Customizing accessors

"Class::InsideOut" supports custom subroutine hooks to modify the behavior of accessors. Hooks are passed as property options: "set_hook" and "get_hook".

The "set_hook" is called when the accessor is called with an argument. The hook subroutine receives the entire argument list. Just before the hook is called, $_ is locally aliased to the first argument for convenience. When the "set_hook" returns, the property is set equal to $_. This feature is useful for on-the-fly modification of the value that will be stored.

  public initials => my %initials, {
     set_hook => sub { $_ = uc $_ }
  };
 
  public tags => my %tags, {
     set_hook => sub { $_ = [ @_ ] } # stores arguments in a reference
  };

If the "set_hook" dies, the error is caught and rethrown with a preamble that includes the name of the accessor. The error should end with a newline to prevent "die" from adding 'at ... filename line N'. The correct location will be added when the error is rethrown with "croak":

  public height  => my %height, {
     set_hook => sub { /^\d+$/ or die "must be a positive integer" }
  };
 
  # dies with "height() must be a positive integer at ..."
  $person->height(3.5);

Note that the return value of the "set_hook" function is ignored. This simplifies syntax in the case where "die" is used to validate input.

The "get_hook" is called when the accessor is called without an argument. Just before the hook is called, $_ is set equal to the property value of the object for convenience. The hook is called in the same context (i.e. list versus scalar) as the accessor. The return value of the hook is passed through as the return value of the accessor.

  public tags => my %tags, {
     set_hook => sub { $_ = [ @_ ] }, # stores arguments in a reference
     get_hook => sub { @$_ }          # return property as a list
  };

Because $_ is a copy, not an alias, of the property value, it can be modified directly, if necessary, without affecting the underlying property.

As with "set_hook", the "get_hook" can die to indicate an error condition and errors are handled similarly. This could be used as a way to implement a protected property:

  sub _protected { 
     die "is protected\n" unless caller(2)->isa(__PACKAGE__)
  }
 
  public hidden => my %hidden, {
     get_hook => \&_protected,
     set_hook => \&_protected,
  }

Accessor hooks can be set as a global default with the "options" function, though they may still be overridden with options passed to specific properties.

Black-box inheritance

Because inside-out objects built with "Class::InsideOut" can use any type of reference for the object, inside-out objects can be built from other objects. This is useful to extend a superclass without needing to know whether it is based on hashes, array, or other types of blessed references.

  use base 'IO::File';
 
  sub new {
    my ($class, $filename) = @_;
 
    my $self = IO::File->new( $filename );
 
    register( $self, $class );
  }

In the example above, "IO::File" is a superclass. The object is an "IO::File" object, re-blessed into the inside-out class. The resulting object can be used directly anywhere an "IO::File" object would be, without interfering with any of its own inside-out functionality.

Classes using black-box inheritance should consider providing a "DEMOLISH" function that calls the black-box class destructor explicitly.

Serialization

"Class::InsideOut" automatically imports "STORABLE_freeze" and "STORABLE_thaw" methods to provide serialization support with Storable.Due to limitations of "Storable", this serialization will only work for objects based on scalars, arrays or hashes.

References to objects within the object being frozen will result in clones upon thawing unless the other references are included in the same freeze operation. (See "Storable" for details.)

   # assume $alice and $bob are objects
   $alice->friends( $bob );
   $bob->friends( $alice );
 
   $alice2 = Storable::dclone( $alice );
 
   # $bob was cloned, too, thanks to the reference
   die if $alice2->has_friend( $bob ); # doesn't die
 
   # get alice2's friend
   ($bob2) = $alice2->friends();
 
   # preserved relationship between bob2 and alice2
   die unless $bob2->has_friend( $alice2 ); # doesn't die

"Class::InsideOut" also allows customizing freeze and thaw hooks. When an object is frozen, if its class or any superclass provides a "FREEZE" method, they are each called with the object as an argument prior to the rest of the freezing process. This allows for custom preparation for freezing, such as writing a cache to disk, closing network connections, or disconnecting database handles.

Likewise, when a serialized object is thawed, if its class or any superclass provides a "THAW" method, they are each called after the object has been thawed with the thawed object as an argument.

"Class::InsideOut" also supports serialization of singleton objects for recent versions of "Storable" (2.14 or later) that support "STORABLE_attach". Users must signal that "STORABLE_attach" should be used instead of "STORABLE_thaw" by adding ":singleton" to their import line as follows:

   use Class::InsideOut qw( :std :singleton );

When attaching, the singleton object will be recreated in one of two ways:

1. If the singleton class contains an "ATTACH" method, it will be called with three arguments: the class name, a flag for whether this is part of a dclone, and a data structure representing the object:

     $data = {
         class => ref $obj,              # class name
         type => $type,                  # object reference type
         contents => $contents,          # object reference contents
         properties => \%property_vals,  # HoH of classes and properties
     }

"contents" is a reference of the same type as "type". "properties" is a multi-level hash, with the names of the class and any superclasses as top-level keys and property labels as second-level keys. This data may be used to reconstruct or reattach to the singleton. The "ATTACH" method should return the singleton.

2. If no "ATTACH" routine is found, but the class has or inherits a "new" method, then "new" will be called with no arguments and the result will be returned as the singleton.

Thread-safety

Because "Class::InsideOut" uses memory addresses as indices to object properties, special handling is necessary for use with threads. When a new thread is created, the Perl interpreter is cloned, and all objects in the new thread will have new memory addresses. Starting with Perl 5.8, if a "CLONE" function exists in a package, it will be called when a thread is created to provide custom responses to thread cloning. (See perlmod for details.) To avoid bugs in the implementation of threading, Perl 5.8.5 or later is strongly recommended.

"Class::InsideOut" itself has a "CLONE" function that automatically fixes up properties in a new thread to reflect the new memory addresses for all classes created with "Class::InsideOut". "register" must be called on all newly constructed inside-out objects to register them for use in "Class::InsideOut::CLONE".

Users are strongly encouraged not to define their own "CLONE" functions as they may interfere with the operation of "Class::InsideOut::CLONE" and leave objects in an undefined state. Future versions may support a user-defined CLONE hook, depending on demand.

Limitations:

"fork" on Perl for Win32 is emulated using threads since Perl 5.6. (See perlfork.) As Perl 5.6 did not support "CLONE", inside-out objects that use memory addresses (e.g. "Class::InsideOut") are not fork-safe for Win32 on Perl 5.6. Win32 Perl 5.8 "fork" is supported.

The technique for thread-safety requires creating weak references using "Scalar::Util::weaken()", which is implemented in XS. If the XS-version of Scalar::Util is not installed or if run on an older version of Perl without support for weak references, "Class::InsideOut" will issue a warning and continue without thread-safety. Also, objects will leak memory unless manually deregistered with a private function:

  # destroying an object when weaken() isn't availalbe
  Class::InsideOut::_deregister( $obj );
  undef $obj;