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FreezeThaw - converting Perl structures to strings and back.
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!"}
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.
None.
freeze thaw cmpStr cmpStrHard safeFreeze.
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). thawing this string leads to a fatal error if arguments to
freeze contained references to GLOBs and CODEs.
safeFreeze
returns a string that encupsulates its arguments (considered as a
group). The result is thawable in the same process. thawing the
result in a different process should result in a fatal error if
arguments to safeFreeze contained references to GLOBs and
CODEs.
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).
FreezeThaw freezes and thaws 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 thawed 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.
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 allocated,
in the second it is instantiated.
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.
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};