time
Depart     time.Time
Charge     float64
CreateTime time.Time
}

我有一个名为orderList的切片来记录订单。现在我有第一个订单，OtaBookID是"A123"，我想复制这个订单，并将其OtaBookID更改为"B123"。ResElement类型有一些其他详细信息来记录这个订单，我将它们隐藏起来，因为这不会影响我的问题。我可以成功更改Res.OtaBookID，但我不知道如何更改Res.StayDetail[0]。Res.OtaBookID

func main() {
var orderList []*Order
res := new(Res)
res.OtaBookID = "A123"
resElt := new(ResElement)
resElt.Res = res
res.StayDetail = append(res.StayDetail, resElt)
order := new(Order)
order.Res = res
orderList = append(orderList, order)
originalOrder := new(Order)
originalOrder.Res = new(Res)
*originalOrder.Res = *order.Res
//originalOrder.Res.StayDetail[0].Res.OtaBookID = "B123" //this will make all become "B123"
originalOrder.Res.OtaBookID = "B123"
orderList = append(orderList, originalOrder)
fmt.Println(orderList[0].Res.OtaBookID)  //A123
fmt.Println(orderList[1].Res.OtaBookID)  //B123
fmt.Println(orderList[0].Res.StayDetail[0].Res.OtaBookID)  //A123
fmt.Println(orderList[1].Res.StayDetail[0].Res.OtaBookID)  //A123, i want this become B123
}

我想要的结果是orderList[0]。Res.OtaBookID和orderList[0]，Res.StayDetail[0]。Res.OtaBookID是"A123"，其他是"B123">

ps：因为我要接手别人的作品。所以三种类型已经固定，无法调整。我想知道在这种情况下是否有任何方法来达到我的目标

Answer 1

根据您的示例和说明，我们可以专注于实现您的目标的主要内容：

type Order struct {
*Res
Status int
}
type Res struct {
ResID            int64
OtaBookID        string
StayDetail       []*ResElement
TotalCharge      float64
CustFName        string
CustLName        string
CreateTime       time.Time
}
type ResElement struct {
Res        *Res
OtaEleID   string
OtaRoomID  string
RoomID     int
Arrival    time.Time
Depart     time.Time
Charge     float64
CreateTime time.Time
}
func main() {
var orderList []*Order
res := new(Res)
res.OtaBookID = "A123"
resElt := new(ResElement)
resElt.Res = res
res.StayDetail = append(res.StayDetail, resElt)
order := new(Order)
order.Res = res
orderList = append(orderList, order)
originalOrder := new(Order)
originalOrder.Res = new(Res)
// The following statement will copy the values in the memory space of order to the memory
// space of originalOrder, but take note that a pointer contains a memory address. Thus,
// following this statement originalOrder.Res.StayDetail will point to the same address
// as order.Res.StayDetail as slices are pointers...
*originalOrder.Res = *order.Res
// We now have to manually copy the slice to ensure it is different from the one in order.
// We must create a new slice to avoid overwriting the source.
originalOrder.Res.StayDetail := make([]*ResElement, len(order.Res.StayDetail))
// Further, this slice contains pointers so we must copy these as well to ensure they do not
// point to the ResElements of order.
for i, v := range order.Res.StayDetail {
re := new(ResElement)
// copy the values of the memory in v to the values in the memory of re
*re = *v
// set re.Res to point to originalOrder.Res as it currently point to order.Res
re.Res = originalOrder.Res
// Now we must place re in the slice of originalOrder as it currently points to an
// empty array of the correct size. Thus, we will not use append (which will grow
// the size), we will just set the index to the correct value. Thus:
originalOrder.StayDetail[i] = re
}
// The pointers and structures have now been properly set up, thus the following are equivalent:
// originalOrder.OtaBookID = "B123" - Res via embedding
// originalOrder.Res.OtaBookID = "B123" - Res directly
// originalOrder.Res.StayDetail[0].Res.OtaBookID = "B123" - Res via ResElement pointer Res
// They all point to the exact same memory space
originalOrder.Res.OtaBookID = "B123"
orderList = append(orderList, originalOrder)
fmt.Println(orderList[0].Res.OtaBookID)  //A123
fmt.Println(orderList[1].Res.OtaBookID)  //B123
fmt.Println(orderList[0].Res.StayDetail[0].Res.OtaBookID)  //A123
fmt.Println(orderList[1].Res.StayDetail[0].Res.OtaBookID)  //B123
}

永远记住，指针只是一个内存地址。如果地址值为0xffab67e1，指针的副本也将包含相同的0xffab670e1值，并有效地指向相同的东西。因此，您必须有意识地在新地址分配新的内存空间，并复制内部值。对于任何包含指针的结构，都必须执行此操作。我们称之为深度复制或克隆。