Making a copy of a struct in Go

A struct variable in Go can be copied to another variable very easily using the assignment statement:

package main

import (
	"fmt"
)

type Dog struct {
	age  int
	name string
}

func main() {
	roger := Dog{5, "Roger"}
	mydog := roger
	if roger == mydog {
		fmt.Println("Roger and mydog are equal structs")
	}
}

play

mydog and roger are two separate variables, but comparing them property-by-property they are exactly the same.

We can now modify every property of those variables without affecting the other object, because the Dog struct contains basic types, which are copied by value.

If we had a reference type in our struct, this was not that easy, because the pointer would be copied, not the value, so it would have been a shallow copy.

For example

type Cat struct {
    age int
    name string
    friends []string
}

The list of friends of our cat is a slice, a reference type (along with pointers, maps, functions and channels), so if you had a cat object and you copy it like we did for the Dog example above, you couldn’t modify the friends property without affecting the other object as well.

A deep copy is needed.

Deep copy a struct

You can perform such deep copy manually. In the case of a slice, you can perform the copy like this:

package main

import (
	"fmt"
)

type Cat struct {
	age     int
	name    string
	friends []string
}

func main() {
	wilson := Cat{7, "Wilson", []string{"Tom", "Tabata", "Willie"}}
	nikita := wilson

	nikita.friends = make([]string, len(wilson.friends))
	copy(nikita.friends, wilson.friends)

	nikita.friends = append(nikita.friends, "Syd")

	fmt.Println(wilson)
	fmt.Println(nikita)
}

play

The above code will result in wilson being {7 Wilson [Tom Tabata Willie]} and nikita being {7 Wilson [Tom Tabata Willie Syd]}

So you can see the slice is a completely different object, not just a reference to the same slice.

Using a deep copy library

https://github.com/jinzhu/copier is (one of the many) deep copy libs/helpers out there. It makes it really easy to deep copy a struct without explicitly managing each field. Here’s an example, all we need is copier.Copy(&nikita, &wilson) and we’re set:

package main

import (
	"fmt"
	"github.com/jinzhu/copier"
)

type Cat struct {
	age     int
	name    string
	friends []string
}

func main() {
	wilson := Cat{7, "Wilson", []string{"Tom", "Tabata", "Willie"}}
	nikita := Cat{}
	copier.Copy(&nikita, &wilson)

	nikita.friends = append(nikita.friends, "Syd")

	fmt.Println(wilson)
	fmt.Println(nikita)
}

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