🌻 Isolating Go Slices: How to Create Independent Slices from an Array Safely

Be careful with slices — modifying a slice can also change the original array.

arr := [...]int{1, 2, 3, 4, 5}
fmt.Println(len(arr), arr) // 5 [1 2 3 4 5]

// slicing the above array
s1 := arr[2:5] // index 2 (inclusive) to index 5 (exclusive)
fmt.Println(len(s1), cap(s1), s1) // 3 3 [3 4 5]

s2 := arr[1:4]
fmt.Println(len(s2), cap(s2), s2) // 3 4 [2 3 4]

s1[0] = 99 // this will modify both the slices and array as well

fmt.Println("arr:", len(arr), arr) // arr: 5 [1 2 99 4 5] 🤯
fmt.Println("s1:", len(s1), cap(s1), s1) // s1: 3 3 [99 4 5]
fmt.Println("s2:", len(s2), cap(s2), s2) // s2: 3 4 [2 99 4] 🤯

The capacity of each slice is calculated from the specified starting point to the end of the underlying array

• s1 starts at index 2, so capacity is 3
• s2 starts at index 1, so capacity is 4

Modifying s1

s1[0] = 99

• Since both s1 and s2 pointing to the same underlying array, modifying s1 affects arr and s2 slice as well.
• Slice is a view of an underlying array. When we create a slice from an array, data will not be copied. It will just create a reference to array. So modifying the array via slice will be visible to all other slices that share the same underlying array.

Solution: Create independent slices using copy func

arr := [...]int{1, 2, 3, 4, 5}

s1 := make([]int, 3)
copy(s1, arr[2:5]) // s1 is now a copy of arr[2:5]

s2 := make([]int, 3)
copy(s2, arr[1:4])

s1[0] = 99

fmt.Println("Original array:", arr) // [1 2 3 4 5] 😌 array is safe now
fmt.Println("s1:", s1)              // [99 4 5]
fmt.Println("s2:", s2)              // [2 3 4]

• When slice is created using make with specified size, it will be initialized with zero values.
• copy function will copy data from array into the slice. So new slice will have their own independent memory and not linked with the original array anymore.

Different ways to create a deep copy of slice

original := []int{1, 2, 3, 4, 5}

// 1️⃣ using append()
duplicate := []int{}
duplicate = append(duplicate, nums...)

// 2️⃣ using copy()
duplicate2 := make([]int, len(nums))
copy(duplicate2, nums)

• Does Go supports Pass By Ref ?
• new vs make in Go
• Does Go supports Inheritance ?
• How to create a simple web server in Go ?

To learn more about such topics along with code examples, feel free to checkout my Github repo where I regularly commit about Go core-concepts & best practices.