什么情况下，实例变量在'use fields'中声明为'_var'为private？

Question

我试图了解fields编译指示的行为，我发现poorly documented，有关以下划线开头的字段。这是文档要说的：

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.

根据我的测试，这不符合它的实际行为。不仅_ - 在子类中可见的前缀字段，它们在外部类中也是可见的（除非我没有看到'可见'的含义）。此外，直接访问受限哈希工作正常。

从哪里可以找到关于fields编译指示行为的更多信息，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

巧得很，我曾经有在~/codescraps/fields/test.pl测试脚本从两年前开始约会时，我正好有这个回答同样的问题尝试。 :)

#!/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; 

当我运行代码，我得到的错误：

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

（第16行是次b中的定义。）你运行这个什么架构上？使用字段杂注的对象不是简单的祝福hashrefs - 它们是有福的arrayrefs，例如当我修改你的构造，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; 
} 

我看到：

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

---

附言，我觉得有必要指出的是，fields编译和base编译那与此同时，都被弃用，强烈要求避免使用它们。如今，如果您打算使用访问器构建一个漂亮的面向对象模块，您可以使用Class::Accessor或直接转至Moose。