将切片从功能参数追加到切片

Question

I am trying to find out, how slice works. Look at the following code

package main

import "fmt"

type ByteSlice []byte

func (p *ByteSlice) Append(data []byte) {
    slice := *p
    *p = append(slice, data...)
}

func main() {
    var a ByteSlice
    a[0] = 3
    b := []byte{4,6,7}
    a.Append(b)
    fmt.Println(b)
}

As you can see in the append function, i try to append slice parameter into call but i've got the following error:

panic: runtime error: index out of range

goroutine 16 [running]:
runtime.panic(0x4a14e0, 0x51333c)
    c:/go/src/pkg/runtime/panic.c:279 +0xe9
main.main()
    D:/Go/samples/src/method.go:16 +0x243

goroutine 17 [runnable]:
runtime.MHeap_Scavenger()
    c:/go/src/pkg/runtime/mheap.c:507
runtime.goexit()
    c:/go/src/pkg/runtime/proc.c:1445

goroutine 18 [runnable]:
bgsweep()
    c:/go/src/pkg/runtime/mgc0.c:1976
runtime.goexit()
    c:/go/src/pkg/runtime/proc.c:1445

goroutine 19 [runnable]:
runfinq()
    c:/go/src/pkg/runtime/mgc0.c:2606
runtime.goexit()
    c:/go/src/pkg/runtime/proc.c:1445
exit status 2

What do i here wrong?

Answer 1

See my comments line by line:

a slice is just a "view" of an array.

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

package main

import "fmt"

type ByteSlice []byte

func (p *ByteSlice) Append(data []byte) {
    *p = append(*p, data...)
}

func main() {
    var a ByteSlice // is just a pointer
    // a[0] = 3  does not have memory allocate
    a = append(a, 3) // append allocates a memory underneath
    fmt.Println(a[0])
    b := []byte{4, 6, 7}
    a.Append(b)
    fmt.Println(a)
}

output

3 [3 4 6 7]