perl - '_var'私有(private)实例变量在什么情况下声明为 'use fields'？

Question

我试图了解 fields 的行为编译指示，我发现 poorly documented , 关于带下划线前缀的字段。这是文档必须说的:

Field names that start with an underscore character are made private to the class and are not visible to subclasses. Inherited fields can be overridden but will generate a warning if used together with the -w switch.

根据我在下面的测试，这与其实际行为不一致。不仅是 _ -前缀字段在子类中可见，它们在外部类中也可见(除非我不明白“可见”的含义)。此外，直接访问受限哈希也可以正常工作。

在哪里可以找到有关 fields 行为的更多信息pragma，缺少源代码？

{
    package Foo;
    use strict;
    use warnings;
    use fields qw/a _b __c/;

    sub new {
        my ( $class ) = @_;
        my Foo $self = fields::new($class);
        $self->a = 1; $self->b = 2; $self->c = 3;
        return $self;
    }

    sub a : lvalue { shift->{a}   }
    sub b : lvalue { shift->{_b}  }
    sub c : lvalue { shift->{__c} }
}
{
    package Bar;
    use base 'Foo';
    use strict;
    use warnings;
    use Data::Dumper;

    my $o = Bar->new;
    print Dumper $o; ##$VAR1 = bless({'_b' => 2, '__c' => 3, 'a' => 1}, 'Foo');

    $o->a = 4; $o->b = 5; $o->c = 6;
    print Dumper $o; ##$VAR1 = bless({'_b' => 5, '__c' => 6, 'a' => 4}, 'Foo');

    $o->{a} = 7; $o->{_b} = 8; $o->{__c} = 9;
    print Dumper $o; ##$VAR1 = bless({'_b' => 8, '__c' => 9, 'a' => 7}, 'Foo');
}

Answer 1

巧合的是，我恰好在 ~/codescraps/fields/test.pl 中有一个测试脚本可以追溯到两年前，当时我尝试回答完全相同的问题。 :)

#!/usr/bin/perl

use strict;
use warnings;

use Data::Dumper;

{
   package Foo;
   use fields qw(foo bar _Foo_private);
   use private qw(_really_private);
   sub new {
       my Foo $self = shift;
       unless (ref $self) {
           $self = fields::new($self);
           $self->{_Foo_private} = "this is Foo's secret";
       }
       $self->{foo} = 10;
       $self->{bar} = 20;
       return $self;
   }
}

my $foo = Foo->new;
$foo->{foo} = 42;

# this will generate an error: field does not exist
#$foo->{zap} = 42;

print "_Foo_private: " . $foo->{_Foo_private} . "\n";
$foo->{_Foo_private} = 1;
print "_Foo_private: " . $foo->{_Foo_private} . "\n";

print "_really_private: " . $foo->{_really_private} . "\n";
$foo->{_really_private} = 1;
print "_really_private: " . $foo->{_really_private} . "\n";

print Dumper($foo);

# subclassing
{
   package Bar;
   use base 'Foo';
   use fields qw(baz _Bar_private);        # these fields not shared with Foo
   sub new {
       my $class = shift;
       my $self = fields::new($class);
       $self->SUPER::new();                # init base fields
       $self->{baz} = 10;                  # init own fields
       $self->{_Bar_private} = "this is Bar's secret";
       return $self;
   }
}

my $bar = Bar->new;
# these work fine
$bar->{foo} = 1;
$bar->{bar} = 1;
$bar->{_Bar_private} = 1;

# this will not work - underscored fields are not visible to children
$bar->{_Foo_private} = 1;

当我运行你的代码时，我得到了错误:

No such pseudo-hash field "_b" at test2.pl line 16.

(第 16 行是 sub b 的定义。)您在什么架构上运行它？使用字段 pragma 的对象不是简单的祝福 hashrefs —— 它们是祝福的 arrayrefs，例如当我修改你的构造函数看起来像这样:

sub new {
    my ( $class ) = @_;
    my Foo $self = fields::new($class);
    $self->{a} = 1; $self->{_b} = 2; $self->{__c} = 3;
    print "I look like: ", Data::Dumper::Dumper($self);
    return $self;
}

我懂了:

I look like: $VAR1 = bless( [
                 bless( {
                          'a' => 1
                        }, 'pseudohash' ),
                 1,
                 2,
                 3
               ], 'Bar' );

作为附言，我觉得有必要指出 fields杂注和 base附带的编译指示都已被弃用，强烈建议避免使用它们。现在，如果你想用访问器构建一个漂亮的 OO 模块，可以使用 Class::Accessor或直接转至 Moose .