How to preallocate and fill a slice of pointers in a go-idiomatic way?

Question

http://play.golang.org/p/j-Y0mQzTdP

package main  import "fmt"  type UselessStruct struct {     a int     b int }  func main() {     mySlice := make([]*UselessStruct, 5)     for i := 0; i != 5; i++ {         mySlice = append(mySlice, &UselessStruct{})     }      fmt.Println(mySlice) } 

Outputs: [<nil> <nil> <nil> <nil> <nil> 0xc010035160 0xc010035170 0xc010035180 0xc010035190 0xc0100351a0]

What i would like to do, is preallocate memory for 5 UselessStructs, stored as pointers. If i declare a slice of struct values eq:

mySlice := make([]UselessStruct, 5) 

then this creates 5 empty structs - appending doesn't replace the empty structs, but rather keeps on adding to the slice, so the end result with this code:

http://play.golang.org/p/zBYqGVO85h

package main  import "fmt"  type UselessStruct struct {     a int     b int }  func main() {     mySlice := make([]UselessStruct, 5)     for i := 0; i != 5; i++ {         mySlice = append(mySlice, UselessStruct{})     }      fmt.Println(mySlice) } 

is: [{0 0} {0 0} {0 0} {0 0} {0 0} {0 0} {0 0} {0 0} {0 0} {0 0}]

What is the the go-idiomatic way to preallocate and fill slices?

Answer 1

For your first example, I would do:

mySlice := make([]*UselessStruct, 5) for i := range mySlice {      mySlice[i] = new(UselessStruct) } 

The issue you are facing in both examples is you are appending to a slice that is already the correct length. If you set mySlice := make([]*UselessStruct, 5), you are asking for a slice of nil pointers of length 5. If you append one pointer, it now has length 6.

Instead, you want to use mySlice := make([]*UselessStruct, 0, 5). This creates a slice of length 0 but capacity 5. Each time you append it will add one to the length but it won't reallocate until you exceed the slice's capacity.

mySlice := make([]*UselessStruct, 0, 5) for i := 0; i != 5; i++ {     mySlice = append(mySlice, &UselessStruct{}) } // mySlice is [0xc010035160 0xc010035170 0xc010035180 0xc010035190 0xc0100351a0] 

Both of my examples will work as you expect but I recommend the first one for purely style reasons.