Correct way to initialize empty slice

To declare an empty slice, with a non-fixed size, is it better to do:

mySlice1 := make([]int, 0) 

or:

mySlice2 := []int{} 

Just wondering which one is the correct way.

5 Answers

The two alternative you gave are semantically identical, but using make([]int, 0) will result in an internal call to runtime.makeslice (Go 1.16).

You also have the option to leave it with a nil value:

var myslice []int 

As written in the Golang.org blog:

a nil slice is functionally equivalent to a zero-length slice, even though it points to nothing. It has length zero and can be appended to, with allocation.

A nil slice will however json.Marshal() into "null" whereas an empty slice will marshal into "[]", as pointed out by @farwayer.

None of the above options will cause any allocation, as pointed out by @ArmanOrdookhani.

They are equivalent. See this code:

mySlice1 := make([]int, 0) mySlice2 := []int{} fmt.Println("mySlice1", cap(mySlice1)) fmt.Println("mySlice2", cap(mySlice2)) 

Output:

mySlice1 0 mySlice2 0 

Both slices have 0 capacity which implies both slices have 0 length (cannot be greater than the capacity) which implies both slices have no elements. This means the 2 slices are identical in every aspect.

See similar questions:

What is the point of having nil slice and empty slice in golang?

nil slices vs non-nil slices vs empty slices in Go language

As an addition to @ANisus' answer...

below is some information from the "Go in action" book, which I think is worth mentioning:

Difference between nil & empty slices

If we think of a slice like this:

[pointer] [length] [capacity] 

then:

nil slice: [nil][0][0] empty slice: [addr][0][0] // points to an address 

nil slice

They’re useful when you want to represent a slice that doesn’t exist, such as when an exception occurs in a function that returns a slice.

// Create a nil slice of integers. var slice []int 

empty slice

Empty slices are useful when you want to represent an empty collection, such as when a database query returns zero results.

// Use make to create an empty slice of integers. slice := make([]int, 0) // Use a slice literal to create an empty slice of integers. slice := []int{} 

Regardless of whether you’re using a nil slice or an empty slice, the built-in functions append, len, and cap work the same.

Go playground example:

package main import ( "fmt" ) func main() { var nil_slice []int var empty_slice = []int{} fmt.Println(nil_slice == nil, len(nil_slice), cap(nil_slice)) fmt.Println(empty_slice == nil, len(empty_slice), cap(empty_slice)) } 

prints:

true 0 0 false 0 0 

Empty slice and nil slice are initialized differently in Go:

var nilSlice []int emptySlice1 := make([]int, 0) emptySlice2 := []int{} fmt.Println(nilSlice == nil) // true fmt.Println(emptySlice1 == nil) // false fmt.Println(emptySlice2 == nil) // false 

As for all three slices, len and cap are 0.

In addition to @ANisus' answer

When using the official Go MongoDb Driver, a nil slice will also marshal into "null" whereas an empty slice will marshal into "[]".

When using using the community supported MGO driver, both nil and empty slices will be marshalled into "[]".

Reference: