[ViewVC] Contents of: cvs/JSON-XS/README

NAME
    JSON::XS - JSON serialising/deserialising, done correctly and fast

SYNOPSIS
     use JSON::XS;

     # exported functions, croak on error

     $utf8_encoded_json_text = to_json $perl_hash_or_arrayref;
     $perl_hash_or_arrayref  = from_json $utf8_encoded_json_text;

     # oo-interface

     $coder = JSON::XS->new->ascii->pretty->allow_nonref;
     $pretty_printed_unencoded = $coder->encode ($perl_scalar);
     $perl_scalar = $coder->decode ($unicode_json_text);

DESCRIPTION
    This module converts Perl data structures to JSON and vice versa. Its
    primary goal is to be *correct* and its secondary goal is to be *fast*.
    To reach the latter goal it was written in C.

    As this is the n-th-something JSON module on CPAN, what was the reason
    to write yet another JSON module? While it seems there are many JSON
    modules, none of them correctly handle all corner cases, and in most
    cases their maintainers are unresponsive, gone missing, or not listening
    to bug reports for other reasons.

    See COMPARISON, below, for a comparison to some other JSON modules.

    See MAPPING, below, on how JSON::XS maps perl values to JSON values and
    vice versa.

  FEATURES
    * correct handling of unicode issues
        This module knows how to handle Unicode, and even documents how and
        when it does so.

    * round-trip integrity
        When you serialise a perl data structure using only datatypes
        supported by JSON, the deserialised data structure is identical on
        the Perl level. (e.g. the string "2.0" doesn't suddenly become "2").

    * strict checking of JSON correctness
        There is no guessing, no generating of illegal JSON strings by
        default, and only JSON is accepted as input by default (the latter
        is a security feature).

    * fast
        Compared to other JSON modules, this module compares favourably in
        terms of speed, too.

    * simple to use
        This module has both a simple functional interface as well as an OO
        interface.

    * reasonably versatile output formats
        You can choose between the most compact guarenteed single-line
        format possible (nice for simple line-based protocols), a pure-ascii
        format (for when your transport is not 8-bit clean), or a
        pretty-printed format (for when you want to read that stuff). Or you
        can combine those features in whatever way you like.

FUNCTIONAL INTERFACE
    The following convinience methods are provided by this module. They are
    exported by default:

    $json_string = to_json $perl_scalar
        Converts the given Perl data structure (a simple scalar or a
        reference to a hash or array) to a UTF-8 encoded, binary string
        (that is, the string contains octets only). Croaks on error.

        This function call is functionally identical to
        "JSON::XS->new->utf8->encode ($perl_scalar)".

    $perl_scalar = from_json $json_string
        The opposite of "to_json": expects an UTF-8 (binary) string and
        tries to parse that as an UTF-8 encoded JSON string, returning the
        resulting simple scalar or reference. Croaks on error.

        This function call is functionally identical to
        "JSON::XS->new->utf8->decode ($json_string)".

OBJECT-ORIENTED INTERFACE
    The object oriented interface lets you configure your own encoding or
    decoding style, within the limits of supported formats.

    $json = new JSON::XS
        Creates a new JSON::XS object that can be used to de/encode JSON
        strings. All boolean flags described below are by default
        *disabled*.

        The mutators for flags all return the JSON object again and thus
        calls can be chained:

           my $json = JSON::XS->new->utf8(1)->space_after(1)->encode ({a => [1,2]})
           => {"a": [1, 2]}

    $json = $json->ascii ([$enable])
        If $enable is true (or missing), then the "encode" method will not
        generate characters outside the code range 0..127. Any unicode
        characters outside that range will be escaped using either a single
        \uXXXX (BMP characters) or a double \uHHHH\uLLLLL escape sequence,
        as per RFC4627.

        If $enable is false, then the "encode" method will not escape
        Unicode characters unless necessary.

          JSON::XS->new->ascii (1)->encode (chr 0x10401)
          => \ud801\udc01

    $json = $json->utf8 ([$enable])
        If $enable is true (or missing), then the "encode" method will
        encode the JSON string into UTF-8, as required by many protocols,
        while the "decode" method expects to be handled an UTF-8-encoded
        string. Please note that UTF-8-encoded strings do not contain any
        characters outside the range 0..255, they are thus useful for
        bytewise/binary I/O.

        If $enable is false, then the "encode" method will return the JSON
        string as a (non-encoded) unicode string, while "decode" expects
        thus a unicode string. Any decoding or encoding (e.g. to UTF-8 or
        UTF-16) needs to be done yourself, e.g. using the Encode module.

        Example, output UTF-16-encoded JSON:

    $json = $json->pretty ([$enable])
        This enables (or disables) all of the "indent", "space_before" and
        "space_after" (and in the future possibly more) flags in one call to
        generate the most readable (or most compact) form possible.

        Example, pretty-print some simple structure:

           my $json = JSON::XS->new->pretty(1)->encode ({a => [1,2]})
           =>
           {
              "a" : [
                 1,
                 2
              ]
           }

    $json = $json->indent ([$enable])
        If $enable is true (or missing), then the "encode" method will use a
        multiline format as output, putting every array member or
        object/hash key-value pair into its own line, identing them
        properly.

        If $enable is false, no newlines or indenting will be produced, and
        the resulting JSON strings is guarenteed not to contain any
        "newlines".

        This setting has no effect when decoding JSON strings.

    $json = $json->space_before ([$enable])
        If $enable is true (or missing), then the "encode" method will add
        an extra optional space before the ":" separating keys from values
        in JSON objects.

        If $enable is false, then the "encode" method will not add any extra
        space at those places.

        This setting has no effect when decoding JSON strings. You will also
        most likely combine this setting with "space_after".

        Example, space_before enabled, space_after and indent disabled:

           {"key" :"value"}

    $json = $json->space_after ([$enable])
        If $enable is true (or missing), then the "encode" method will add
        an extra optional space after the ":" separating keys from values in
        JSON objects and extra whitespace after the "," separating key-value
        pairs and array members.

        If $enable is false, then the "encode" method will not add any extra
        space at those places.

        This setting has no effect when decoding JSON strings.

        Example, space_before and indent disabled, space_after enabled:

           {"key": "value"}

    $json = $json->canonical ([$enable])
        If $enable is true (or missing), then the "encode" method will
        output JSON objects by sorting their keys. This is adding a
        comparatively high overhead.

        If $enable is false, then the "encode" method will output key-value
        pairs in the order Perl stores them (which will likely change
        between runs of the same script).

        This option is useful if you want the same data structure to be
        encoded as the same JSON string (given the same overall settings).
        If it is disabled, the same hash migh be encoded differently even if
        contains the same data, as key-value pairs have no inherent ordering
        in Perl.

        This setting has no effect when decoding JSON strings.

    $json = $json->allow_nonref ([$enable])
        If $enable is true (or missing), then the "encode" method can
        convert a non-reference into its corresponding string, number or
        null JSON value, which is an extension to RFC4627. Likewise,
        "decode" will accept those JSON values instead of croaking.

        If $enable is false, then the "encode" method will croak if it isn't
        passed an arrayref or hashref, as JSON strings must either be an
        object or array. Likewise, "decode" will croak if given something
        that is not a JSON object or array.

        Example, encode a Perl scalar as JSON value with enabled
        "allow_nonref", resulting in an invalid JSON text:

           JSON::XS->new->allow_nonref->encode ("Hello, World!")
           => "Hello, World!"

    $json = $json->shrink ([$enable])
        Perl usually over-allocates memory a bit when allocating space for
        strings. This flag optionally resizes strings generated by either
        "encode" or "decode" to their minimum size possible. This can save
        memory when your JSON strings are either very very long or you have
        many short strings. It will also try to downgrade any strings to
        octet-form if possible: perl stores strings internally either in an
        encoding called UTF-X or in octet-form. The latter cannot store
        everything but uses less space in general.

        If $enable is true (or missing), the string returned by "encode"
        will be shrunk-to-fit, while all strings generated by "decode" will
        also be shrunk-to-fit.

        If $enable is false, then the normal perl allocation algorithms are
        used. If you work with your data, then this is likely to be faster.

        In the future, this setting might control other things, such as
        converting strings that look like integers or floats into integers
        or floats internally (there is no difference on the Perl level),
        saving space.

    $json_string = $json->encode ($perl_scalar)
        Converts the given Perl data structure (a simple scalar or a
        reference to a hash or array) to its JSON representation. Simple
        scalars will be converted into JSON string or number sequences,
        while references to arrays become JSON arrays and references to
        hashes become JSON objects. Undefined Perl values (e.g. "undef")
        become JSON "null" values. Neither "true" nor "false" values will be
        generated.

    $perl_scalar = $json->decode ($json_string)
        The opposite of "encode": expects a JSON string and tries to parse
        it, returning the resulting simple scalar or reference. Croaks on
        error.

        JSON numbers and strings become simple Perl scalars. JSON arrays
        become Perl arrayrefs and JSON objects become Perl hashrefs. "true"
        becomes 1, "false" becomes 0 and "null" becomes "undef".

MAPPING
    This section describes how JSON::XS maps Perl values to JSON values and
    vice versa. These mappings are designed to "do the right thing" in most
    circumstances automatically, preserving round-tripping characteristics
    (what you put in comes out as something equivalent).

    For the more enlightened: note that in the following descriptions,
    lowercase *perl* refers to the Perl interpreter, while uppcercase *Perl*
    refers to the abstract Perl language itself.

  JSON -> PERL
    object
        A JSON object becomes a reference to a hash in Perl. No ordering of
        object keys is preserved.

    array
        A JSON array becomes a reference to an array in Perl.

    string
        A JSON string becomes a string scalar in Perl - Unicode codepoints
        in JSON are represented by the same codepoints in the Perl string,
        so no manual decoding is necessary.

    number
        A JSON number becomes either an integer or numeric (floating point)
        scalar in perl, depending on its range and any fractional parts. On
        the Perl level, there is no difference between those as Perl handles
        all the conversion details, but an integer may take slightly less
        memory and might represent more values exactly than (floating point)
        numbers.

    true, false
        These JSON atoms become 0, 1, respectively. Information is lost in
        this process. Future versions might represent those values
        differently, but they will be guarenteed to act like these integers
        would normally in Perl.

    null
        A JSON null atom becomes "undef" in Perl.

  PERL -> JSON
    The mapping from Perl to JSON is slightly more difficult, as Perl is a
    truly typeless language, so we can only guess which JSON type is meant
    by a Perl value.

    hash references
        Perl hash references become JSON objects. As there is no inherent
        ordering in hash keys, they will usually be encoded in a
        pseudo-random order that can change between runs of the same program
        but stays generally the same within the single run of a program.
        JSON::XS can optionally sort the hash keys (determined by the
        *canonical* flag), so the same datastructure will serialise to the
        same JSON text (given same settings and version of JSON::XS), but
        this incurs a runtime overhead.

    array references
        Perl array references become JSON arrays.

    blessed objects
        Blessed objects are not allowed. JSON::XS currently tries to encode
        their underlying representation (hash- or arrayref), but this
        behaviour might change in future versions.

    simple scalars
        Simple Perl scalars (any scalar that is not a reference) are the
        most difficult objects to encode: JSON::XS will encode undefined
        scalars as JSON null value, scalars that have last been used in a
        string context before encoding as JSON strings and anything else as
        number value:

           # dump as number
           to_json [2]                      # yields [2]
           to_json [-3.0e17]                # yields [-3e+17]
           my $value = 5; to_json [$value]  # yields [5]

           # used as string, so dump as string
           print $value;
           to_json [$value]                 # yields ["5"]

           # undef becomes null
           to_json [undef]                  # yields [null]

        You can force the type to be a string by stringifying it:

           my $x = 3.1; # some variable containing a number
           "$x";        # stringified
           $x .= "";    # another, more awkward way to stringify
           print $x;    # perl does it for you, too, quite often

        You can force the type to be a number by numifying it:

           my $x = "3"; # some variable containing a string
           $x += 0;     # numify it, ensuring it will be dumped as a number
           $x *= 1;     # same thing, the choise is yours.

        You can not currently output JSON booleans or force the type in
        other, less obscure, ways. Tell me if you need this capability.

    circular data structures
        Those will be encoded until memory or stackspace runs out.

COMPARISON
    As already mentioned, this module was created because none of the
    existing JSON modules could be made to work correctly. First I will
    describe the problems (or pleasures) I encountered with various existing
    JSON modules, followed by some benchmark values. JSON::XS was designed
    not to suffer from any of these problems or limitations.

    JSON 1.07
        Slow (but very portable, as it is written in pure Perl).

        Undocumented/buggy Unicode handling (how JSON handles unicode values
        is undocumented. One can get far by feeding it unicode strings and
        doing en-/decoding oneself, but unicode escapes are not working
        properly).

        No roundtripping (strings get clobbered if they look like numbers,
        e.g. the string 2.0 will encode to 2.0 instead of "2.0", and that
        will decode into the number 2.

    JSON::PC 0.01
        Very fast.

        Undocumented/buggy Unicode handling.

        No roundtripping.

        Has problems handling many Perl values (e.g. regex results and other
        magic values will make it croak).

        Does not even generate valid JSON ("{1,2}" gets converted to "{1:2}"
        which is not a valid JSON string.

        Unmaintained (maintainer unresponsive for many months, bugs are not
        getting fixed).

    JSON::Syck 0.21
        Very buggy (often crashes).

        Very inflexible (no human-readable format supported, format pretty
        much undocumented. I need at least a format for easy reading by
        humans and a single-line compact format for use in a protocol, and
        preferably a way to generate ASCII-only JSON strings).

        Completely broken (and confusingly documented) Unicode handling
        (unicode escapes are not working properly, you need to set
        ImplicitUnicode to *different* values on en- and decoding to get
        symmetric behaviour).

        No roundtripping (simple cases work, but this depends on wether the
        scalar value was used in a numeric context or not).

        Dumping hashes may skip hash values depending on iterator state.

        Unmaintained (maintainer unresponsive for many months, bugs are not
        getting fixed).

        Does not check input for validity (i.e. will accept non-JSON input
        and return "something" instead of raising an exception. This is a
        security issue: imagine two banks transfering money between each
        other using JSON. One bank might parse a given non-JSON request and
        deduct money, while the other might reject the transaction with a
        syntax error. While a good protocol will at least recover, that is
        extra unnecessary work and the transaction will still not succeed).

    JSON::DWIW 0.04
        Very fast. Very natural. Very nice.

        Undocumented unicode handling (but the best of the pack. Unicode
        escapes still don't get parsed properly).

        Very inflexible.

        No roundtripping.

        Does not generate valid JSON (key strings are often unquoted, empty
        keys result in nothing being output)

        Does not check input for validity.

  SPEED
    It seems that JSON::XS is surprisingly fast, as shown in the following
    tables. They have been generated with the help of the "eg/bench" program
    in the JSON::XS distribution, to make it easy to compare on your own
    system.

    First is a comparison between various modules using a very simple JSON
    string, showing the number of encodes/decodes per second (JSON::XS is
    the functional interface, while JSON::XS/2 is the OO interface with
    pretty-printing and hashkey sorting enabled).

       module     |     encode |     decode |
       -----------|------------|------------|
       JSON       |      14006 |       6820 |
       JSON::DWIW |     200937 |     120386 |
       JSON::PC   |      85065 |     129366 |
       JSON::Syck |      59898 |      44232 |
       JSON::XS   |    1171478 |     342435 |
       JSON::XS/2 |     730760 |     328714 |
       -----------+------------+------------+

    That is, JSON::XS is 6 times faster than than JSON::DWIW and about 80
    times faster than JSON, even with pretty-printing and key sorting.

    Using a longer test string (roughly 8KB, generated from Yahoo! Locals
    search API (http://nanoref.com/yahooapis/mgPdGg):

       module     |     encode |     decode |
       -----------|------------|------------|
       JSON       |        673 |         38 |
       JSON::DWIW |       5271 |        770 |
       JSON::PC   |       9901 |       2491 |
       JSON::Syck |       2360 |        786 |
       JSON::XS   |      37398 |       3202 |
       JSON::XS/2 |      13765 |       3153 |
       -----------+------------+------------+

    Again, JSON::XS leads by far in the encoding case, while still beating
    every other module in the decoding case.

RESOURCE LIMITS
    JSON::XS does not impose any limits on the size of JSON texts or Perl
    values they represent - if your machine can handle it, JSON::XS will
    encode or decode it. Future versions might optionally impose structure
    depth and memory use resource limits.

BUGS
    While the goal of this module is to be correct, that unfortunately does
    not mean its bug-free, only that I think its design is bug-free. It is
    still very young and not well-tested. If you keep reporting bugs they
    will be fixed swiftly, though.

AUTHOR
     Marc Lehmann <schmorp@schmorp.de>
     http://home.schmorp.de/

Revision:	1.4
Committed:	Fri Mar 23 18:33:50 2007 UTC (17 years, 1 month ago) by root
Branch:	MAIN
CVS Tags:	rel-0_3
Changes since 1.3:	+211 -48 lines
Log Message:	* empty log message *
#	Content
1	NAME
2	JSON::XS - JSON serialising/deserialising, done correctly and fast
3
4	SYNOPSIS
5	use JSON::XS;
6
7	# exported functions, croak on error
8
9	$utf8_encoded_json_text = to_json $perl_hash_or_arrayref;
10	$perl_hash_or_arrayref = from_json $utf8_encoded_json_text;
11
12	# oo-interface
13
14	$coder = JSON::XS->new->ascii->pretty->allow_nonref;
15	$pretty_printed_unencoded = $coder->encode ($perl_scalar);
16	$perl_scalar = $coder->decode ($unicode_json_text);
17
18	DESCRIPTION
19	This module converts Perl data structures to JSON and vice versa. Its
20	primary goal is to be correct and its secondary goal is to be fast.
21	To reach the latter goal it was written in C.
22
23	As this is the n-th-something JSON module on CPAN, what was the reason
24	to write yet another JSON module? While it seems there are many JSON
25	modules, none of them correctly handle all corner cases, and in most
26	cases their maintainers are unresponsive, gone missing, or not listening
27	to bug reports for other reasons.
28
29	See COMPARISON, below, for a comparison to some other JSON modules.
30
31	See MAPPING, below, on how JSON::XS maps perl values to JSON values and
32	vice versa.
33
34	FEATURES
35	* correct handling of unicode issues
36	This module knows how to handle Unicode, and even documents how and
37	when it does so.
38
39	* round-trip integrity
40	When you serialise a perl data structure using only datatypes
41	supported by JSON, the deserialised data structure is identical on
42	the Perl level. (e.g. the string "2.0" doesn't suddenly become "2").
43
44	* strict checking of JSON correctness
45	There is no guessing, no generating of illegal JSON strings by
46	default, and only JSON is accepted as input by default (the latter
47	is a security feature).
48
49	* fast
50	Compared to other JSON modules, this module compares favourably in
51	terms of speed, too.
52
53	* simple to use
54	This module has both a simple functional interface as well as an OO
55	interface.
56
57	* reasonably versatile output formats
58	You can choose between the most compact guarenteed single-line
59	format possible (nice for simple line-based protocols), a pure-ascii
60	format (for when your transport is not 8-bit clean), or a
61	pretty-printed format (for when you want to read that stuff). Or you
62	can combine those features in whatever way you like.
63
64	FUNCTIONAL INTERFACE
65	The following convinience methods are provided by this module. They are
66	exported by default:
67
68	$json_string = to_json $perl_scalar
69	Converts the given Perl data structure (a simple scalar or a
70	reference to a hash or array) to a UTF-8 encoded, binary string
71	(that is, the string contains octets only). Croaks on error.
72
73	This function call is functionally identical to
74	"JSON::XS->new->utf8->encode ($perl_scalar)".
75
76	$perl_scalar = from_json $json_string
77	The opposite of "to_json": expects an UTF-8 (binary) string and
78	tries to parse that as an UTF-8 encoded JSON string, returning the
79	resulting simple scalar or reference. Croaks on error.
80
81	This function call is functionally identical to
82	"JSON::XS->new->utf8->decode ($json_string)".
83
84	OBJECT-ORIENTED INTERFACE
85	The object oriented interface lets you configure your own encoding or
86	decoding style, within the limits of supported formats.
87
88	$json = new JSON::XS
89	Creates a new JSON::XS object that can be used to de/encode JSON
90	strings. All boolean flags described below are by default
91	disabled.
92
93	The mutators for flags all return the JSON object again and thus
94	calls can be chained:
95
96	my $json = JSON::XS->new->utf8(1)->space_after(1)->encode ({a => [1,2]})
97	=> {"a": [1, 2]}
98
99	$json = $json->ascii ([$enable])
100	If $enable is true (or missing), then the "encode" method will not
101	generate characters outside the code range 0..127. Any unicode
102	characters outside that range will be escaped using either a single
103	\uXXXX (BMP characters) or a double \uHHHH\uLLLLL escape sequence,
104	as per RFC4627.
105
106	If $enable is false, then the "encode" method will not escape
107	Unicode characters unless necessary.
108
109	JSON::XS->new->ascii (1)->encode (chr 0x10401)
110	=> \ud801\udc01
111
112	$json = $json->utf8 ([$enable])
113	If $enable is true (or missing), then the "encode" method will
114	encode the JSON string into UTF-8, as required by many protocols,
115	while the "decode" method expects to be handled an UTF-8-encoded
116	string. Please note that UTF-8-encoded strings do not contain any
117	characters outside the range 0..255, they are thus useful for
118	bytewise/binary I/O.
119
120	If $enable is false, then the "encode" method will return the JSON
121	string as a (non-encoded) unicode string, while "decode" expects
122	thus a unicode string. Any decoding or encoding (e.g. to UTF-8 or
123	UTF-16) needs to be done yourself, e.g. using the Encode module.
124
125	Example, output UTF-16-encoded JSON:
126
127	$json = $json->pretty ([$enable])
128	This enables (or disables) all of the "indent", "space_before" and
129	"space_after" (and in the future possibly more) flags in one call to
130	generate the most readable (or most compact) form possible.
131
132	Example, pretty-print some simple structure:
133
134	my $json = JSON::XS->new->pretty(1)->encode ({a => [1,2]})
135	=>
136	{
137	"a" : [
138	1,
139	2
140	]
141	}
142
143	$json = $json->indent ([$enable])
144	If $enable is true (or missing), then the "encode" method will use a
145	multiline format as output, putting every array member or
146	object/hash key-value pair into its own line, identing them
147	properly.
148
149	If $enable is false, no newlines or indenting will be produced, and
150	the resulting JSON strings is guarenteed not to contain any
151	"newlines".
152
153	This setting has no effect when decoding JSON strings.
154
155	$json = $json->space_before ([$enable])
156	If $enable is true (or missing), then the "encode" method will add
157	an extra optional space before the ":" separating keys from values
158	in JSON objects.
159
160	If $enable is false, then the "encode" method will not add any extra
161	space at those places.
162
163	This setting has no effect when decoding JSON strings. You will also
164	most likely combine this setting with "space_after".
165
166	Example, space_before enabled, space_after and indent disabled:
167
168	{"key" :"value"}
169
170	$json = $json->space_after ([$enable])
171	If $enable is true (or missing), then the "encode" method will add
172	an extra optional space after the ":" separating keys from values in
173	JSON objects and extra whitespace after the "," separating key-value
174	pairs and array members.
175
176	If $enable is false, then the "encode" method will not add any extra
177	space at those places.
178
179	This setting has no effect when decoding JSON strings.
180
181	Example, space_before and indent disabled, space_after enabled:
182
183	{"key": "value"}
184
185	$json = $json->canonical ([$enable])
186	If $enable is true (or missing), then the "encode" method will
187	output JSON objects by sorting their keys. This is adding a
188	comparatively high overhead.
189
190	If $enable is false, then the "encode" method will output key-value
191	pairs in the order Perl stores them (which will likely change
192	between runs of the same script).
193
194	This option is useful if you want the same data structure to be
195	encoded as the same JSON string (given the same overall settings).
196	If it is disabled, the same hash migh be encoded differently even if
197	contains the same data, as key-value pairs have no inherent ordering
198	in Perl.
199
200	This setting has no effect when decoding JSON strings.
201
202	$json = $json->allow_nonref ([$enable])
203	If $enable is true (or missing), then the "encode" method can
204	convert a non-reference into its corresponding string, number or
205	null JSON value, which is an extension to RFC4627. Likewise,
206	"decode" will accept those JSON values instead of croaking.
207
208	If $enable is false, then the "encode" method will croak if it isn't
209	passed an arrayref or hashref, as JSON strings must either be an
210	object or array. Likewise, "decode" will croak if given something
211	that is not a JSON object or array.
212
213	Example, encode a Perl scalar as JSON value with enabled
214	"allow_nonref", resulting in an invalid JSON text:
215
216	JSON::XS->new->allow_nonref->encode ("Hello, World!")
217	=> "Hello, World!"
218
219	$json = $json->shrink ([$enable])
220	Perl usually over-allocates memory a bit when allocating space for
221	strings. This flag optionally resizes strings generated by either
222	"encode" or "decode" to their minimum size possible. This can save
223	memory when your JSON strings are either very very long or you have
224	many short strings. It will also try to downgrade any strings to
225	octet-form if possible: perl stores strings internally either in an
226	encoding called UTF-X or in octet-form. The latter cannot store
227	everything but uses less space in general.
228
229	If $enable is true (or missing), the string returned by "encode"
230	will be shrunk-to-fit, while all strings generated by "decode" will
231	also be shrunk-to-fit.
232
233	If $enable is false, then the normal perl allocation algorithms are
234	used. If you work with your data, then this is likely to be faster.
235
236	In the future, this setting might control other things, such as
237	converting strings that look like integers or floats into integers
238	or floats internally (there is no difference on the Perl level),
239	saving space.
240
241	$json_string = $json->encode ($perl_scalar)
242	Converts the given Perl data structure (a simple scalar or a
243	reference to a hash or array) to its JSON representation. Simple
244	scalars will be converted into JSON string or number sequences,
245	while references to arrays become JSON arrays and references to
246	hashes become JSON objects. Undefined Perl values (e.g. "undef")
247	become JSON "null" values. Neither "true" nor "false" values will be
248	generated.
249
250	$perl_scalar = $json->decode ($json_string)
251	The opposite of "encode": expects a JSON string and tries to parse
252	it, returning the resulting simple scalar or reference. Croaks on
253	error.
254
255	JSON numbers and strings become simple Perl scalars. JSON arrays
256	become Perl arrayrefs and JSON objects become Perl hashrefs. "true"
257	becomes 1, "false" becomes 0 and "null" becomes "undef".
258
259	MAPPING
260	This section describes how JSON::XS maps Perl values to JSON values and
261	vice versa. These mappings are designed to "do the right thing" in most
262	circumstances automatically, preserving round-tripping characteristics
263	(what you put in comes out as something equivalent).
264
265	For the more enlightened: note that in the following descriptions,
266	lowercase perl refers to the Perl interpreter, while uppcercase Perl
267	refers to the abstract Perl language itself.
268
269	JSON -> PERL
270	object
271	A JSON object becomes a reference to a hash in Perl. No ordering of
272	object keys is preserved.
273
274	array
275	A JSON array becomes a reference to an array in Perl.
276
277	string
278	A JSON string becomes a string scalar in Perl - Unicode codepoints
279	in JSON are represented by the same codepoints in the Perl string,
280	so no manual decoding is necessary.
281
282	number
283	A JSON number becomes either an integer or numeric (floating point)
284	scalar in perl, depending on its range and any fractional parts. On
285	the Perl level, there is no difference between those as Perl handles
286	all the conversion details, but an integer may take slightly less
287	memory and might represent more values exactly than (floating point)
288	numbers.
289
290	true, false
291	These JSON atoms become 0, 1, respectively. Information is lost in
292	this process. Future versions might represent those values
293	differently, but they will be guarenteed to act like these integers
294	would normally in Perl.
295
296	null
297	A JSON null atom becomes "undef" in Perl.
298
299	PERL -> JSON
300	The mapping from Perl to JSON is slightly more difficult, as Perl is a
301	truly typeless language, so we can only guess which JSON type is meant
302	by a Perl value.
303
304	hash references
305	Perl hash references become JSON objects. As there is no inherent
306	ordering in hash keys, they will usually be encoded in a
307	pseudo-random order that can change between runs of the same program
308	but stays generally the same within the single run of a program.
309	JSON::XS can optionally sort the hash keys (determined by the
310	canonical flag), so the same datastructure will serialise to the
311	same JSON text (given same settings and version of JSON::XS), but
312	this incurs a runtime overhead.
313
314	array references
315	Perl array references become JSON arrays.
316
317	blessed objects
318	Blessed objects are not allowed. JSON::XS currently tries to encode
319	their underlying representation (hash- or arrayref), but this
320	behaviour might change in future versions.
321
322	simple scalars
323	Simple Perl scalars (any scalar that is not a reference) are the
324	most difficult objects to encode: JSON::XS will encode undefined
325	scalars as JSON null value, scalars that have last been used in a
326	string context before encoding as JSON strings and anything else as
327	number value:
328
329	# dump as number
330	to_json [2] # yields [2]
331	to_json [-3.0e17] # yields [-3e+17]
332	my $value = 5; to_json [$value] # yields [5]
333
334	# used as string, so dump as string
335	print $value;
336	to_json [$value] # yields ["5"]
337
338	# undef becomes null
339	to_json [undef] # yields [null]
340
341	You can force the type to be a string by stringifying it:
342
343	my $x = 3.1; # some variable containing a number
344	"$x"; # stringified
345	$x .= ""; # another, more awkward way to stringify
346	print $x; # perl does it for you, too, quite often
347
348	You can force the type to be a number by numifying it:
349
350	my $x = "3"; # some variable containing a string
351	$x += 0; # numify it, ensuring it will be dumped as a number
352	$x *= 1; # same thing, the choise is yours.
353
354	You can not currently output JSON booleans or force the type in
355	other, less obscure, ways. Tell me if you need this capability.
356
357	circular data structures
358	Those will be encoded until memory or stackspace runs out.
359
360	COMPARISON
361	As already mentioned, this module was created because none of the
362	existing JSON modules could be made to work correctly. First I will
363	describe the problems (or pleasures) I encountered with various existing
364	JSON modules, followed by some benchmark values. JSON::XS was designed
365	not to suffer from any of these problems or limitations.
366
367	JSON 1.07
368	Slow (but very portable, as it is written in pure Perl).
369
370	Undocumented/buggy Unicode handling (how JSON handles unicode values
371	is undocumented. One can get far by feeding it unicode strings and
372	doing en-/decoding oneself, but unicode escapes are not working
373	properly).
374
375	No roundtripping (strings get clobbered if they look like numbers,
376	e.g. the string 2.0 will encode to 2.0 instead of "2.0", and that
377	will decode into the number 2.
378
379	JSON::PC 0.01
380	Very fast.
381
382	Undocumented/buggy Unicode handling.
383
384	No roundtripping.
385
386	Has problems handling many Perl values (e.g. regex results and other
387	magic values will make it croak).
388
389	Does not even generate valid JSON ("{1,2}" gets converted to "{1:2}"
390	which is not a valid JSON string.
391
392	Unmaintained (maintainer unresponsive for many months, bugs are not
393	getting fixed).
394
395	JSON::Syck 0.21
396	Very buggy (often crashes).
397
398	Very inflexible (no human-readable format supported, format pretty
399	much undocumented. I need at least a format for easy reading by
400	humans and a single-line compact format for use in a protocol, and
401	preferably a way to generate ASCII-only JSON strings).
402
403	Completely broken (and confusingly documented) Unicode handling
404	(unicode escapes are not working properly, you need to set
405	ImplicitUnicode to different values on en- and decoding to get
406	symmetric behaviour).
407
408	No roundtripping (simple cases work, but this depends on wether the
409	scalar value was used in a numeric context or not).
410
411	Dumping hashes may skip hash values depending on iterator state.
412
413	Unmaintained (maintainer unresponsive for many months, bugs are not
414	getting fixed).
415
416	Does not check input for validity (i.e. will accept non-JSON input
417	and return "something" instead of raising an exception. This is a
418	security issue: imagine two banks transfering money between each
419	other using JSON. One bank might parse a given non-JSON request and
420	deduct money, while the other might reject the transaction with a
421	syntax error. While a good protocol will at least recover, that is
422	extra unnecessary work and the transaction will still not succeed).
423
424	JSON::DWIW 0.04
425	Very fast. Very natural. Very nice.
426
427	Undocumented unicode handling (but the best of the pack. Unicode
428	escapes still don't get parsed properly).
429
430	Very inflexible.
431
432	No roundtripping.
433
434	Does not generate valid JSON (key strings are often unquoted, empty
435	keys result in nothing being output)
436
437	Does not check input for validity.
438
439	SPEED
440	It seems that JSON::XS is surprisingly fast, as shown in the following
441	tables. They have been generated with the help of the "eg/bench" program
442	in the JSON::XS distribution, to make it easy to compare on your own
443	system.
444
445	First is a comparison between various modules using a very simple JSON
446	string, showing the number of encodes/decodes per second (JSON::XS is
447	the functional interface, while JSON::XS/2 is the OO interface with
448	pretty-printing and hashkey sorting enabled).
449
450	module \| encode \| decode \|
451	-----------\|------------\|------------\|
452	JSON \| 14006 \| 6820 \|
453	JSON::DWIW \| 200937 \| 120386 \|
454	JSON::PC \| 85065 \| 129366 \|
455	JSON::Syck \| 59898 \| 44232 \|
456	JSON::XS \| 1171478 \| 342435 \|
457	JSON::XS/2 \| 730760 \| 328714 \|
458	-----------+------------+------------+
459
460	That is, JSON::XS is 6 times faster than than JSON::DWIW and about 80
461	times faster than JSON, even with pretty-printing and key sorting.
462
463	Using a longer test string (roughly 8KB, generated from Yahoo! Locals
464	search API (http://nanoref.com/yahooapis/mgPdGg):
465
466	module \| encode \| decode \|
467	-----------\|------------\|------------\|
468	JSON \| 673 \| 38 \|
469	JSON::DWIW \| 5271 \| 770 \|
470	JSON::PC \| 9901 \| 2491 \|
471	JSON::Syck \| 2360 \| 786 \|
472	JSON::XS \| 37398 \| 3202 \|
473	JSON::XS/2 \| 13765 \| 3153 \|
474	-----------+------------+------------+
475
476	Again, JSON::XS leads by far in the encoding case, while still beating
477	every other module in the decoding case.
478
479	RESOURCE LIMITS
480	JSON::XS does not impose any limits on the size of JSON texts or Perl
481	values they represent - if your machine can handle it, JSON::XS will
482	encode or decode it. Future versions might optionally impose structure
483	depth and memory use resource limits.
484
485	BUGS
486	While the goal of this module is to be correct, that unfortunately does
487	not mean its bug-free, only that I think its design is bug-free. It is
488	still very young and not well-tested. If you keep reporting bugs they
489	will be fixed swiftly, though.
490
491	AUTHOR
492	Marc Lehmann <schmorp@schmorp.de>
493	http://home.schmorp.de/
494