Dereferencing a map index in Golang

Question

I'm learning Go currently and I made this simple and crude inventory program just to tinker with structs and methods to understand how they work. In the driver file I try to call a method from and item type from the items map of the Cashier type. My method have pointer reciever to use the structs directly instead of making copies. When I run the program I get this error .\driver.go:11: cannot call pointer method on f[0] .\driver.go:11: cannot take the address of f[0]

Inventory.go:

package inventory


type item struct{
    itemName string
    amount int
}

type Cashier struct{
    items map[int]item
    cash int
}

func (c *Cashier) Buy(itemNum int){
    item, pass := c.items[itemNum]

    if pass{
        if item.amount == 1{
            delete(c.items, itemNum)
        } else{
            item.amount--
            c.items[itemNum] = item 
        }
        c.cash++
    }
}


func (c *Cashier) AddItem(name string, amount int){
    if c.items == nil{
        c.items = make(map[int]item)
    }
    temp := item{name, amount}
    index := len(c.items)
    c.items[index] = temp
}

func (c *Cashier) GetItems() map[int]item{
    return c.items;
}

func (i *item) GetName() string{
    return i.itemName
}

func (i *item) GetAmount() int{
    return i.amount
}

Driver.go:

package main

import "fmt"
import "inventory"

func main() {
    x := inventory.Cashier{}
    x.AddItem("item1", 13)
    f := x.GetItems()

    fmt.Println(f[0].GetAmount())
}

The part of the code that really pertains to my problem is the GetAmount function in inventory.go and print statement in the driver.go

Answer 1

A map entry cannot be addressed (as its address might change during map growth/shrink), so you cannot call pointer receiver methods on them.

Detail here: https://groups.google.com/forum/?fromgroups=#!topic/golang-nuts/4_pabWnsMp0

Answer 2

As Volker said in his answer - you can't get address of an item in the map. What you should do - is to store pointers to items in your map, instead of storing item values:

package main

import "fmt"

type item struct {
    itemName string
    amount   int
}

type Cashier struct {
    items map[int]*item
    cash  int
}

func (c *Cashier) Buy(itemNum int) {
    item, pass := c.items[itemNum]

    if pass {
        if item.amount == 1 {
            delete(c.items, itemNum)
        } else {
            item.amount--
        }
        c.cash++
    }
}

func (c *Cashier) AddItem(name string, amount int) {
    if c.items == nil {
        c.items = make(map[int]*item)
    }
    temp := &item{name, amount}
    index := len(c.items)
    c.items[index] = temp
}

func (c *Cashier) GetItems() map[int]*item {
    return c.items
}

func (i *item) GetName() string {
    return i.itemName
}

func (i *item) GetAmount() int {
    return i.amount
}

func main() {
    x := Cashier{}
    x.AddItem("item1", 13)
    f := x.GetItems()
    fmt.Println(f[0].GetAmount()) // 13
    x.Buy(0)
    f = x.GetItems()
    fmt.Println(f[0].GetAmount()) // 12
}

http://play.golang.org/p/HkIg668fjN

Answer 3

While the other answers are useful, I think in this case it is best just to make non-mutating functions not take a pointer:

func (i item) GetName() string{
    return i.itemName
}

func (i item) GetAmount() int{
    return i.amount
}