Understanding shallow copy vs deep copy in Go

In Go, copying generally does a copy by value. So modifying the destination, doesn’t modify the source. However, that holds good only if you are copying value types not reference types (i.e. pointer, slice, map etc.)

package main

import (
	"fmt"
)

func main() {

	type Cat struct {
		age     int
		name    string
		Friends []string //reference type
	}
	wilson := Cat{7, "Wilson", []string{"Tom", "Tabata", "Willie"}}
	// shallow copy
	nikita := wilson
	// modifies both nikita and wilson
	nikita.Friends[0] = "cruise"
	fmt.Println("nikita:", wilson)
	fmt.Println("wilson:", wilson)
	// Here I am modifying the slice too, but whats stored in nikita.Friends is the address of new slice which append returns not wilson.Friends
	nikita.Friends = append(wilson.Friends, "jerry")
	fmt.Println("nikita:", nikita)
	fmt.Println("wilson:", wilson)
	// If the above doesn't makes sense this should. Created a new slice data
	data := []string{"popeye"}
	// shallow copy data to nikita Friends
	nikita.Friends = data
	// changing nikita Friends should change data too
	nikita.Friends[0] = "pluto"
	fmt.Println("nikita:", nikita)
	fmt.Println("wilson:", wilson)
	fmt.Println("data:", data)
}

This code explains one of the gotchas that I ran into. nikita.Friends = append(wilson.Friends, "jerry") I was expecting wilson.Friends will also have jerry but since i did shallow copy nikita := wilson. However, that’s not the case. I realized nikita.Friends = append(wilson.Friends, "jerry") actually creates a new slice and stores that’s reference. The data should clarify that further.

Another simpler example to understand the side effects of shallow copy in reference types

package main

import (
	"fmt"
)

func main() {

	type Cat struct {
		age     *int
		name    string
		Friends []string //reference type
	}
	catAge := 4
	wilson := Cat{&catAge, "Wilson", []string{"Tom", "Tabata", "Willie"}}
	// shallow copy
	nikita := wilson
	*nikita.age = 3
	fmt.Println("nikita:", *nikita.age)
	fmt.Println("wilson:", *wilson.age)
	type Cats struct {
		age     int
		name    string
		Friends []string //reference type
	}
	wilson1 := Cats{4, "Wilson", []string{"Tom", "Tabata", "Willie"}}
	// shallow copy
	nikita1 := wilson1
	nikita1.age = 3
	fmt.Println("nikita1:", nikita1.age)
	fmt.Println("wilson1:", wilson1.age)
}

• Go