nimforum mirror - Can wasMoved be overloaded?

demotomohiro (orginal) [2022-12-05T03:08:56+01:00] view original

Can wasMoved be overloaded?

wasMoved cannot be overloaded? I tried to overload wasMoved proc for my object type but it is ignored.

# testmove.nim
type
  Foo = object
    name: string
    x: int

proc `=destroy`(x: var Foo) =
  echo "Destory ", x.name, ": ", x.x

proc `=sink`(x: var Foo; y: Foo) =
  echo "Sink to ", x.name, " from ", y.name
  `=destroy`(x)
  x.x = y.x

proc wasMoved(x: var Foo) =
  echo "wasMoved ", x.name
  x.x = -1

proc test =
  var
    a = Foo(name: "a", x: 1)
    b = Foo(name: "b", x: 2)
  
  a = b
  
  echo a

test()

Compiled with following command:

nim c -r --mm:arc --expandArc:test testmove.nim

Program output:

Sink to a from b
Destory a: 1
(name: "a", x: 2)
Destory : 0
Destory a: 2

--expandArc: test

var
  a
  b
  :tmpD
try:
  a = Foo(name: "a", x: 1)
  b = Foo(name: "b", x: 2)
  `=sink`(a, b)
  wasMoved(b)
  echo [
    :tmpD = `$`(a)
    :tmpD]
finally:
  `=destroy`(:tmpD)
  `=destroy_1`(b)
  `=destroy_1`(a)

There is wasMoved(b) in expandArc code, but compiler says testmove.nim(14, 6) Hint: 'wasMoved' is declared but not used [XDeclaredButNotUsed].

Background: I'm trying to wrap GMP. (I know there is already wrappers for the GMP: nim-gmp, bignum. But they don't use destructors.)

type
  MPZ* = object
    mpz: mpz_t

proc `=destroy`*(x: var MPZ) =
  mpz_clear(x.mpz)

proc `=sink`*(x: var MPZ; y: MPZ) =
  mpz_clear(x.mpz)
  x = y

proc initMPZ*(): MPZ =
  mpz_init(result.mpz)

GMP provides multiple precision integer type mpz_t. GMP manual says initialize mpz_t variable with the special initialization functions (mpz_init, mpz_init_set or etc) before using it and clear it out with mpz_clear when you’re done with a variable. It also says that the actual fields in mpz_t are for internal use only and should not be accessed directly by code that expects to be compatible with future GMP releases.

So just clearing all bits of mpz_t is directly writing the fields in mpz_t without using functions in GMP. In this case, I think wasMoved should use mpz_init instead of clearing all bits.

Move constructor in C++ class interface calls mpz_init(z.mp) to source object after it is moved: https://gmplib.org/repo/gmp/file/tip/gmpxx.h (ctrl + f with __gmp_expr(__gmp_expr &&z))

__gmp_expr(__gmp_expr &&z) noexcept
{ *__get_mp() = *z.__get_mp(); mpz_init(z.mp); }

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9694 :: Can wasMoved be overloaded?