使用值接收器附加到具有足够容量的切片

Question

can someone help me understand what happens here?

package main

import (
    "fmt"
)

func appendString(slice []string, newString string) {
    slice = append(slice, newString)
}

func main() {
    slice := make([]string, 0, 1)
    appendString(slice, "a")
    fmt.Println(slice)
}

I know about the slice header and the need to use a pointer receiver. But here, as the underlying array has enough capacity I would expect append to work anyways (just adding the new value to the underlying array and the original [copied] header working as expected)

What is wrong with my assumptions?

Answer 1

Let's add a final print statement to see the result:

slice := make([]string, 0, 1)
fmt.Println(cap(slice))
appendString(slice, "a")
fmt.Println(slice)

And the output will be (try it on the Go Playground ):

1
[]

Which is correct. One could expect the output to be:

1
[a]

The reason why this is not the case is because even though a new backing array will not be allocated, the slice header in the slice variable inside main() is not changed , it will still hold length = 0 . Only the slice header stored in the slice local variable inside appendString() (the parameter) is changed, but this variable is independent from main's slice .

If you were to reslice main's slice , you will see that the backing array does contain the new string:

slice := make([]string, 0, 1)
fmt.Println(cap(slice))
appendString(slice, "a")
fmt.Println(slice)

slice = slice[:1]
fmt.Println(slice)

Now output will be (try it on the Go Playground ):

1
[]
[a]

This is why the builtin append() has to return the new slice: because even if no new backing array is needed, the slice header (which contains the length) will have to be changed (increased) if more than 0 elements are appended.

This is why appendString() should also return the new slice:

func appendString(slice []string, newString string) []string {
    slice = append(slice, newString)
    return slice
}

Or short:

func appendString(slice []string, newString string) []string {
    return append(slice, newString)
}

Which you have to reassign where you use it:

slice := make([]string, 0, 1)
fmt.Println(cap(slice))
slice = appendString(slice, "a")
fmt.Println(slice)

And then you get the expected outcome right away (try it on the Go Playground ):

1
[a]