arrays - Re-slicing slices in Golang

Question

I recently picked up the Go language, and now I am confused with the following code:

package main

import "fmt"

func main() {
    a := make([]int, 5)
    printSlice("a", a)
    b := make([]int, 0, 5)
    printSlice("b", b)
    c := b[:2]
    printSlice("c", c)
    d := c[2:5]
    printSlice("d", d)
}

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

And the result:

a len=5 cap=5 [0 0 0 0 0]
b len=0 cap=5 []
c len=2 cap=5 [0 0] //why the capacity of c not 2 but 5 instead
d len=3 cap=3 [0 0 0]

Answer 1

c is a slice taken from the array b. This isn't a copy, but just a window over the 2 first elements of b.

As b has a capacity of 5, c could be extended to take the 3 other places (in fact it makes a new slice but over the same place in memory).

The maximal capacity of the slice is the capacity of the underlying array minus the position of the start of the slice in the array :

 array : [0 0 0 0 0 0 0 0 0 0 0 0]
 array :  <----   capacity   --->
 slice :     [0 0 0 0]
 slice :      <---- capacity ---> 

Maybe this program will make it more clear that c and d are just windows over b :

func main() {
    b := make([]int, 0, 5)
    c := b[:2]
    d := c[1:5] // this is equivalent to d := b[1:5]
    d[0] = 1
    printSlice("c", c)
    printSlice("d", d)
}

Output :

c len=2 cap=5 [0 1] // modifying d has modified c
d len=4 cap=4 [1 0 0 0]