将数组作为函数参数传递

Question

I expected the following code snippet either to produce a as {0x01, 0x02} (pass by value) or as {0x03, 0x02, 0x01} (pass by reference). Strangely, it produces the output as {0x03, 0x02}. Could you please help me understand why is that?

package main

import "fmt"

func test1(t []byte) {
    t[0] = 0x03
    t = append(t, 0x01 )
}

func main() {
    a := []byte{0x01, 0x02 }
    test1(a)
    _ = a
    fmt.Printf("%v", a)
}

Answer 1

In Go []byte is not an array, it's a byte slice.

What happened is that the size of a was 2, and because of that, after its first element was changed to 3, append allocated a new bigger slice - and t was set to the address of the new slice that was allocated, but that doesn't affect the a in main.

From A Tour of Go about append:

If the backing array of s is too small to fit all the given values a bigger array will be allocated. The returned slice will point to the newly allocated array.