Re^4: Accessing Arguments inside Subroutines via @

Well you asked for the "why" not for the "how" ... :)

I took a look into the Panther book and experimented with Devel::Peek and the implementation of aliases seems simpler than a magic data structure of type alias.

Arrays (AVs) are composed from C-structures with pointers to C-structure(s) realizing scalars (SVs).

So $_[0]=666 updates the underlying scalar ( what I called "assigning" ) while @_=(666) creates the pointer to a new scalar ( "setting" ).

In the following dump the scalar value for $x is held within SV = IV(0x88f2498) at 0x88f249c

The array's first slot will point to that same structure before and after updating with $_[0]=666 (#markers added)

  DB<126> use  Devel::Peek

  DB<127> sub tst_update { Dump(\@_); $_[0]=666; Dump(\@_) }

  DB<128> my $x=42; Dump($x); tst_update $x
SV = IV(0x88f2498) at 0x88f249c                    # <--- SV for $x
  REFCNT = 1
  FLAGS = (PADMY,IOK,pIOK)
  IV = 42
SV = IV(0x8d02430) at 0x8d02434
  REFCNT = 1
  FLAGS = (ROK)
  RV = 0x8a5cbbc
  SV = PVAV(0x89be074) at 0x8a5cbbc
    REFCNT = 3
    FLAGS = ()
    ARRAY = 0x86d4500
    FILL = 0
    MAX = 3
    ARYLEN = 0x0
    FLAGS = (REAL)
    Elt No. 0
    SV = IV(0x88f2498) at 0x88f249c                # <--- SV for $x
      REFCNT = 2
      FLAGS = (PADMY,IOK,pIOK)
      IV = 42
SV = IV(0x8d02430) at 0x8d02434
  REFCNT = 1
  FLAGS = (ROK)
  RV = 0x8a5cbbc
  SV = PVAV(0x89be074) at 0x8a5cbbc
    REFCNT = 3
    FLAGS = ()
    ARRAY = 0x86d4500
    FILL = 0
    MAX = 3
    ARYLEN = 0x0
    FLAGS = (REAL)
    Elt No. 0
    SV = IV(0x88f2498) at 0x88f249c                # <--- SV for $x
      REFCNT = 2
      FLAGS = (PADMY,IOK,pIOK)
      IV = 666
[download]

But creating new entries with @_=(666) will replace the pointer in this AV-slot to a new SV.

  DB<129> sub tst_create { Dump(\@_); @_=(666); Dump(\@_) }

  DB<130> my $x=42; Dump($x); tst_create $x
SV = IV(0x88f2740) at 0x88f2744                    # <--- SV for $x
  REFCNT = 1
  FLAGS = (PADMY,IOK,pIOK)
  IV = 42
SV = IV(0x8d019a4) at 0x8d019a8
  REFCNT = 1
  FLAGS = (ROK)
  RV = 0x8a5cbbc
  SV = PVAV(0x89be074) at 0x8a5cbbc
    REFCNT = 3
    FLAGS = ()
    ARRAY = 0x86d4500
    FILL = 0
    MAX = 3
    ARYLEN = 0x0
    FLAGS = (REAL)
    Elt No. 0
    SV = IV(0x88f2740) at 0x88f2744                # <--- SV for $x
      REFCNT = 2
      FLAGS = (PADMY,IOK,pIOK)
      IV = 42
SV = IV(0x8d019a4) at 0x8d019a8
  REFCNT = 1
  FLAGS = (ROK)
  RV = 0x8a5cbbc
  SV = PVAV(0x89be074) at 0x8a5cbbc
    REFCNT = 3
    FLAGS = ()
    ARRAY = 0x86d4500
    FILL = 0
    MAX = 3
    ARYLEN = 0x0
    FLAGS = (REAL)
    Elt No. 0
    SV = IV(0x88f2718) at 0x88f271c                # <--- SV for liter
+al 666
      REFCNT = 1
      FLAGS = (IOK,pIOK)
      IV = 666
[download]

I hope the "how" is clearer now! :)

Cheers Rolf
_{(addicted to the Perl Programming Language and ☆☆☆☆ :)}

PS: Je suis Charlie!

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