引用类型在Go语言中令人困惑

Question

I tried to make Trie data structures by Go Language, but somehow it stuck with References problem, Here it is. http://play.golang.org/p/ASSGF5Oe9R

// Package main provides ...
package main

import "fmt"

type RootTrie []Trie

type Trie struct {
    subtrie []Trie
    index   byte
}

func (trie *Trie) Insert(data string) *Trie {
    if data != "" {
        if trie.index == 0 {
            trie.index = data[0]
        }
        if next := trie.containsIndex(data[1:]); next != nil {
            //Problem Point
            fmt.Println(string(data[1]), "found follwing", string(data[0]))
            next.Insert(data[1:])
        } else {
            nt := &Trie{}
            trie.subtrie = append(trie.subtrie, *nt.Insert(data[1:]))
        }
    }

    return trie
}
func (trie *Trie) containsIndex(next string) *Trie {
    if next != "" {
        for _, st := range trie.subtrie {
            if st.index == next[0] {
                return &st
            }
        }
    }
    return nil
}

func main() {
    t := &Trie{}
    t = t.Insert("hanyang")
    fmt.Println("result:", t)
    t = t.Insert("hanyKk")
    fmt.Println("result:", t)
    t.Insert("hanyK")
}

The following problems happen in second "Insert", the where I put, //Problem Point

I made containsIndex method for searching next linked trie, and it searched well actually. But when I updated next property which containsIndex given, its not affected its mother struct trie though.

What I don't understand is I gave it reference type when returning containsIndex, but its still act liked 'value copied', Why does it not affected its mother structure(trie)?

Thanks!

Answer 1

The problem is in method containsIndex. Golang range by default creates copy each element in slice and assigns copy of this value to st (in your example). Usually to preserve reference to element in slice you should use original slice and its index. In you case method containsIndex should look something like this:

func (trie *Trie) containsIndex(next string) *Trie {
    if next != "" {
        for i, st := range trie.subtrie {
            if st.index == next[0] {
                return &trie.subtrie[i]
            }
        }
    }
    return nil
}