如何创建包装容器/ list.List的golang结构？

Question

I am scratching my head over an unexpected difference between having my struct contain list.List vs *list.List. Why doesn't the following work?

type listHolder struct {
    id     int
    mylist list.List
}

func newListHolder(id int, text string) listHolder {
    var newLH listHolder
    newLH.mylist = *list.New()
    newLH.id = id
    newLH.mylist.PushBack(text)
    return newLH
}

func (l *listHolder) pushBack(text string) {
    l.mylist.PushBack(text)
}

func (l *listHolder) printAll() {
    for temp := l.mylist.Front(); temp != nil; temp = temp.Next() {
        fmt.Println(temp.Value)
    }
}


func main() {
    a := newListHolder(1, "first") 
    a.pushBack("second")
    fmt.Printf("listHolder %d length %d Front()= %v, Back()=%v
",
         a.id, a.mylist.Len(), a.mylist.Front().Value, a.mylist.Back().Value)
    a.printAll()
}

This outputs the following, showing that the length is as expected but the Front() and Back() methods don't work.

listHolder 1 length 2 Front()= `<nil>`, Back()=<nil>
<nil>

If I define the struct as

// Same thing with a pointer
type listHolderPtr struct {
    id     int
    mylist *list.List
}

func newListHolderPtr(id int, text string) listHolderPtr {
    var newLH listHolderPtr
    newLH.mylist = list.New()
    newLH.id = id
    newLH.mylist.PushBack(text)
    return newLH
}

that works as expected, but of course, any copies of the listHolder struct share a reference to the same list, which is not what I want. I need to be able to copy the surrounding object and get a new copy of the internal list. Is that possible?

See https://play.golang.org/p/KCtTwuvaS1R for a simplified example of what I'm trying to do. In the real use case, I'll be pushing onto the back and popping off the front of each listHolder in a slice of listHolder in a complicated nested loop.

Answer 1

I think @JimB's suggestion to make a local copy of your list whenever you're pushing a value to listHolder.mylist could be a good solution to your problem (given, that I understand the underlying issue correctly). I tried to come up with an implementation that looks like the following:

package main

import (
    "container/list"
    "fmt"
)

type listHolder struct {
    id     int
    mylist list.List
}

func newListHolder(id int) listHolder { // don't push back when constructing a new listHolder
    var newLH listHolder
    newLH.mylist = *list.New()
    newLH.id = id
    return newLH
}

func (l *listHolder) pushBack(text string) {
    // create a temporary list to copy all old and the new value to
    tmpList := list.New()

    // copy all existing values from l.mylist
    for e := l.mylist.Front(); e != nil; e = e.Next() {
        fmt.Printf("pushing back '%v' from old list
", e.Value)
        tmpList.PushBack(e.Value)
    }

    // push back the new value
    tmpList.PushBack(text)

    // print the new tmpList for debugging purposes
    for ele := tmpList.Front(); ele != nil; ele = ele.Next() {
        fmt.Printf("creating new list element: %v
", ele.Value)
    }

    // replace l.mylist with tmpList
    l.mylist = *tmpList
    // another version of this solution could be to return a new (i.e. copied) 
    // *listHolder with all the old values and the new 'text' value
}

func (l *listHolder) printAll() {
    for temp := l.mylist.Front(); temp != nil; temp = temp.Next() {
        fmt.Println(temp.Value)
    }
}

func main() {
    a := newListHolder(1)
    a.pushBack("first")  // push a value to a
    a.pushBack("second") // push another value to a
    fmt.Printf("listHolder %d length %d Front()=%v, Back()=%v
",
        a.id, a.mylist.Len(), a.mylist.Front().Value, a.mylist.Back().Value)
    a.printAll()
}

This code outputs:

creating new list element: first                         // nothing to copy, only creating a new list element
pushing back 'first' from old list                       // copy element ...
creating new list element: first                         // ... from old list
creating new list element: second                        // and push new element to 'tmpList'
listHolder 1 length 2 Front()=first, Back()=second       // print a summary
first                                                    // of the
second                                                   // new list

If I had some mock data, I could do some more testing/debugging. At least this code works without a *list.List.