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.
6 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.
8 Comments
Grzegorz Żur
Also mention on wiki github.com/golang/go/wiki/…
farwayer
Be care:
json.Marshal() will return null for var myslice []int and [] for initialized slice myslice := []int{}
asgaines
Also be careful:
reflect.DeepEqual makes a distinction between nil slices and non-nil slices: a := []int{}, var b []int, reflect.DeepEqual(a, b) // returns false
Arman Ordookhani
Why you think it would do an allocation? Cap is zero so nothing is allocated. All pointers to zero length things point to the same location in memory: play.golang.org/p/MPOKKl_sYvw
ANisus
@КонстантинВан As of Go 1.16, they don't produce identical code, as you can see here. Using make calls
runtime.makeslice, even for a zero-length slice, while the empty slice literal directly uses runtime.zerobase. Meaning, the literal is slightly faster in Go 1.16. |
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?
Comments
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 []intempty 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, andcapwork the same.
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.
2 Comments
Andrzej Rehmann
make([]int, 0) is the best because Jetbrains GoLand does not complain about it being "unnecessary" as it does in the case of []int{}. This is useful in writing unit tests.
bool3max
I don't think we should decide best practices based on what IDE developers' opinions.
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: https://jira.mongodb.org/browse/GODRIVER-971
Comments
This is the shortest way to initialize empty slice.
list := []int{}
keys := make([]int, 0, len(m)); for k, v := range m { keys := append(keys,k) }