NAME¶

FreezeThaw - converting Perl structures to strings and back.

SYNOPSIS¶

  use FreezeThaw qw(freeze thaw cmpStr safeFreeze cmpStrHard);
  $string = freeze $data1, $data2, $data3;
  ...
  ($olddata1, $olddata2, $olddata3) = thaw $string;
  if (cmpStr($olddata2,$data2) == 0) {print "OK!"}

DESCRIPTION¶

Converts data to/from stringified form, appropriate for saving-to/reading-from permanent storage.

Deals with objects, circular lists, repeated appearence of the same refence. Does not deal with overloaded stringify operator yet.

EXPORT¶

Default

None.

Exportable

"freeze thaw cmpStr cmpStrHard safeFreeze".

User API¶

"cmpStr"

analogue of "cmp" for data. Takes two arguments and compares them as separate entities.

"cmpStrHard"

analogue of "cmp" for data. Takes two arguments and compares them considered as a group.

"freeze"

returns a string that encupsulates its arguments (considered as a group). "thaw"ing this string leads to a fatal error if arguments to "freeze" contained references to "GLOB"s and "CODE"s.

"safeFreeze"

returns a string that encupsulates its arguments (considered as a group). The result is "thaw"able in the same process. "thaw"ing the result in a different process should result in a fatal error if arguments to "safeFreeze" contained references to "GLOB"s and "CODE"s.

"thaw"

takes one string argument and returns an array. The elements of the array are "equivalent" to arguments of the "freeze" command that created the string. Can result in a fatal error (see above).

Developer API¶

"FreezeThaw" "freeze"s and "thaw"s data blessed in some package by calling methods "Freeze" and "Thaw" in the package. The fallback methods are provided by the "FreezeThaw" itself. The fallback "Freeze" freezes the "content" of blessed object (from Perl point of view). The fallback "Thaw" blesses the "thaw"ed data back into the package.

So the package needs to define its own methods only if the fallback methods will fail (for example, for a lot of data the "content" of an object is an address of some C data). The methods are called like

  $newcooky = $obj->Freeze($cooky);
  $obj = Package->Thaw($content,$cooky);

To save and restore the data the following method are applicable:

  $cooky->FreezeScalar($data,$ignorePackage,$noduplicate);

during Freeze()ing, and

  $data = $cooky->ThawScalar;

Two optional arguments $ignorePackage and $noduplicate regulate whether the freezing should not call the methods even if $data is a reference to a blessed object, and whether the data should not be marked as seen already even if it was seen before. The default methods

  sub UNIVERSAL::Freeze {
    my ($obj, $cooky) = (shift, shift);
    $cooky->FreezeScalar($obj,1,1);
  }
  sub UNIVERSAL::Thaw {
    my ($package, $cooky) = (shift, shift);
    my $obj = $cooky->ThawScalar;
    bless $obj, $package;
  }

call the "FreezeScalar" method of the $cooky since the freezing engine will see the data the second time during this call. Indeed, it is the freezing engine who calls UNIVERSAL::Freeze(), and it calls it because it needs to freeze $obj. The above call to $cooky->FreezeScalar() handles the same data back to engine, but because flags are different, the code does not cycle.

Freezing and thawing $cooky also allows the following additional methods:

  $cooky->isSafe;

to find out whether the current freeze was initiated by "freeze" or "safeFreeze" command. Analogous method for thaw $cooky returns whether the current thaw operation is considered safe (i.e., either does not contain cached elsewhere data, or comes from the same application). You can use

  $cooky->makeSafe;

to prohibit cached data for the duration of the rest of freezing or thawing of current object.

Two methods

  $value = $cooky->repeatedOK;
  $cooky->noRepeated;           # Now repeated are prohibited

allow to find out/change the current setting for allowing repeated references.

If you want to flush the cache of saved objects you can use

  FreezeThaw->flushCache;

this can invalidate some frozen string, so that thawing them will result in fatal error.

  $pre_object_ref = Package->Allocate($pre_pre_object_ref);
        # Returns reference
  Package->Instantiate($pre_object_ref,$cooky);
        # Converts into reference to blessed object

  $object_ref->FreezeEmpty($cooky);
  $object_ref->FreezeInstance($cooky);

  $o1->FreezeEmpty($cooky)

  FreezeThaw::copyContents $to, $from;

BUGS and LIMITATIONS¶

A lot of objects are blessed in some obscure packages by XSUB typemaps. It is not clear how to (automatically) prevent the "UNIVERSAL" methods to be called for objects in these packages.

The objects which can survive freeze()/thaw() cycle must also survive a change of a "member" to an equal member. Say, after

  $a = [a => 3];
  $a->{b} = \ $a->{a};

$a satisfies

  $a->{b} == \ $a->{a}

This property will be broken by freeze()/thaw(), but it is also broken by

  $a->{a} = delete $a->{a};