{"id":635,"date":"2022-08-30T15:04:38","date_gmt":"2022-08-30T15:04:38","guid":{"rendered":"https:\/\/unknownerror.org\/index.php\/2013\/11\/09\/what-is-the-default-parameter-for-autoload-in-perl-collection-of-common-programming-errors\/"},"modified":"2022-08-30T15:04:38","modified_gmt":"2022-08-30T15:04:38","slug":"what-is-the-default-parameter-for-autoload-in-perl-collection-of-common-programming-errors","status":"publish","type":"post","link":"https:\/\/unknownerror.org\/index.php\/2022\/08\/30\/what-is-the-default-parameter-for-autoload-in-perl-collection-of-common-programming-errors\/","title":{"rendered":"What is the default parameter for AUTOLOAD in Perl?-Collection of common programming errors"},"content":{"rendered":"

Using Autoload is inherently difficult. If you want a solid object system that makes accessors for you then please use Moose, Mouse, Moo<\/strong>, or just loop over your fields and install the accessors yourself:<\/p>\n

BEGIN {\n  my @fields = qw\/foo bar baz\/;\n  for my $field (@fields) {\n    no strict 'refs';\n    # install a closure in the package stash.\n    *{ __PACKAGE__ . \"::\" . $field } = sub {\n      my $self = shift;\n      $self->{$field} = shift if @_;\n      return $self->{$field};\n    };\n  }\n}\n<\/code><\/pre>\n
If a class that can AUTOLOAD<\/code> encounters an undefined method, the AUTOLOAD<\/code> sub is called with the arguments of the missing sub. The fully qualified name of the requested sub is passed in the $AUTOLOAD<\/code> package variable.<\/p>\n
A typical Autoload sub would look like:<\/p>\n
use Carp;\n\nmy %fields_allowed = map {$_ => 1} qw\/foo bar baz\/;\n\nsub AUTOLOAD {\n  my $field = our $AUTOLOAD;\n  $field =~ s\/.*::\/\/; # strip the package name\n  $fields_allowed{$field}\n    or croak qq(Can't locate object method $field via package \"@{[__PACKAGE__]}\");\n  my $self = shift;\n  $self->{$field} = shift if @_;\n  return $self->{$field};\n}\n<\/code><\/pre>\n
There remain two problems:<\/p>\n
    \n
  • When the reference count of an object drops to zero, or when a thread terminates, the DESTROY<\/code> method is called on the object if it provides one. We can prevent autoloading of DESTROY<\/code> by providing an empty implementation: sub DESTROY {}<\/code>.<\/li>\n
  • We can ask any object if it can perform a certain method, like say \"Good dog\" if $dog->can(\"roll\")<\/code>. Therefore, we have to override can<\/code> to support our autoloading. The can<\/code> method is useful for safe duck typing. Every object inherits from UNIVERSAL<\/code>, which provides default implementations for can<\/code> and isa<\/code>.<\/li>\n<\/ul>\n
    The contract of can<\/code> is that it takes the name of a method. It will return undef<\/code> when the object cannot perform the method, or a code reference to that method if it can. A suitable implementation would be<\/p>\n
    sub can {\n  my ($self, $name) = @_;\n\n  # check if it's a field of ours\n  if ($fields_allowed{$name}) {\n    return sub {\n      my $self = shift;\n      $self->{$name} = shift if @_;\n      return $self->{$name};\n    };\n  }\n\n  # Ask SUPER implementation of can if we can do $name\n  if (my $meth = $self->SUPER::can($name)) {\n    return $meth;\n  }\n  return; # no method found\n}\n<\/code><\/pre>\n
    We can now simplify AUTOLOAD<\/code> to<\/p>\n
    sub AUTOLOAD {\n  my $field = our $AUTOLOAD;\n  $field =~ s\/.*::\/\/; # strip the package name\n  my $code = $self->can($field)\n    or croak qq(Can't locate object method $field via package \"@{[__PACKAGE__]}\");\n  goto &$code; # tail call; invisible via `caller()`.\n}\n<\/code><\/pre>\n
    This is a lot of complexity to get right. Verdict: Don’t use Autoload because you think it might be less work. It never is. It is quite useful for implementing a proxy pattern, but that is a bit advanced.<\/p>\n
    I urge you to dabble around with OO basics, and the Moose object system, before diving deep into Perl’s unique and strange features.<\/p>\n
    Originally posted 2013-11-09 21:09:20. <\/small><\/p>","protected":false},"excerpt":{"rendered":"
    Using Autoload is inherently difficult. If you want a solid object system that makes accessors for you then please use Moose, Mouse, Moo, or just loop over your fields and install the accessors yourself: BEGIN { my @fields = qw\/foo bar baz\/; for my $field (@fields) { no strict ‘refs’; # install a closure in […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1],"tags":[],"class_list":["post-635","post","type-post","status-publish","format-standard","hentry","category-uncategorized"],"_links":{"self":[{"href":"https:\/\/unknownerror.org\/index.php\/wp-json\/wp\/v2\/posts\/635","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/unknownerror.org\/index.php\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/unknownerror.org\/index.php\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/unknownerror.org\/index.php\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/unknownerror.org\/index.php\/wp-json\/wp\/v2\/comments?post=635"}],"version-history":[{"count":0,"href":"https:\/\/unknownerror.org\/index.php\/wp-json\/wp\/v2\/posts\/635\/revisions"}],"wp:attachment":[{"href":"https:\/\/unknownerror.org\/index.php\/wp-json\/wp\/v2\/media?parent=635"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/unknownerror.org\/index.php\/wp-json\/wp\/v2\/categories?post=635"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/unknownerror.org\/index.php\/wp-json\/wp\/v2\/tags?post=635"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}