unsafe_cast.go

package main

import (
	"fmt"
	"unsafe"
)

type URLString string
type URLStrings []URLString
type Strings []string

func unsafeCastToStringSlice(x interface{}) []string {
	return *(*([]string))(unsafe.Pointer(&x))
}

func (urls URLStrings) AsStrings() []string {
	return unsafeCastToStringSlice(urls)
}

func (ss Strings) AsURLStrings() []URLString {
	return *(*([]URLString))(unsafe.Pointer(&ss))
}

// Patern A. Strings to URLStrings
func main1() {
	ss := Strings{"a", "b", "c"}
	urls := ss.AsURLStrings()
	fmt.Println(urls)
	// Output:
	// [a b c]
}

// Pattern B. URLStrings to Strings
func main2() {
	urls := URLStrings{"a", "b", "c"}
	ss := urls.AsStrings()
	fmt.Println(ss)
	// panic: runtime error: invalid memory address or nil pointer dereference
}

func main() {
	main1()
	main2()
}