为什么Perl的autovivification在这种情况下有效？

Question

有人可以帮助我理解这个Perl程序的输出:

use Data::Dumper;
my %hash;
$hash{hello} = "foo";
$hash{hello}{world} = "bar";
print $hash{hello} . "\n";
print $hash{hello}{world} . "\n";
print Dumper(\%hash);

并输出:

foo
bar
$VAR1 = {
          'hello' => 'foo'
        };

"foo"来自哪里？怎么没有翻斗车打印出来？

请注意,如果我交换作业的顺序:

use Data::Dumper;
my %hash;
$hash{hello}{world} = "bar";
$hash{hello} = "foo";
print $hash{hello} . "\n";
print $hash{hello}{world} . "\n";
print Dumper(\%hash);

我的输出是我所期望的:

foo

$VAR1 = {
          'hello' => 'foo'
        };

编辑:我知道use strict;会抓住这个,但我更感兴趣的是知道如何打印字符串"foo".

Answer 1

你的代码丢失了

use strict;

C:\Temp> hui
Can't use string ("foo") as a HASH ref while "strict refs" in use at 
C:\Temp\hui.pl line 7.

确保所有脚本都以:

use strict;
use warnings;

鉴于:

$hash{hello} = "foo";

$hash{hello}是NOT散列引用.

$hash{hello}{world} = "bar";

将字符串"foo"视为哈希引用并创建哈希%main::foo并设置$foo{world}为"bar".

当你这样做时:

print Dumper \%hash;

它只打印内容%hash.然而,当你这样做的时候

print $hash{hello}{world} . "\n";

它打印$foo{world}.

没有strict,你不会发现脚本已经遍布包名称空间.

添加一个

print Dumper \%main::;

要么

print Dumper \%main::foo;

运行此命令后检查符号表.