Golang：追加或不分配切片

Question

Go's append() function only allocates new slice data, when the capacity of the given slice is not sufficient (see also: https://stackoverflow.com/a/28143457/802833). This can lead to unexpected behavior (at least for me as a golang newbie):

package main

import (
    "fmt"
)

func main() {

    a1 := make([][]int, 3)
    a2 := make([][]int, 3)
    b := [][]int{{1, 1, 1}, {2, 2, 2}, {3, 3, 3}}
    common1 := make([]int, 0)
    common2 := make([]int, 0, 12) // provide sufficient capacity
    common1 = append(common1, []int{10, 20}...)
    common2 = append(common2, []int{10, 20}...)

    idx := 0
    for _, k := range b {
        a1[idx] = append(common1, k...) // new slice is allocated
        a2[idx] = append(common2, k...) // no allocation
        idx++
    }

    fmt.Println(a1)
    fmt.Println(a2) // surprise!!!
}

output:

[[10 20 1 1 1] [10 20 2 2 2] [10 20 3 3 3]]

[[10 20 3 3 3] [10 20 3 3 3] [10 20 3 3 3]]

https://play.golang.org/p/8PEqFxAsMt

So, what ist the (idomatic) way in Go to force allocation of new slice data or more precisely to make sure that the slice argument to append() remains unchanged?

Answer 1

Thanx for your feedback.

So the solution to gain control of the memory allocation is to do it explicitely (which remembers me that Go is a more a system language than other (scripting) langs):

package main

import (
    "fmt"
)

func main() {

    a1 := make([][]int, 3)
    a2 := make([][]int, 3)
    b := [][]int{{1, 1, 1}, {2, 2, 2}, {3, 3, 3}}
    common1 := make([]int, 0)
    common2 := make([]int, 0, 12) // provide sufficient capacity
    common1 = append(common1, []int{10, 20}...)
    common2 = append(common2, []int{10, 20}...)

    idx := 0
    for _, k := range b {
        a1[idx] = append(common1, k...) // new slice is allocated

        a2[idx] = make([]int, len(common2), len(common2)+len(k))
        copy(a2[idx], common2)      // copy & append could probably be
        a2[idx] = append(a2[idx], k...) // combined into a single copy step

        idx++
    }

    fmt.Println(a1)
    fmt.Println(a2)
}

output:

[[10 20 1 1 1] [10 20 2 2 2] [10 20 3 3 3]]

[[10 20 1 1 1] [10 20 2 2 2] [10 20 3 3 3]]

https://play.golang.org/p/Id_wSZwb84