sharedstring.nim

type
  SharedString* = object
    str: cstring

proc newSharedString*(s: string): SharedString =
  result = SharedString(str: cast[cstring](allocShared(s.len + 1)))
  copyMem(result.str, s.cstring, s.len)

proc deallocSharedString*(s: SharedString) =
  deallocShared(s.str)

proc deallocSharedString*(s: var SharedString) =
  deallocShared(s.str)
  s.str = nil

proc `:=`*(ss: var SharedString, s: string) =
  # reassign `var SharedString` to new string value
  if ss.str == nil: ss.str = cast[cstring](allocShared(s.len + 1))
  else: ss.str = cast[cstring](reallocShared(ss.str, s.len + 1))
  copyMem(ss.str, s.cstring, s.len)

proc `$`*(s: SharedString): string = 
 if s.str == nil: "" else: $s.str

converter asString*(s: SharedString): string = $s

converter fromString*(s: string): SharedString = newSharedString(s)

let s = "foo"

let s1 = newSharedString(s)
echo "s1 is " & s1

let s2: SharedString = "bar"
echo "s2 is " & s2

var s3: SharedString
echo "s3 is " & s3

var s4: SharedString = "baz"
echo "s4 is " & s4
s4 := "quux"
echo "s4 is " & s4

var s5: SharedString = "baz"
echo "s5 is " & s5
s5 := s5 & "quux"
echo "s5 is " & s5

var s6: SharedString = "something"
let s7: string = s6 # copy back
s6 := "something_else"
deallocSharedString(s6)
echo "s6 is " & s6
echo "s7 is " & s7