Go slice容量在追加时如何变化？ [重复]

Question

This question already has an answer here:

• How the slice is enlarged by append? Is the capacity always doubled? 1 answer

Running the example The Go Tour on server (currently on version 1.12.7), I find the capacity of slice doubling to the next power of 2, if the new slice length is larger than current backing array's length.

If I run the same program on my machine (version 1.10.3 on windows), the slice capacity changes to next multiple of two.

Why are they different? Is it because of Go version or run-time implementations? Is the capacity change deterministic?

The output on remote server is this

len=0 cap=0 []
len=1 cap=2 [0]
len=2 cap=2 [0 1]
len=5 cap=8 [0 1 2 3 4]

The output on local machine is this

len=0 cap=0 []
len=1 cap=1 [0]
len=2 cap=2 [0 1]
len=5 cap=6 [0 1 2 3 4]

This is the code for reference

package main

import "fmt"

func main() {
    var s []int
    printSlice(s)

    // append works on nil slices.
    s = append(s, 0)
    printSlice(s)

    // The slice grows as needed.
    s = append(s, 1)
    printSlice(s)

    // We can add more than one element at a time.
    s = append(s, 2, 3, 4)
    printSlice(s)
}

func printSlice(s []int) {
    fmt.Printf("len=%d cap=%d %v
", len(s), cap(s), s)
}

</div>

Answer 1

TL;DR: It depends on the size of the elements stored in the array

The implementation can be seen here:

https://github.com/golang/go/blob/master/src/runtime/slice.go

But as you can see looking into the history it can't be relied to stay the same over time.

That might also explain the difference you may note on different versions of Go.

Making some tests show how a 0-size struct will increment capacity by just 1 element, and int or string will duplicate on each growth, and a 3-byte struct "roughly" doubles on each growth.

You can execute code like this using different types to see these different cases in action:

arr := []struct{}{}
oldCap := 0
for i := 0; i < 100; i++ {
    arr = append(arr, struct{}{})
    if cap(arr) != oldCap {
        oldCap = cap(arr)
        fmt.Println("arr", cap(arr))
    }
}

Playground showing the cases described above:

https://play.golang.org/p/OKtCFskbp2t