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Data::Dumper(3)        Perl Programmers Reference Guide        Data::Dumper(3)

       Data::Dumper - stringified perl data structures, suitable for both printing and "eval"

           use Data::Dumper;

           # simple procedural interface
           print Dumper($foo, $bar);

           # extended usage with names
           print Data::Dumper->Dump([$foo, $bar], [qw(foo *ary)]);

           # configuration variables
             local $Data::Dumper::Purity = 1;
             eval Data::Dumper->Dump([$foo, $bar], [qw(foo *ary)]);

           # OO usage
           $d = Data::Dumper->new([$foo, $bar], [qw(foo *ary)]);
           print $d->Dump;
           eval $d->Dump;

       Given a list of scalars or reference variables, writes out their contents in perl syntax. The references can
       also be objects.  The contents of each variable is output in a single Perl statement.  Handles self-referential
       structures correctly.

       The return value can be "eval"ed to get back an identical copy of the original reference structure.

       Any references that are the same as one of those passed in will be named $VARn (where n is a numeric suffix),
       and other duplicate references to substructures within $VARn will be appropriately labeled using arrow nota-
       tion.  You can specify names for individual values to be dumped if you use the "Dump()" method, or you can
       change the default $VAR prefix to something else.  See $Data::Dumper::Varname and $Data::Dumper::Terse below.

       The default output of self-referential structures can be "eval"ed, but the nested references to $VARn will be
       undefined, since a recursive structure cannot be constructed using one Perl statement.  You should set the
       "Purity" flag to 1 to get additional statements that will correctly fill in these references.  Moreover, if
       "eval"ed when strictures are in effect, you need to ensure that any variables it accesses are previously

       In the extended usage form, the references to be dumped can be given user-specified names.  If a name begins
       with a "*", the output will describe the dereferenced type of the supplied reference for hashes and arrays, and
       coderefs.  Output of names will be avoided where possible if the "Terse" flag is set.

       In many cases, methods that are used to set the internal state of the object will return the object itself, so
       method calls can be conveniently chained together.

       Several styles of output are possible, all controlled by setting the "Indent" flag.  See "Configuration Vari-
       ables or Methods" below for details.


           Returns a newly created "Data::Dumper" object.  The first argument is an anonymous array of values to be
           dumped.  The optional second argument is an anonymous array of names for the values.  The names need not
           have a leading "$" sign, and must be comprised of alphanumeric characters.  You can begin a name with a "*"
           to specify that the dereferenced type must be dumped instead of the reference itself, for ARRAY and HASH

           The prefix specified by $Data::Dumper::Varname will be used with a numeric suffix if the name for a value
           is undefined.

           Data::Dumper will catalog all references encountered while dumping the values. Cross-references (in the
           form of names of substructures in perl syntax) will be inserted at all possible points, preserving any
           structural interdependencies in the original set of values.  Structure traversal is depth-first,  and pro-
           ceeds in order from the first supplied value to the last.

       $OBJ->Dump  or  PACKAGE->Dump(ARRAYREF [, ARRAYREF])
           Returns the stringified form of the values stored in the object (preserving the order in which they were
           supplied to "new"), subject to the configuration options below.  In a list context, it returns a list of
           strings corresponding to the supplied values.

           The second form, for convenience, simply calls the "new" method on its arguments before dumping the object

           Queries or adds to the internal table of already encountered references.  You must use "Reset" to explic-
           itly clear the table if needed.  Such references are not dumped; instead, their names are inserted wherever
           they are encountered subsequently.  This is useful especially for properly dumping subroutine references.

           Expects an anonymous hash of name => value pairs.  Same rules apply for names as in "new".  If no argument
           is supplied, will return the "seen" list of name => value pairs, in a list context.  Otherwise, returns the
           object itself.

           Queries or replaces the internal array of values that will be dumped.  When called without arguments,
           returns the values.  Otherwise, returns the object itself.

           Queries or replaces the internal array of user supplied names for the values that will be dumped.  When
           called without arguments, returns the names.  Otherwise, returns the object itself.

           Clears the internal table of "seen" references and returns the object itself.


           Returns the stringified form of the values in the list, subject to the configuration options below.  The
           values will be named $VARn in the output, where n is a numeric suffix.  Will return a list of strings in a
           list context.

       Configuration Variables or Methods

       Several configuration variables can be used to control the kind of output generated when using the procedural
       interface.  These variables are usually "local"ized in a block so that other parts of the code are not affected
       by the change.

       These variables determine the default state of the object created by calling the "new" method, but cannot be
       used to alter the state of the object thereafter.  The equivalent method names should be used instead to query
       or set the internal state of the object.

       The method forms return the object itself when called with arguments, so that they can be chained together

       ?   $Data::Dumper::Indent  or  $OBJ->Indent([NEWVAL])

           Controls the style of indentation.  It can be set to 0, 1, 2 or 3.  Style 0 spews output without any new-
           lines, indentation, or spaces between list items.  It is the most compact format possible that can still be
           called valid perl.  Style 1 outputs a readable form with newlines but no fancy indentation (each level in
           the structure is simply indented by a fixed amount of whitespace).  Style 2 (the default) outputs a very
           readable form which takes into account the length of hash keys (so the hash value lines up).  Style 3 is
           like style 2, but also annotates the elements of arrays with their index (but the comment is on its own
           line, so array output consumes twice the number of lines).  Style 2 is the default.

       ?   $Data::Dumper::Purity  or  $OBJ->Purity([NEWVAL])

           Controls the degree to which the output can be "eval"ed to recreate the supplied reference structures.
           Setting it to 1 will output additional perl statements that will correctly recreate nested references.  The
           default is 0.

       ?   $Data::Dumper::Pad  or  $OBJ->Pad([NEWVAL])

           Specifies the string that will be prefixed to every line of the output.  Empty string by default.

       ?   $Data::Dumper::Varname  or  $OBJ->Varname([NEWVAL])

           Contains the prefix to use for tagging variable names in the output. The default is "VAR".

       ?   $Data::Dumper::Useqq  or  $OBJ->Useqq([NEWVAL])

           When set, enables the use of double quotes for representing string values.  Whitespace other than space
           will be represented as "[\n\t\r]", "unsafe" characters will be backslashed, and unprintable characters will
           be output as quoted octal integers.  Since setting this variable imposes a performance penalty, the default
           is 0.  "Dump()" will run slower if this flag is set, since the fast XSUB implementation doesn't support it

       ?   $Data::Dumper::Terse  or  $OBJ->Terse([NEWVAL])

           When set, Data::Dumper will emit single, non-self-referential values as atoms/terms rather than statements.
           This means that the $VARn names will be avoided where possible, but be advised that such output may not
           always be parseable by "eval".

       ?   $Data::Dumper::Freezer  or  $OBJ->Freezer([NEWVAL])

           Can be set to a method name, or to an empty string to disable the feature.  Data::Dumper will invoke that
           method via the object before attempting to stringify it.  This method can alter the contents of the object
           (if, for instance, it contains data allocated from C), and even rebless it in a different package.  The
           client is responsible for making sure the specified method can be called via the object, and that the
           object ends up containing only perl data types after the method has been called.  Defaults to an empty

           If an object does not support the method specified (determined using UNIVERSAL::can()) then the call will
           be skipped.  If the method dies a warning will be generated.

       ?   $Data::Dumper::Toaster  or  $OBJ->Toaster([NEWVAL])

           Can be set to a method name, or to an empty string to disable the feature.  Data::Dumper will emit a method
           call for any objects that are to be dumped using the syntax "bless(DATA, CLASS)->METHOD()".  Note that this
           means that the method specified will have to perform any modifications required on the object (like creat-
           ing new state within it, and/or reblessing it in a different package) and then return it.  The client is
           responsible for making sure the method can be called via the object, and that it returns a valid object.
           Defaults to an empty string.

       ?   $Data::Dumper::Deepcopy  or  $OBJ->Deepcopy([NEWVAL])

           Can be set to a boolean value to enable deep copies of structures.  Cross-referencing will then only be
           done when absolutely essential (i.e., to break reference cycles).  Default is 0.

       ?   $Data::Dumper::Quotekeys  or  $OBJ->Quotekeys([NEWVAL])

           Can be set to a boolean value to control whether hash keys are quoted.  A false value will avoid quoting
           hash keys when it looks like a simple string.  Default is 1, which will always enclose hash keys in quotes.

       ?   $Data::Dumper::Bless  or  $OBJ->Bless([NEWVAL])

           Can be set to a string that specifies an alternative to the "bless" builtin operator used to create
           objects.  A function with the specified name should exist, and should accept the same arguments as the
           builtin.  Default is "bless".

       ?   $Data::Dumper::Pair  or  $OBJ->Pair([NEWVAL])

           Can be set to a string that specifies the separator between hash keys and values. To dump nested hash,
           array and scalar values to JavaScript, use: "$Data::Dumper::Pair = ' : ';". Implementing "bless" in
           JavaScript is left as an exercise for the reader.  A function with the specified name exists, and accepts
           the same arguments as the builtin.

           Default is: " => ".

       ?   $Data::Dumper::Maxdepth  or  $OBJ->Maxdepth([NEWVAL])

           Can be set to a positive integer that specifies the depth beyond which which we don't venture into a struc-
           ture.  Has no effect when "Data::Dumper::Purity" is set.  (Useful in debugger when we often don't want to
           see more than enough).  Default is 0, which means there is no maximum depth.

       ?   $Data::Dumper::Useperl  or  $OBJ->Useperl([NEWVAL])

           Can be set to a boolean value which controls whether the pure Perl implementation of "Data::Dumper" is
           used. The "Data::Dumper" module is a dual implementation, with almost all functionality written in both
           pure Perl and also in XS ('C'). Since the XS version is much faster, it will always be used if possible.
           This option lets you override the default behavior, usually for testing purposes only. Default is 0, which
           means the XS implementation will be used if possible.

       ?   $Data::Dumper::Sortkeys  or  $OBJ->Sortkeys([NEWVAL])

           Can be set to a boolean value to control whether hash keys are dumped in sorted order. A true value will
           cause the keys of all hashes to be dumped in Perl's default sort order. Can also be set to a subroutine
           reference which will be called for each hash that is dumped. In this case "Data::Dumper" will call the sub-
           routine once for each hash, passing it the reference of the hash. The purpose of the subroutine is to
           return a reference to an array of the keys that will be dumped, in the order that they should be dumped.
           Using this feature, you can control both the order of the keys, and which keys are actually used. In other
           words, this subroutine acts as a filter by which you can exclude certain keys from being dumped. Default is
           0, which means that hash keys are not sorted.

       ?   $Data::Dumper::Deparse  or  $OBJ->Deparse([NEWVAL])

           Can be set to a boolean value to control whether code references are turned into perl source code. If set
           to a true value, "B::Deparse" will be used to get the source of the code reference. Using this option will
           force using the Perl implementation of the dumper, since the fast XSUB implementation doesn't support it.

           Caution : use this option only if you know that your coderefs will be properly reconstructed by



       Run these code snippets to get a quick feel for the behavior of this module.  When you are through with these
       examples, you may want to add or change the various configuration variables described above, to see their
       behavior.  (See the testsuite in the Data::Dumper distribution for more examples.)

           use Data::Dumper;

           package Foo;
           sub new {bless {'a' => 1, 'b' => sub { return "foo" }}, $_[0]};

           package Fuz;                       # a weird REF-REF-SCALAR object
           sub new {bless \($_ = \ 'fu\'z'), $_[0]};

           package main;
           $foo = Foo->new;
           $fuz = Fuz->new;
           $boo = [ 1, [], "abcd", \*foo,
                    {1 => 'a', 023 => 'b', 0x45 => 'c'},
                    \\"p\q\'r", $foo, $fuz];

           # simple usage

           $bar = eval(Dumper($boo));
           print($@) if $@;
           print Dumper($boo), Dumper($bar);  # pretty print (no array indices)

           $Data::Dumper::Terse = 1;          # don't output names where feasible
           $Data::Dumper::Indent = 0;         # turn off all pretty print
           print Dumper($boo), "\n";

           $Data::Dumper::Indent = 1;         # mild pretty print
           print Dumper($boo);

           $Data::Dumper::Indent = 3;         # pretty print with array indices
           print Dumper($boo);

           $Data::Dumper::Useqq = 1;          # print strings in double quotes
           print Dumper($boo);

           $Data::Dumper::Pair = " : ";       # specify hash key/value separator
           print Dumper($boo);

           # recursive structures

           @c = ('c');
           $c = \@c;
           $b = {};
           $a = [1, $b, $c];
           $b->{a} = $a;
           $b->{b} = $a->[1];
           $b->{c} = $a->[2];
           print Data::Dumper->Dump([$a,$b,$c], [qw(a b c)]);

           $Data::Dumper::Purity = 1;         # fill in the holes for eval
           print Data::Dumper->Dump([$a, $b], [qw(*a b)]); # print as @a
           print Data::Dumper->Dump([$b, $a], [qw(*b a)]); # print as %b

           $Data::Dumper::Deepcopy = 1;       # avoid cross-refs
           print Data::Dumper->Dump([$b, $a], [qw(*b a)]);

           $Data::Dumper::Purity = 0;         # avoid cross-refs
           print Data::Dumper->Dump([$b, $a], [qw(*b a)]);

           # deep structures

           $a = "pearl";
           $b = [ $a ];
           $c = { 'b' => $b };
           $d = [ $c ];
           $e = { 'd' => $d };
           $f = { 'e' => $e };
           print Data::Dumper->Dump([$f], [qw(f)]);

           $Data::Dumper::Maxdepth = 3;       # no deeper than 3 refs down
           print Data::Dumper->Dump([$f], [qw(f)]);

           # object-oriented usage

           $d = Data::Dumper->new([$a,$b], [qw(a b)]);
           $d->Seen({'*c' => $c});            # stash a ref without printing it
           print $d->Dump;
           $d->Reset->Purity(0);              # empty the seen cache
           print join "----\n", $d->Dump;

           # persistence

           package Foo;
           sub new { bless { state => 'awake' }, shift }
           sub Freeze {
               my $s = shift;
               print STDERR "preparing to sleep\n";
               $s->{state} = 'asleep';
               return bless $s, 'Foo::ZZZ';

           package Foo::ZZZ;
           sub Thaw {
               my $s = shift;
               print STDERR "waking up\n";
               $s->{state} = 'awake';
               return bless $s, 'Foo';

           package Foo;
           use Data::Dumper;
           $a = Foo->new;
           $b = Data::Dumper->new([$a], ['c']);
           $c = $b->Dump;
           print $c;
           $d = eval $c;
           print Data::Dumper->Dump([$d], ['d']);

           # symbol substitution (useful for recreating CODE refs)

           sub foo { print "foo speaking\n" }
           *other = \&foo;
           $bar = [ \&other ];
           $d = Data::Dumper->new([\&other,$bar],['*other','bar']);
           $d->Seen({ '*foo' => \&foo });
           print $d->Dump;

           # sorting and filtering hash keys

           $Data::Dumper::Sortkeys = \&my_filter;
           my $foo = { map { (ord, "$_$_$_") } 'I'..'Q' };
           my $bar = { %$foo };
           my $baz = { reverse %$foo };
           print Dumper [ $foo, $bar, $baz ];

           sub my_filter {
               my ($hash) = @_;
               # return an array ref containing the hash keys to dump
               # in the order that you want them to be dumped
               return [
                 # Sort the keys of %$foo in reverse numeric order
                   $hash eq $foo ? (sort {$b <=> $a} keys %$hash) :
                 # Only dump the odd number keys of %$bar
                   $hash eq $bar ? (grep {$_ % 2} keys %$hash) :
                 # Sort keys in default order for all other hashes
                   (sort keys %$hash)

       Due to limitations of Perl subroutine call semantics, you cannot pass an array or hash.  Prepend it with a "\"
       to pass its reference instead.  This will be remedied in time, now that Perl has subroutine prototypes.  For
       now, you need to use the extended usage form, and prepend the name with a "*" to output it as a hash or array.

       "Data::Dumper" cheats with CODE references.  If a code reference is encountered in the structure being pro-
       cessed (and if you haven't set the "Deparse" flag), an anonymous subroutine that contains the string '"DUMMY"'
       will be inserted in its place, and a warning will be printed if "Purity" is set.  You can "eval" the result,
       but bear in mind that the anonymous sub that gets created is just a placeholder.  Someday, perl will have a
       switch to cache-on-demand the string representation of a compiled piece of code, I hope.  If you have prior
       knowledge of all the code refs that your data structures are likely to have, you can use the "Seen" method to
       pre-seed the internal reference table and make the dumped output point to them, instead.  See "EXAMPLES" above.

       The "Useqq" and "Deparse" flags makes Dump() run slower, since the XSUB implementation does not support them.

       SCALAR objects have the weirdest looking "bless" workaround.

       Pure Perl version of "Data::Dumper" escapes UTF-8 strings correctly only in Perl 5.8.0 and later.


       Starting from Perl 5.8.1 different runs of Perl will have different ordering of hash keys.  The change was done
       for greater security, see "Algorithmic Complexity Attacks" in perlsec.  This means that different runs of Perl
       will have different Data::Dumper outputs if the data contains hashes.  If you need to have identical
       Data::Dumper outputs from different runs of Perl, use the environment variable PERL_HASH_SEED, see
       "PERL_HASH_SEED" in perlrun.  Using this restores the old (platform-specific) ordering: an even prettier solu-
       tion might be to use the "Sortkeys" filter of Data::Dumper.

       Gurusamy Sarathy

       Copyright (c) 1996-98 Gurusamy Sarathy. All rights reserved.  This program is free software; you can redis-
       tribute it and/or modify it under the same terms as Perl itself.

       Version 2.121  (Aug 24 2003)


perl v5.8.8                       2001-09-21                   Data::Dumper(3)