perl - 在 perl 中，这些没有印记的变量是什么？

Question

我正在尝试了解高阶 Perl 的平面文件数据库。

在 FlatDB 包中，sub new，第 141 页，他有声明:

bless { FH => $fh, FIELDS =>\@field, FIELDNUM =>\%fieldnum, FIELDSEP => $FIELDSEP } => $class;

其中 $class == FlatDB。

没有印记的这些变量(FH、FIELDNUM 等)是什么？如何打印它们的值、查找它们的类型或使用它们？

FlatDB 似乎是某种散列的散列，但我所有打印值的尝试都导致错误，例如:

printhash(\%fieldnum,"at37:\\%fieldnum=");
# prints the hash properly, BUT
printhash(FlatDB{FIELDNUM),"at38:FlatDB{FIELDNUM}=");
printhash(FlatDB->FIELDNUM,"at39:FlatDB->FIELDNUM=");
# print the error:
# Can't locate object method "FIELDNUM" via package "FlatDB"

为什么它认为 FIELDNUM 是一个方法，当它被定义为一个散列时？

仅供引用:printhash 子是:

sub printhash
 { my $href=shift;  # a REFERENCE to a hash
   my $msg=shift;   # a text message
   my %h = %{$href};
   my $len = keys %h;;
   print "\n$msg, length=$len";
   foreach my $k ( keys %h )
   { print "\nkey: $k, value: $h{$k}";  }
   print "\n";
 }

Answer 1

它们不是变量。

它们是受祝福的哈希引用中的键(即 an object )。 “fat comma ”(=> 运算符)允许将左侧的​​裸词解释为字符串，如果它以字母或下划线开头并且仅包含字母数字字符和下划线。 (这与对象无关，但在初始化散列或散列引用时，或者每当有人明确尝试指示关联时，您通常会看到这种语法。)

回答你的问题:

Why does it think FIELDNUM is a method, when it is defined as a hash?

因为您将它作为一种方法来调用。语法接近 dereference operator ，但不完全一样。事实上，documentation甚至说了这样的话:

Calling a method on an object is written as $object->method.

The left hand side of the method invocation (or arrow) operator is the object (or class name), and the right hand side is the method name.

my $pod = File->new( 'perlobj.pod', $data );
$pod->save();

The -> syntax is also used when dereferencing a reference. It looks like the same operator, but these are two different operations.

和:

"->" is an infix dereference operator, just as it is in C and C++. If the right side is either a [...], {...}, or a (...) subscript, then the left side must be either a hard or symbolic reference to an array, a hash, or a subroutine respectively. (Or technically speaking, a location capable of holding a hard reference, if it's an array or hash reference being used for assignment.) See perlreftut and perlref.

Otherwise, the right side is a method name or a simple scalar variable containing either the method name or a subroutine reference, and the left side must be either an object (a blessed reference) or a class name (that is, a package name). See perlobj.

但实际上，您可能想要的是该类的一个实例，它看起来像这样:

my $obj = FlatDB->new;

然后您可以像这样访问对象成员:

$obj->{FH};
@{$obj->{FIELDS}};
%{$obj->{FIELDNUM}};
$obj->{FIELDSEP};

但是你不应该因为封装。不幸的是，解释该原则和其他 OOP 原则超出了本答案的范围。