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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/read-
       ing-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 com-
                   pares them as separate entities.

       "cmpStrHard"
                   analogue of "cmp" for data. Takes two arguments and com-
                   pares them considered as a group.

       "freeze"    returns a string that encupsulates its arguments (consid-
                   ered 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 (consid-
                   ered 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" con-
                   tained references to "GLOB"s and "CODE"s.

       "thaw"      takes one string argument and returns an array. The ele-
                   ments 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 meth-
       ods 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() han-
       dles 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 meth-
       ods:

         $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 ref-
       erences.

       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 ref-
       erences. 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 simpli-
       fication 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 "copyCon-
                   tents" 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 v5.8.8                       2002-03-29                     FreezeThaw(3)

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