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FreezeThaw(3)                        User Contributed Perl Documentation                       FreezeThaw(3)



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.

   Instantiating
       Sometimes, when an object from a package is recreated in presense of repeated references, it is not
       safe to recreate the internal structure of an object in one step. In such a situation recreation of
       an object is carried out in two steps: in the first the object is "allocate"d, in the second it is
       "instantiate"d.

       The restriction is that during the allocation step you cannot use any reference to any Perl object
       that can be referenced from any other place. This restriction is applied since that object may not
       exist yet.

       Correspondingly, during instantiation step the previosly allocated object should be "filled", i.e.,
       it can be changed in any way such that the references to this object remain valid.

       The methods are called like this:

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

       The reverse operations are

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

       during these calls object can "freezeScalar" some information (in a usual way) that will be used
       during "Allocate" and "Instantiate" calls (via "thawScalar"). Note that the return value of
       "FreezeEmpty" is cached during the phase of creation of uninialized objects. This must be used like
       this: the return value is the reference to the created object, so it is not destructed until other
       objects are created, thus the frozen values of the different objects will not share the same
       references. Example of bad result:

         $o1->FreezeEmpty($cooky)

       freezes "{}", and "$o2->FreezeEmpty($cooky)" makes the same. Now nobody guaranties that that these
       two copies of "{}" are different, unless a reference to the first one is preserved during the call to
       "$o2->FreezeEmpty($cooky)". If "$o1->FreezeEmpty($cooky)" returns the value of "{}" it uses, it will
       be preserved by the engine.

       The helper function "FreezeThaw::copyContents" is provided for simplification of instantiation. The
       syntax is

         FreezeThaw::copyContents $to, $from;

       The function copies contents the object $from point to into what the object $to points to (including
       package for blessed references). Both arguments should be references.

       The default methods are provided. They do the following:

       "FreezeEmpty"
                   Freezes an empty object of underlying type.

       "FreezeInstance"
                   Calls "Freeze".

       "Allocate"  Thaws what was frozen by "FreezeEmpty".

       "Instantiate"
                   Thaws what was frozen by "FreezeInstance", uses "copyContents" to transfer this to the
                   $pre_object.

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};



perl v5.16.2                                     2010-04-03                                    FreezeThaw(3)