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 argu

ments and compares them as separate entities.

"cmpStrHard"

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

"freeze" returns a string that encupsulates its argu

ments (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 argu
ments (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 "equiva
lent" 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() 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 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 prohib

ited

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 recre
ate 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_ob

ject_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 informa
tion (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) pre
vent 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 v5.8.0 2002-03-30 FreezeThaw(3)