go map, key is string, value is pointer to a struct

Question

type Country struct {
    Code string
    Name string
}

var store = map[string]*Country{}

in this go code piece, key is string, value is pointer to a struct. What's the benefit to use pointer of Contry here? Can I remove the "*" and achieve the same behaviour? Such as:

 var store = map[string]Country

Thanks.

I'm not knowledgeable on go specifically, but my immediate thought is that this would make rebalancing the map (if necessary) a more costly operation (would require moving more memory). Getting and setting data would also become more costly for the same reason (requires copying the full struct, instead of a pointer to the struct) — Dave
– Dave, Commented Apr 25, 2015 at 17:20

IamNaN · Accepted Answer · 2015-04-25 17:50:00Z

8

You can achieve the same behavior using either pointer or value.

package main

import (
    "fmt"
)

type Country struct {
    Code string
    Name string
}

func main() {
    var store = make(map[string]*Country)
    var store2 = make(map[string]Country)

    c1 := Country{"US", "United States"}

    store["country1"] = &c1
    store2["country1"] = c1

    fmt.Println(store["country1"].Name)  // prints "United States"
    fmt.Println(store2["country1"].Name) // prints "United States"

}

Using a pointer will store the address of the struct in the map instead of a copy of the entire struct. With small structs like in your example this won't make much of a difference. With larger structs it might impact performance.

answered Apr 25, 2015 at 17:50

IamNaN

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2 Comments

Dave C Over a year ago

In particular, with a small struct like this, not using a pointer will perform better. As always, if/when performance is a concern, don't guess, benchmark.

OneOfOne Over a year ago

Only caveat is that you can't directly assign to the value without a pointer (store2["country1"].Name = "X").

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