perl hashref/perl语法

Question

今天通过的这段代码的变体(由perl编码器编写),令人困惑:

   my $k = {3,5,6,8};
   my $y = {%$k};

为什么？那是做什么的？这似乎与此相同:

   my $y = $k;

使用dbi模块调用上下文:

               while (my $x = $db->fetchrow_hashref )
               {  $y{something} = {%$x};  }

Answer 1

不同之处在于它在不引用相同内存的情况下克隆数据结构.

例如:

use strict;
use warnings;
use Data::Dumper;

my $h={'a'=>1,'b'=>2};
my $exact_copy=$h; #$exact_copy references the same memory as $h
$h->{b}++; #$h maps b to 3

print Dumper($exact_copy) . "\n"; #a=>1,b=>3

my $clone={%$h}; #We dereference $h and then make a new reference
$h->{a}++; #h now maps a to 2

print Dumper($clone) . "\n"; #a=>1,b=>3 so this clone doesn't shadow $h

顺便说一下,使用所有逗号(如同my $k = {3,5,6,8})手动初始化哈希是非常非常难看的.