使用undef作为哈希值是否可以在Perl中保存任何内存?

用于从数组中删除重复值的Perl习惯用法:

@uniq = keys %{{map{$_=>1}@list}}

使用这个版本更便宜:

@uniq = keys %{{map{$_=>undef}@list}}

我用这些单行测试了它,似乎在某些版本的Perl上都是如此:

perl -e 'my %x; $x{$_} = 1 for 0..1000_000; system "ps -ovsz $$"' 
perl -e 'my %x; $x{$_} = undef for 0..1000_000; system "ps -ovsz $$"'
好吧,undef应该是一个flyweight值,这意味着对它的所有引用都指向相同的数据.你没有得到其他文字.您仍然需要引用它的插槽的开销.但是,我没有看到它为我在Mac OS X上的Perl 5.10或5.11保存任何内存.虽然perl可能没有在unf情况下使用更多内存,但我敢打赌它预计会使用更多内存,所以它无论如何都要抓住它.但是,我现在并不热衷于调查内部的内存使用情况.

Devel::Peek对于展示这些东西非常方便:

#!perl

use Devel::Peek;

my $a = undef;
my $b = undef;

Dump( $a );
Dump( $b );


my %hash = map { $_, undef } 1 .. 3;
$hash{4} = 'Hello';
Dump( \%hash );

输出看起来有点吓人,但你看到undef值是NULL(0x0)而不是单个字符串值(PV):

SV = NULL(0x0) at 0x100208708
  REFCNT = 1
  FLAGS = (PADMY)
SV = NULL(0x0) at 0x100208738
  REFCNT = 1
  FLAGS = (PADMY)
SV = RV(0x100805018) at 0x100805008
  REFCNT = 1
  FLAGS = (TEMP,ROK)
  RV = 0x100208780
  SV = PVHV(0x100809ed8) at 0x100208780
    REFCNT = 2
    FLAGS = (PADMY,SHAREKEYS)
    ARRAY = 0x100202200  (0:5, 1:2, 2:1)
    hash quality = 91.7%
    KEYS = 4
    FILL = 3
    MAX = 7
    RITER = -1
    EITER = 0x0
    Elt "4" HASH = 0xb803eff9
    SV = PV(0x100801c78) at 0x100804ed0
      REFCNT = 1
      FLAGS = (POK,pPOK)
      PV = 0x100202a30 "Hello"\0
      CUR = 5
      LEN = 8
    Elt "1" HASH = 0x806b80c9
    SV = NULL(0x0) at 0x100820db0
      REFCNT = 1
      FLAGS = ()
    Elt "3" HASH = 0xa400c7f3
    SV = NULL(0x0) at 0x100820df8
      REFCNT = 1
      FLAGS = ()
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