phpMan > man > Data::Dumper(3pm)

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

SYNOPSIS
        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;
           ...
        $d->Purity(1)->Terse(1)->Deepcopy(1);
        eval $d->Dump;

DESCRIPTION
    Given a list of scalars or reference variables, writes out their contents in perl syntax. The
    references can also be objects. The content 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. (Please do consider the security implications of eval'ing code from untrusted
    sources!)

    Any references that are the same as one of those passed in will be named $VAR*n* (where *n* is a
    numeric suffix), and other duplicate references to substructures within $VAR*n* 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
    $VAR*n* 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 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 conveniently chained together.

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

  Methods
    *PACKAGE*->new(*ARRAYREF [*, *ARRAYREF]*)
        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 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"), 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 immediately.

    *$OBJ*->Seen(*[HASHREF]*)
        Queries or adds to the internal table of already encountered references. 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 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.

    *$OBJ*->Values(*[ARRAYREF]*)
        Queries or replaces the internal array of values that will be dumped. When called without
        arguments, returns the values as a list. When called with a reference to an array of
        replacement values, returns the object itself. When called with any other type of argument,
        dies.

    *$OBJ*->Names(*[ARRAYREF]*)
        Queries or replaces the internal array of user supplied names for the values that will be
        dumped. When called without arguments, returns the names. When called with an array of
        replacement names, returns the object itself. If the number of replacement names exceeds the
        number of values to be named, the excess names will not be used. If the number of
        replacement names falls short of the number of values to be named, the list of replacement
        names will be exhausted and remaining values will not be renamed. When called with any other
        type of argument, dies.

    *$OBJ*->Reset
        Clears the internal table of "seen" references and returns the object itself.

  Functions
    Dumper(*LIST*)
        Returns the stringified form of the values in the list, subject to the configuration options
        below. The values will be named $VAR*n* 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 nicely.

    *   $Data::Dumper::Indent *or* *$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 lines up the
        hash keys. 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.

    *   $Data::Dumper::Trailingcomma *or* *$OBJ*->Trailingcomma(*[NEWVAL]*)

        Controls whether a comma is added after the last element of an array or hash. Even when
        true, no comma is added between the last element of an array or hash and a closing bracket
        when they appear on the same line. The default is false.

    *   $Data::Dumper::Purity *or* *$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. The default is 0.

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

        When set, Data::Dumper will emit single, non-self-referential values as atoms/terms rather
        than statements. This means that the $VAR*n* 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 string.

        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 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.

    *   $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 defined 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 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.

    *   $Data::Dumper::Maxrecurse *or* $*OBJ*->Maxrecurse(*[NEWVAL]*)

        Can be set to a positive integer that specifies the depth beyond which recursion into a
        structure will throw an exception. This is intended as a security measure to prevent perl
        running out of stack space when dumping an excessively deep structure. Can be set to 0 to
        remove the limit. Default is 1000.

    *   $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 subroutine 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. In older versions, using this option imposed a significant performance penalty
        when dumping parts of a data structure other than code references, but that is no longer the
        case.

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

    *   $Data::Dumper::Sparseseen *or* $*OBJ*->Sparseseen(*[NEWVAL]*)

        By default, Data::Dumper builds up the "seen" hash of scalars that it has encountered during
        serialization. This is very expensive. This seen hash is necessary to support and even just
        detect circular references. It is exposed to the user via the "Seen()" call both for writing
        and reading.

        If you, as a user, do not need explicit access to the "seen" hash, then you can set the
        "Sparseseen" option to allow Data::Dumper to eschew building the "seen" hash for scalars
        that are known not to possess more than one reference. This speeds up serialization
        considerably if you use the XS implementation.

        Note: If you turn on "Sparseseen", then you must not rely on the content of the seen hash
        since its contents will be an implementation detail!

  Exports
    Dumper

EXAMPLES
    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
        $d->Indent(3);
        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 main;
        use Data::Dumper;
        $a = Foo->new;
        $b = Data::Dumper->new([$a], ['c']);
        $b->Freezer('Freeze');
        $b->Toaster('Thaw');
        $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)
            ];
        }

BUGS
    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. Even using the "Deparse" flag will in some cases produce results
    that behave differently after being passed to "eval"; see the documentation for B::Deparse.

    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.

  NOTE
    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 solution might be to use the "Sortkeys"
    filter of Data::Dumper.

AUTHOR
    Gurusamy Sarathy gsar AT activestate.com

    Copyright (c) 1996-2019 Gurusamy Sarathy. All rights reserved. This program is free software;
    you can redistribute it and/or modify it under the same terms as Perl itself.

VERSION
    Version 2.179

SEE ALSO
    perl(1)