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03 Apr 2010 22:28:52 UTC
- Distribution: FreezeThaw
- Module version: 0.5001
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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 tofreeze
contained references toGLOB
s andCODE
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 tosafeFreeze
contained references toGLOB
s andCODE
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 andthaw
s data blessed in some package by calling methodsFreeze
andThaw
in the package. The fallback methods are provided by theFreezeThaw
itself. The fallbackFreeze
freezes the "content" of blessed object (from Perl point of view). The fallbackThaw
blesses thethaw
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
orsafeFreeze
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 isinstantiate
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 duringAllocate
andInstantiate
calls (viathawScalar
). Note that the return value ofFreezeEmpty
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 isFreezeThaw::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
, usescopyContents
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};
Module Install Instructions
To install FreezeThaw, copy and paste the appropriate command in to your terminal.
cpanm FreezeThaw
perl -MCPAN -e shell install FreezeThaw
For more information on module installation, please visit the detailed CPAN module installation guide.