1FreezeThaw(3) User Contributed Perl Documentation FreezeThaw(3)
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6 FreezeThaw - converting Perl structures to strings and back.
7
9 use FreezeThaw qw(freeze thaw cmpStr safeFreeze cmpStrHard);
10 $string = freeze $data1, $data2, $data3;
11 ...
12 ($olddata1, $olddata2, $olddata3) = thaw $string;
13 if (cmpStr($olddata2,$data2) == 0) {print "OK!"}
14
16 Converts data to/from stringified form, appropriate for
17 saving-to/reading-from permanent storage.
18 Deals with objects, circular lists, repeated appearence of the same
20 refence. Does not deal with overloaded stringify operator yet.
21
23 Default None.
24 Exportable "freeze thaw cmpStr cmpStrHard safeFreeze".
26
28 "cmpStr" analogue of "cmp" for data. Takes two arguments and
29 compares them as separate entities.
30 "cmpStrHard"
32 analogue of "cmp" for data. Takes two arguments and
33 compares them considered as a group.
34 "freeze" returns a string that encupsulates its arguments
36 (considered as a group). "thaw"ing this string leads to a
37 fatal error if arguments to "freeze" contained references
38 to "GLOB"s and "CODE"s.
39 "safeFreeze"
41 returns a string that encupsulates its arguments
42 (considered as a group). The result is "thaw"able in the
43 same process. "thaw"ing the result in a different process
44 should result in a fatal error if arguments to "safeFreeze"
45 contained references to "GLOB"s and "CODE"s.
46 "thaw" takes one string argument and returns an array. The
48 elements of the array are "equivalent" to arguments of the
49 "freeze" command that created the string. Can result in a
50 fatal error (see above).
51
53 "FreezeThaw" "freeze"s and "thaw"s data blessed in some package by
54 calling methods "Freeze" and "Thaw" in the package. The fallback
55 methods are provided by the "FreezeThaw" itself. The fallback "Freeze"
56 freezes the "content" of blessed object (from Perl point of view). The
57 fallback "Thaw" blesses the "thaw"ed data back into the package.
58 So the package needs to define its own methods only if the fallback
60 methods will fail (for example, for a lot of data the "content" of an
61 object is an address of some C data). The methods are called like
62 $newcooky = $obj->Freeze($cooky);
64 $obj = Package->Thaw($content,$cooky);
65 To save and restore the data the following method are applicable:
67 $cooky->FreezeScalar($data,$ignorePackage,$noduplicate);
69 during Freeze()ing, and
71 $data = $cooky->ThawScalar;
73 Two optional arguments $ignorePackage and $noduplicate regulate whether
75 the freezing should not call the methods even if $data is a reference
76 to a blessed object, and whether the data should not be marked as seen
77 already even if it was seen before. The default methods
78 sub UNIVERSAL::Freeze {
80 my ($obj, $cooky) = (shift, shift);
81 $cooky->FreezeScalar($obj,1,1);
82 }
83 sub UNIVERSAL::Thaw {
85 my ($package, $cooky) = (shift, shift);
86 my $obj = $cooky->ThawScalar;
87 bless $obj, $package;
88 }
89 call the "FreezeScalar" method of the $cooky since the freezing engine
91 will see the data the second time during this call. Indeed, it is the
92 freezing engine who calls UNIVERSAL::Freeze(), and it calls it because
93 it needs to freeze $obj. The above call to $cooky->FreezeScalar()
94 handles the same data back to engine, but because flags are different,
95 the code does not cycle.
96 Freezing and thawing $cooky also allows the following additional
98 methods:
99 $cooky->isSafe;
101 to find out whether the current freeze was initiated by "freeze" or
103 "safeFreeze" command. Analogous method for thaw $cooky returns whether
104 the current thaw operation is considered safe (i.e., either does not
105 contain cached elsewhere data, or comes from the same application). You
106 can use
107 $cooky->makeSafe;
109 to prohibit cached data for the duration of the rest of freezing or
111 thawing of current object.
112 Two methods
114 $value = $cooky->repeatedOK;
116 $cooky->noRepeated; # Now repeated are prohibited
117 allow to find out/change the current setting for allowing repeated
119 references.
120 If you want to flush the cache of saved objects you can use
122 FreezeThaw->flushCache;
124 this can invalidate some frozen string, so that thawing them will
126 result in fatal error.
127 Instantiating
129 Sometimes, when an object from a package is recreated in presense of
130 repeated references, it is not safe to recreate the internal structure
131 of an object in one step. In such a situation recreation of an object
132 is carried out in two steps: in the first the object is "allocate"d, in
133 the second it is "instantiate"d.
134 The restriction is that during the allocation step you cannot use any
136 reference to any Perl object that can be referenced from any other
137 place. This restriction is applied since that object may not exist yet.
138 Correspondingly, during instantiation step the previosly allocated
140 object should be "filled", i.e., it can be changed in any way such that
141 the references to this object remain valid.
142 The methods are called like this:
144 $pre_object_ref = Package->Allocate($pre_pre_object_ref);
146 # Returns reference
147 Package->Instantiate($pre_object_ref,$cooky);
148 # Converts into reference to blessed object
149 The reverse operations are
151 $object_ref->FreezeEmpty($cooky);
153 $object_ref->FreezeInstance($cooky);
154 during these calls object can "freezeScalar" some information (in a
156 usual way) that will be used during "Allocate" and "Instantiate" calls
157 (via "thawScalar"). Note that the return value of "FreezeEmpty" is
158 cached during the phase of creation of uninialized objects. This must
159 be used like this: the return value is the reference to the created
160 object, so it is not destructed until other objects are created, thus
161 the frozen values of the different objects will not share the same
162 references. Example of bad result:
163 $o1->FreezeEmpty($cooky)
165 freezes "{}", and "$o2->FreezeEmpty($cooky)" makes the same. Now nobody
167 guaranties that that these two copies of "{}" are different, unless a
168 reference to the first one is preserved during the call to
169 "$o2->FreezeEmpty($cooky)". If "$o1->FreezeEmpty($cooky)" returns the
170 value of "{}" it uses, it will be preserved by the engine.
171 The helper function "FreezeThaw::copyContents" is provided for
173 simplification of instantiation. The syntax is
174 FreezeThaw::copyContents $to, $from;
176 The function copies contents the object $from point to into what the
178 object $to points to (including package for blessed references). Both
179 arguments should be references.
180 The default methods are provided. They do the following:
182 "FreezeEmpty"
184 Freezes an empty object of underlying type.
185 "FreezeInstance"
187 Calls "Freeze".
188 "Allocate" Thaws what was frozen by "FreezeEmpty".
190 "Instantiate"
192 Thaws what was frozen by "FreezeInstance", uses
193 "copyContents" to transfer this to the $pre_object.
194
196 A lot of objects are blessed in some obscure packages by XSUB typemaps.
197 It is not clear how to (automatically) prevent the "UNIVERSAL" methods
198 to be called for objects in these packages.
199 The objects which can survive freeze()/thaw() cycle must also survive a
201 change of a "member" to an equal member. Say, after
202 $a = [a => 3];
204 $a->{b} = \ $a->{a};
205 $a satisfies
207 $a->{b} == \ $a->{a}
209 This property will be broken by freeze()/thaw(), but it is also broken
211 by
212 $a->{a} = delete $a->{a};
214perl v5.34.0 2021-07-22 FreezeThaw(3)