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 con-
       tents 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 refer-
       ences to substructures within $VARn will be appropriately labeled using
       arrow notation.  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 cor-
       rectly fill in these references.  Moreover, if "eval"ed when strictures
       are in effect, you need to ensure that any variables it accesses are
       previously declared.

       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 conve-
       niently chained together.

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


           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 alphanu-
           meric 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 references.

           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 proceeds 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"), sub-
           ject 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 immediately.

           Queries or adds to the internal table of already encountered refer-
           ences.  You must use "Reset" to explicitly clear the table if
           needed.  Such references are not dumped; instead, their names are
           inserted wherever they are encountered subsequently.  This is use-
           ful 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.  Other-
           wise, 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 out-
       put 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 nicely.

       o   $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 newlines, 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.

       o   $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 ref-
           erences.  The default is 0.

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

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

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

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

       o   $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 unprint-
           able 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 yet.

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

           When set, Data::Dumper will emit single, non-self-referential val-
           ues 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".

       o   $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 con-
           tents 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

       o   $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 creating 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.

       o   $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.

       o   $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.

       o   $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 spec-
           ified name should exist, and should accept the same arguments as
           the builtin.  Default is "bless".

       o   $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 argu-
           ments as the builtin.

           Default is: " => ".

       o   $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 structure.  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.

       o   $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

       o   $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 subroutine
           once for each hash, passing it the reference of the hash. The pur-
           pose 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.

       o   $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 "B::Deparse".



       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 processed (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 con-
       tains 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 (plat-
       form-specific) ordering: an even prettier solution might be to use the
       "Sortkeys" filter of Data::Dumper.

       Gurusamy Sarathy

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

       Version 2.121  (Aug 24 2003)


perl v5.8.6                       2001-09-21                   Data::Dumper(3)