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1	NAME	1	NAME
2	JSON::XS - JSON serialising/deserialising, done correctly and fast	2	JSON::XS - JSON serialising/deserialising, done correctly and fast
3		3
		4	JSON::XS - 正しくて高速な JSON シリアライザ/デシリアライザ
		5	(http://fleur.hio.jp/perldoc/mix/lib/JSON/XS.html)
		6
4	SYNOPSIS	7	SYNOPSIS
5	use JSON::XS;	8	use JSON::XS;
		9
		10	# exported functions, they croak on error
		11	# and expect/generate UTF-8
		12
		13	$utf8_encoded_json_text = encode_json $perl_hash_or_arrayref;
		14	$perl_hash_or_arrayref = decode_json $utf8_encoded_json_text;
		15
		16	# OO-interface
		17
		18	$coder = JSON::XS->new->ascii->pretty->allow_nonref;
		19	$pretty_printed_unencoded = $coder->encode ($perl_scalar);
		20	$perl_scalar = $coder->decode ($unicode_json_text);
		21
		22	# Note that JSON version 2.0 and above will automatically use JSON::XS
		23	# if available, at virtually no speed overhead either, so you should
		24	# be able to just:
		25
		26	use JSON;
		27
		28	# and do the same things, except that you have a pure-perl fallback now.
6		29
7	DESCRIPTION	30	DESCRIPTION
8	This module converts Perl data structures to JSON and vice versa. Its	31	This module converts Perl data structures to JSON and vice versa. Its
9	primary goal is to be *correct* and its secondary goal is to be *fast*.	32	primary goal is to be *correct* and its secondary goal is to be *fast*.
10	To reach the latter goal it was written in C.	33	To reach the latter goal it was written in C.
		34
		35	Beginning with version 2.0 of the JSON module, when both JSON and
		36	JSON::XS are installed, then JSON will fall back on JSON::XS (this can
		37	be overridden) with no overhead due to emulation (by inheriting
		38	constructor and methods). If JSON::XS is not available, it will fall
		39	back to the compatible JSON::PP module as backend, so using JSON instead
		40	of JSON::XS gives you a portable JSON API that can be fast when you need
		41	and doesn't require a C compiler when that is a problem.
11		42
12	As this is the n-th-something JSON module on CPAN, what was the reason	43	As this is the n-th-something JSON module on CPAN, what was the reason
13	to write yet another JSON module? While it seems there are many JSON	44	to write yet another JSON module? While it seems there are many JSON
14	modules, none of them correctly handle all corner cases, and in most	45	modules, none of them correctly handle all corner cases, and in most
15	cases their maintainers are unresponsive, gone missing, or not listening	46	cases their maintainers are unresponsive, gone missing, or not listening
16	to bug reports for other reasons.	47	to bug reports for other reasons.
17		48
18	See COMPARISON, below, for a comparison to some other JSON modules.	49	See MAPPING, below, on how JSON::XS maps perl values to JSON values and
		50	vice versa.
19		51
20	FEATURES	52	FEATURES
21	* correct handling of unicode issues	53	* correct Unicode handling
		54
22	This module knows how to handle Unicode, and even documents how it	55	This module knows how to handle Unicode, documents how and when it
23	does so.	56	does so, and even documents what "correct" means.
24		57
25	* round-trip integrity	58	* round-trip integrity
		59
26	When you serialise a perl data structure using only datatypes	60	When you serialise a perl data structure using only data types
27	supported by JSON, the deserialised data structure is identical on	61	supported by JSON and Perl, the deserialised data structure is
28	the Perl level. (e.g. the string "2.0" doesn't suddenly become "2").	62	identical on the Perl level. (e.g. the string "2.0" doesn't suddenly
		63	become "2" just because it looks like a number). There *are* minor
		64	exceptions to this, read the MAPPING section below to learn about
		65	those.
29		66
30	* strict checking of JSON correctness	67	* strict checking of JSON correctness
		68
31	There is no guessing, no generating of illegal JSON strings by	69	There is no guessing, no generating of illegal JSON texts by
32	default, and only JSON is accepted as input (the latter is a	70	default, and only JSON is accepted as input by default (the latter
33	security feature).	71	is a security feature).
34		72
35	* fast	73	* fast
36	compared to other JSON modules, this module compares favourably.
37		74
		75	Compared to other JSON modules and other serialisers such as
		76	Storable, this module usually compares favourably in terms of speed,
		77	too.
		78
38	* simple to use	79	* simple to use
		80
39	This module has both a simple functional interface as well as an OO	81	This module has both a simple functional interface as well as an
40	interface.	82	object oriented interface.
41		83
42	* reasonably versatile output formats	84	* reasonably versatile output formats
		85
43	You can choose between the most compact format possible, a	86	You can choose between the most compact guaranteed-single-line
44	pure-ascii format, or a pretty-printed format. Or you can combine	87	format possible (nice for simple line-based protocols), a pure-ASCII
		88	format (for when your transport is not 8-bit clean, still supports
		89	the whole Unicode range), or a pretty-printed format (for when you
		90	want to read that stuff). Or you can combine those features in
45	those features in whatever way you like.	91	whatever way you like.
46		92
47	FUNCTIONAL INTERFACE	93	FUNCTIONAL INTERFACE
48	The following convinience methods are provided by this module. They are	94	The following convenience methods are provided by this module. They are
49	exported by default:	95	exported by default:
50		96
51	$json_string = to_json $perl_scalar	97	$json_text = encode_json $perl_scalar
52	Converts the given Perl data structure (a simple scalar or a	98	Converts the given Perl data structure to a UTF-8 encoded, binary
53	reference to a hash or array) to a UTF-8 encoded, binary string
54	(that is, the string contains octets only). Croaks on error.	99	string (that is, the string contains octets only). Croaks on error.
55		100
56	This function call is functionally identical to "JSON::XS->new->utf8	101	This function call is functionally identical to:
57	(1)->encode ($perl_scalar)".
58		102
		103	$json_text = JSON::XS->new->utf8->encode ($perl_scalar)
		104
		105	Except being faster.
		106
59	$perl_scalar = from_json $json_string	107	$perl_scalar = decode_json $json_text
60	The opposite of "to_json": expects an UTF-8 (binary) string and	108	The opposite of "encode_json": expects an UTF-8 (binary) string and
61	tries to parse that as an UTF-8 encoded JSON string, returning the	109	tries to parse that as an UTF-8 encoded JSON text, returning the
62	resulting simple scalar or reference. Croaks on error.	110	resulting reference. Croaks on error.
63		111
64	This function call is functionally identical to "JSON::XS->new->utf8	112	This function call is functionally identical to:
65	(1)->decode ($json_string)".	113
		114	$perl_scalar = JSON::XS->new->utf8->decode ($json_text)
		115
		116	Except being faster.
		117
		118	A FEW NOTES ON UNICODE AND PERL
		119	Since this often leads to confusion, here are a few very clear words on
		120	how Unicode works in Perl, modulo bugs.
		121
		122	1. Perl strings can store characters with ordinal values > 255.
		123	This enables you to store Unicode characters as single characters in
		124	a Perl string - very natural.
		125
		126	2. Perl does *not* associate an encoding with your strings.
		127	... until you force it to, e.g. when matching it against a regex, or
		128	printing the scalar to a file, in which case Perl either interprets
		129	your string as locale-encoded text, octets/binary, or as Unicode,
		130	depending on various settings. In no case is an encoding stored
		131	together with your data, it is *use* that decides encoding, not any
		132	magical meta data.
		133
		134	3. The internal utf-8 flag has no meaning with regards to the encoding
		135	of your string.
		136	Just ignore that flag unless you debug a Perl bug, a module written
		137	in XS or want to dive into the internals of perl. Otherwise it will
		138	only confuse you, as, despite the name, it says nothing about how
		139	your string is encoded. You can have Unicode strings with that flag
		140	set, with that flag clear, and you can have binary data with that
		141	flag set and that flag clear. Other possibilities exist, too.
		142
		143	If you didn't know about that flag, just the better, pretend it
		144	doesn't exist.
		145
		146	4. A "Unicode String" is simply a string where each character can be
		147	validly interpreted as a Unicode code point.
		148	If you have UTF-8 encoded data, it is no longer a Unicode string,
		149	but a Unicode string encoded in UTF-8, giving you a binary string.
		150
		151	5. A string containing "high" (> 255) character values is *not* a UTF-8
		152	string.
		153	It's a fact. Learn to live with it.
		154
		155	I hope this helps :)
66		156
67	OBJECT-ORIENTED INTERFACE	157	OBJECT-ORIENTED INTERFACE
68	The object oriented interface lets you configure your own encoding or	158	The object oriented interface lets you configure your own encoding or
69	decoding style, within the limits of supported formats.	159	decoding style, within the limits of supported formats.
70		160
…		…
74	*disabled*.	164	*disabled*.
75		165
76	The mutators for flags all return the JSON object again and thus	166	The mutators for flags all return the JSON object again and thus
77	calls can be chained:	167	calls can be chained:
78		168
79	my $json = JSON::XS->new->utf8(1)->space_after(1)->encode ({a => [1,2]})	169	my $json = JSON::XS->new->utf8->space_after->encode ({a => [1,2]})
80	=> {"a": [1, 2]}	170	=> {"a": [1, 2]}
81		171
82	$json = $json->ascii ($enable)	172	$json = $json->ascii ([$enable])
		173	$enabled = $json->get_ascii
83	If $enable is true, then the "encode" method will not generate	174	If $enable is true (or missing), then the "encode" method will not
84	characters outside the code range 0..127. Any unicode characters	175	generate characters outside the code range 0..127 (which is ASCII).
85	outside that range will be escaped using either a single \uXXXX (BMP	176	Any Unicode characters outside that range will be escaped using
86	characters) or a double \uHHHH\uLLLLL escape sequence, as per	177	either a single \uXXXX (BMP characters) or a double \uHHHH\uLLLLL
87	RFC4627.	178	escape sequence, as per RFC4627. The resulting encoded JSON text can
		179	be treated as a native Unicode string, an ascii-encoded,
		180	latin1-encoded or UTF-8 encoded string, or any other superset of
		181	ASCII.
88		182
89	If $enable is false, then the "encode" method will not escape	183	If $enable is false, then the "encode" method will not escape
90	Unicode characters unless necessary.	184	Unicode characters unless required by the JSON syntax or other
		185	flags. This results in a faster and more compact format.
91		186
		187	See also the section *ENCODING/CODESET FLAG NOTES* later in this
		188	document.
		189
		190	The main use for this flag is to produce JSON texts that can be
		191	transmitted over a 7-bit channel, as the encoded JSON texts will not
		192	contain any 8 bit characters.
		193
92	JSON::XS->new->ascii (1)->encode (chr 0x10401)	194	JSON::XS->new->ascii (1)->encode ([chr 0x10401])
93	=> \ud801\udc01	195	=> ["\ud801\udc01"]
94		196
		197	$json = $json->latin1 ([$enable])
		198	$enabled = $json->get_latin1
		199	If $enable is true (or missing), then the "encode" method will
		200	encode the resulting JSON text as latin1 (or iso-8859-1), escaping
		201	any characters outside the code range 0..255. The resulting string
		202	can be treated as a latin1-encoded JSON text or a native Unicode
		203	string. The "decode" method will not be affected in any way by this
		204	flag, as "decode" by default expects Unicode, which is a strict
		205	superset of latin1.
		206
		207	If $enable is false, then the "encode" method will not escape
		208	Unicode characters unless required by the JSON syntax or other
		209	flags.
		210
		211	See also the section *ENCODING/CODESET FLAG NOTES* later in this
		212	document.
		213
		214	The main use for this flag is efficiently encoding binary data as
		215	JSON text, as most octets will not be escaped, resulting in a
		216	smaller encoded size. The disadvantage is that the resulting JSON
		217	text is encoded in latin1 (and must correctly be treated as such
		218	when storing and transferring), a rare encoding for JSON. It is
		219	therefore most useful when you want to store data structures known
		220	to contain binary data efficiently in files or databases, not when
		221	talking to other JSON encoders/decoders.
		222
		223	JSON::XS->new->latin1->encode (["\x{89}\x{abc}"]
		224	=> ["\x{89}\\u0abc"] # (perl syntax, U+abc escaped, U+89 not)
		225
95	$json = $json->utf8 ($enable)	226	$json = $json->utf8 ([$enable])
		227	$enabled = $json->get_utf8
96	If $enable is true, then the "encode" method will encode the JSON	228	If $enable is true (or missing), then the "encode" method will
97	string into UTF-8, as required by many protocols, while the "decode"	229	encode the JSON result into UTF-8, as required by many protocols,
98	method expects to be handled an UTF-8-encoded string. Please note	230	while the "decode" method expects to be handled an UTF-8-encoded
99	that UTF-8-encoded strings do not contain any characters outside the	231	string. Please note that UTF-8-encoded strings do not contain any
100	range 0..255, they are thus useful for bytewise/binary I/O.	232	characters outside the range 0..255, they are thus useful for
		233	bytewise/binary I/O. In future versions, enabling this option might
		234	enable autodetection of the UTF-16 and UTF-32 encoding families, as
		235	described in RFC4627.
101		236
102	If $enable is false, then the "encode" method will return the JSON	237	If $enable is false, then the "encode" method will return the JSON
103	string as a (non-encoded) unicode string, while "decode" expects	238	string as a (non-encoded) Unicode string, while "decode" expects
104	thus a unicode string. Any decoding or encoding (e.g. to UTF-8 or	239	thus a Unicode string. Any decoding or encoding (e.g. to UTF-8 or
105	UTF-16) needs to be done yourself, e.g. using the Encode module.	240	UTF-16) needs to be done yourself, e.g. using the Encode module.
106		241
		242	See also the section *ENCODING/CODESET FLAG NOTES* later in this
		243	document.
		244
		245	Example, output UTF-16BE-encoded JSON:
		246
		247	use Encode;
		248	$jsontext = encode "UTF-16BE", JSON::XS->new->encode ($object);
		249
		250	Example, decode UTF-32LE-encoded JSON:
		251
		252	use Encode;
		253	$object = JSON::XS->new->decode (decode "UTF-32LE", $jsontext);
		254
107	$json = $json->pretty ($enable)	255	$json = $json->pretty ([$enable])
108	This enables (or disables) all of the "indent", "space_before" and	256	This enables (or disables) all of the "indent", "space_before" and
109	"space_after" (and in the future possibly more) flags in one call to	257	"space_after" (and in the future possibly more) flags in one call to
110	generate the most readable (or most compact) form possible.	258	generate the most readable (or most compact) form possible.
		259
		260	Example, pretty-print some simple structure:
111		261
112	my $json = JSON::XS->new->pretty(1)->encode ({a => [1,2]})	262	my $json = JSON::XS->new->pretty(1)->encode ({a => [1,2]})
113	=>	263	=>
114	{	264	{
115	"a" : [	265	"a" : [
116	1,	266	1,
117	2	267	2
118	]	268	]
119	}	269	}
120		270
121	$json = $json->indent ($enable)	271	$json = $json->indent ([$enable])
		272	$enabled = $json->get_indent
122	If $enable is true, then the "encode" method will use a multiline	273	If $enable is true (or missing), then the "encode" method will use a
123	format as output, putting every array member or object/hash	274	multiline format as output, putting every array member or
124	key-value pair into its own line, identing them properly.	275	object/hash key-value pair into its own line, indenting them
		276	properly.
125		277
126	If $enable is false, no newlines or indenting will be produced, and	278	If $enable is false, no newlines or indenting will be produced, and
127	the resulting JSON strings is guarenteed not to contain any	279	the resulting JSON text is guaranteed not to contain any "newlines".
128	"newlines".
129		280
130	This setting has no effect when decoding JSON strings.	281	This setting has no effect when decoding JSON texts.
131		282
132	$json = $json->space_before ($enable)	283	$json = $json->space_before ([$enable])
		284	$enabled = $json->get_space_before
133	If $enable is true, then the "encode" method will add an extra	285	If $enable is true (or missing), then the "encode" method will add
134	optional space before the ":" separating keys from values in JSON	286	an extra optional space before the ":" separating keys from values
135	objects.	287	in JSON objects.
136		288
137	If $enable is false, then the "encode" method will not add any extra	289	If $enable is false, then the "encode" method will not add any extra
138	space at those places.	290	space at those places.
139		291
140	This setting has no effect when decoding JSON strings. You will also	292	This setting has no effect when decoding JSON texts. You will also
141	most likely combine this setting with "space_after".	293	most likely combine this setting with "space_after".
142		294
		295	Example, space_before enabled, space_after and indent disabled:
		296
		297	{"key" :"value"}
		298
143	$json = $json->space_after ($enable)	299	$json = $json->space_after ([$enable])
		300	$enabled = $json->get_space_after
144	If $enable is true, then the "encode" method will add an extra	301	If $enable is true (or missing), then the "encode" method will add
145	optional space after the ":" separating keys from values in JSON	302	an extra optional space after the ":" separating keys from values in
146	objects and extra whitespace after the "," separating key-value	303	JSON objects and extra whitespace after the "," separating key-value
147	pairs and array members.	304	pairs and array members.
148		305
149	If $enable is false, then the "encode" method will not add any extra	306	If $enable is false, then the "encode" method will not add any extra
150	space at those places.	307	space at those places.
151		308
152	This setting has no effect when decoding JSON strings.	309	This setting has no effect when decoding JSON texts.
153		310
		311	Example, space_before and indent disabled, space_after enabled:
		312
		313	{"key": "value"}
		314
		315	$json = $json->relaxed ([$enable])
		316	$enabled = $json->get_relaxed
		317	If $enable is true (or missing), then "decode" will accept some
		318	extensions to normal JSON syntax (see below). "encode" will not be
		319	affected in anyway. *Be aware that this option makes you accept
		320	invalid JSON texts as if they were valid!*. I suggest only to use
		321	this option to parse application-specific files written by humans
		322	(configuration files, resource files etc.)
		323
		324	If $enable is false (the default), then "decode" will only accept
		325	valid JSON texts.
		326
		327	Currently accepted extensions are:
		328
		329	* list items can have an end-comma
		330
		331	JSON *separates* array elements and key-value pairs with commas.
		332	This can be annoying if you write JSON texts manually and want
		333	to be able to quickly append elements, so this extension accepts
		334	comma at the end of such items not just between them:
		335
		336	[
		337	1,
		338	2, <- this comma not normally allowed
		339	]
		340	{
		341	"k1": "v1",
		342	"k2": "v2", <- this comma not normally allowed
		343	}
		344
		345	* shell-style '#'-comments
		346
		347	Whenever JSON allows whitespace, shell-style comments are
		348	additionally allowed. They are terminated by the first
		349	carriage-return or line-feed character, after which more
		350	white-space and comments are allowed.
		351
		352	[
		353	1, # this comment not allowed in JSON
		354	# neither this one...
		355	]
		356
154	$json = $json->canonical ($enable)	357	$json = $json->canonical ([$enable])
		358	$enabled = $json->get_canonical
155	If $enable is true, then the "encode" method will output JSON	359	If $enable is true (or missing), then the "encode" method will
156	objects by sorting their keys. This is adding a comparatively high	360	output JSON objects by sorting their keys. This is adding a
157	overhead.	361	comparatively high overhead.
158		362
159	If $enable is false, then the "encode" method will output key-value	363	If $enable is false, then the "encode" method will output key-value
160	pairs in the order Perl stores them (which will likely change	364	pairs in the order Perl stores them (which will likely change
161	between runs of the same script).	365	between runs of the same script, and can change even within the same
		366	run from 5.18 onwards).
162		367
163	This option is useful if you want the same data structure to be	368	This option is useful if you want the same data structure to be
164	encoded as the same JSON string (given the same overall settings).	369	encoded as the same JSON text (given the same overall settings). If
165	If it is disabled, the same hash migh be encoded differently even if	370	it is disabled, the same hash might be encoded differently even if
166	contains the same data, as key-value pairs have no inherent ordering	371	contains the same data, as key-value pairs have no inherent ordering
167	in Perl.	372	in Perl.
168		373
169	This setting has no effect when decoding JSON strings.	374	This setting has no effect when decoding JSON texts.
170		375
		376	This setting has currently no effect on tied hashes.
		377
171	$json = $json->allow_nonref ($enable)	378	$json = $json->allow_nonref ([$enable])
		379	$enabled = $json->get_allow_nonref
172	If $enable is true, then the "encode" method can convert a	380	If $enable is true (or missing), then the "encode" method can
173	non-reference into its corresponding string, number or null JSON	381	convert a non-reference into its corresponding string, number or
174	value, which is an extension to RFC4627. Likewise, "decode" will	382	null JSON value, which is an extension to RFC4627. Likewise,
175	accept those JSON values instead of croaking.	383	"decode" will accept those JSON values instead of croaking.
176		384
177	If $enable is false, then the "encode" method will croak if it isn't	385	If $enable is false, then the "encode" method will croak if it isn't
178	passed an arrayref or hashref, as JSON strings must either be an	386	passed an arrayref or hashref, as JSON texts must either be an
179	object or array. Likewise, "decode" will croak if given something	387	object or array. Likewise, "decode" will croak if given something
180	that is not a JSON object or array.	388	that is not a JSON object or array.
181		389
		390	Example, encode a Perl scalar as JSON value with enabled
		391	"allow_nonref", resulting in an invalid JSON text:
		392
		393	JSON::XS->new->allow_nonref->encode ("Hello, World!")
		394	=> "Hello, World!"
		395
		396	$json = $json->allow_unknown ([$enable])
		397	$enabled = $json->get_allow_unknown
		398	If $enable is true (or missing), then "encode" will *not* throw an
		399	exception when it encounters values it cannot represent in JSON (for
		400	example, filehandles) but instead will encode a JSON "null" value.
		401	Note that blessed objects are not included here and are handled
		402	separately by c<allow_nonref>.
		403
		404	If $enable is false (the default), then "encode" will throw an
		405	exception when it encounters anything it cannot encode as JSON.
		406
		407	This option does not affect "decode" in any way, and it is
		408	recommended to leave it off unless you know your communications
		409	partner.
		410
		411	$json = $json->allow_blessed ([$enable])
		412	$enabled = $json->get_allow_blessed
		413	See "OBJECT SERIALISATION" for details.
		414
		415	If $enable is true (or missing), then the "encode" method will not
		416	barf when it encounters a blessed reference that it cannot convert
		417	otherwise. Instead, a JSON "null" value is encoded instead of the
		418	object.
		419
		420	If $enable is false (the default), then "encode" will throw an
		421	exception when it encounters a blessed object that it cannot convert
		422	otherwise.
		423
		424	This setting has no effect on "decode".
		425
		426	$json = $json->convert_blessed ([$enable])
		427	$enabled = $json->get_convert_blessed
		428	See "OBJECT SERIALISATION" for details.
		429
		430	If $enable is true (or missing), then "encode", upon encountering a
		431	blessed object, will check for the availability of the "TO_JSON"
		432	method on the object's class. If found, it will be called in scalar
		433	context and the resulting scalar will be encoded instead of the
		434	object.
		435
		436	The "TO_JSON" method may safely call die if it wants. If "TO_JSON"
		437	returns other blessed objects, those will be handled in the same
		438	way. "TO_JSON" must take care of not causing an endless recursion
		439	cycle (== crash) in this case. The name of "TO_JSON" was chosen
		440	because other methods called by the Perl core (== not by the user of
		441	the object) are usually in upper case letters and to avoid
		442	collisions with any "to_json" function or method.
		443
		444	If $enable is false (the default), then "encode" will not consider
		445	this type of conversion.
		446
		447	This setting has no effect on "decode".
		448
		449	$json = $json->allow_tags ([$enable])
		450	$enabled = $json->allow_tags
		451	See "OBJECT SERIALISATION" for details.
		452
		453	If $enable is true (or missing), then "encode", upon encountering a
		454	blessed object, will check for the availability of the "FREEZE"
		455	method on the object's class. If found, it will be used to serialise
		456	the object into a nonstandard tagged JSON value (that JSON decoders
		457	cannot decode).
		458
		459	It also causes "decode" to parse such tagged JSON values and
		460	deserialise them via a call to the "THAW" method.
		461
		462	If $enable is false (the default), then "encode" will not consider
		463	this type of conversion, and tagged JSON values will cause a parse
		464	error in "decode", as if tags were not part of the grammar.
		465
		466	$json = $json->filter_json_object ([$coderef->($hashref)])
		467	When $coderef is specified, it will be called from "decode" each
		468	time it decodes a JSON object. The only argument is a reference to
		469	the newly-created hash. If the code references returns a single
		470	scalar (which need not be a reference), this value (i.e. a copy of
		471	that scalar to avoid aliasing) is inserted into the deserialised
		472	data structure. If it returns an empty list (NOTE: *not* "undef",
		473	which is a valid scalar), the original deserialised hash will be
		474	inserted. This setting can slow down decoding considerably.
		475
		476	When $coderef is omitted or undefined, any existing callback will be
		477	removed and "decode" will not change the deserialised hash in any
		478	way.
		479
		480	Example, convert all JSON objects into the integer 5:
		481
		482	my $js = JSON::XS->new->filter_json_object (sub { 5 });
		483	# returns [5]
		484	$js->decode ('[{}]')
		485	# throw an exception because allow_nonref is not enabled
		486	# so a lone 5 is not allowed.
		487	$js->decode ('{"a":1, "b":2}');
		488
		489	$json = $json->filter_json_single_key_object ($key [=>
		490	$coderef->($value)])
		491	Works remotely similar to "filter_json_object", but is only called
		492	for JSON objects having a single key named $key.
		493
		494	This $coderef is called before the one specified via
		495	"filter_json_object", if any. It gets passed the single value in the
		496	JSON object. If it returns a single value, it will be inserted into
		497	the data structure. If it returns nothing (not even "undef" but the
		498	empty list), the callback from "filter_json_object" will be called
		499	next, as if no single-key callback were specified.
		500
		501	If $coderef is omitted or undefined, the corresponding callback will
		502	be disabled. There can only ever be one callback for a given key.
		503
		504	As this callback gets called less often then the
		505	"filter_json_object" one, decoding speed will not usually suffer as
		506	much. Therefore, single-key objects make excellent targets to
		507	serialise Perl objects into, especially as single-key JSON objects
		508	are as close to the type-tagged value concept as JSON gets (it's
		509	basically an ID/VALUE tuple). Of course, JSON does not support this
		510	in any way, so you need to make sure your data never looks like a
		511	serialised Perl hash.
		512
		513	Typical names for the single object key are "__class_whatever__", or
		514	"$__dollars_are_rarely_used__$" or "}ugly_brace_placement", or even
		515	things like "__class_md5sum(classname)__", to reduce the risk of
		516	clashing with real hashes.
		517
		518	Example, decode JSON objects of the form "{ "__widget__" => <id> }"
		519	into the corresponding $WIDGET{<id>} object:
		520
		521	# return whatever is in $WIDGET{5}:
		522	JSON::XS
		523	->new
		524	->filter_json_single_key_object (__widget__ => sub {
		525	$WIDGET{ $_[0] }
		526	})
		527	->decode ('{"__widget__": 5')
		528
		529	# this can be used with a TO_JSON method in some "widget" class
		530	# for serialisation to json:
		531	sub WidgetBase::TO_JSON {
		532	my ($self) = @_;
		533
		534	unless ($self->{id}) {
		535	$self->{id} = ..get..some..id..;
		536	$WIDGET{$self->{id}} = $self;
		537	}
		538
		539	{ __widget__ => $self->{id} }
		540	}
		541
		542	$json = $json->shrink ([$enable])
		543	$enabled = $json->get_shrink
		544	Perl usually over-allocates memory a bit when allocating space for
		545	strings. This flag optionally resizes strings generated by either
		546	"encode" or "decode" to their minimum size possible. This can save
		547	memory when your JSON texts are either very very long or you have
		548	many short strings. It will also try to downgrade any strings to
		549	octet-form if possible: perl stores strings internally either in an
		550	encoding called UTF-X or in octet-form. The latter cannot store
		551	everything but uses less space in general (and some buggy Perl or C
		552	code might even rely on that internal representation being used).
		553
		554	The actual definition of what shrink does might change in future
		555	versions, but it will always try to save space at the expense of
		556	time.
		557
		558	If $enable is true (or missing), the string returned by "encode"
		559	will be shrunk-to-fit, while all strings generated by "decode" will
		560	also be shrunk-to-fit.
		561
		562	If $enable is false, then the normal perl allocation algorithms are
		563	used. If you work with your data, then this is likely to be faster.
		564
		565	In the future, this setting might control other things, such as
		566	converting strings that look like integers or floats into integers
		567	or floats internally (there is no difference on the Perl level),
		568	saving space.
		569
		570	$json = $json->max_depth ([$maximum_nesting_depth])
		571	$max_depth = $json->get_max_depth
		572	Sets the maximum nesting level (default 512) accepted while encoding
		573	or decoding. If a higher nesting level is detected in JSON text or a
		574	Perl data structure, then the encoder and decoder will stop and
		575	croak at that point.
		576
		577	Nesting level is defined by number of hash- or arrayrefs that the
		578	encoder needs to traverse to reach a given point or the number of
		579	"{" or "[" characters without their matching closing parenthesis
		580	crossed to reach a given character in a string.
		581
		582	Setting the maximum depth to one disallows any nesting, so that
		583	ensures that the object is only a single hash/object or array.
		584
		585	If no argument is given, the highest possible setting will be used,
		586	which is rarely useful.
		587
		588	Note that nesting is implemented by recursion in C. The default
		589	value has been chosen to be as large as typical operating systems
		590	allow without crashing.
		591
		592	See SECURITY CONSIDERATIONS, below, for more info on why this is
		593	useful.
		594
		595	$json = $json->max_size ([$maximum_string_size])
		596	$max_size = $json->get_max_size
		597	Set the maximum length a JSON text may have (in bytes) where
		598	decoding is being attempted. The default is 0, meaning no limit.
		599	When "decode" is called on a string that is longer then this many
		600	bytes, it will not attempt to decode the string but throw an
		601	exception. This setting has no effect on "encode" (yet).
		602
		603	If no argument is given, the limit check will be deactivated (same
		604	as when 0 is specified).
		605
		606	See SECURITY CONSIDERATIONS, below, for more info on why this is
		607	useful.
		608
182	$json_string = $json->encode ($perl_scalar)	609	$json_text = $json->encode ($perl_scalar)
183	Converts the given Perl data structure (a simple scalar or a	610	Converts the given Perl value or data structure to its JSON
184	reference to a hash or array) to its JSON representation. Simple	611	representation. Croaks on error.
185	scalars will be converted into JSON string or number sequences,
186	while references to arrays become JSON arrays and references to
187	hashes become JSON objects. Undefined Perl values (e.g. "undef")
188	become JSON "null" values. Neither "true" nor "false" values will be
189	generated.
190		612
191	$perl_scalar = $json->decode ($json_string)	613	$perl_scalar = $json->decode ($json_text)
192	The opposite of "encode": expects a JSON string and tries to parse	614	The opposite of "encode": expects a JSON text and tries to parse it,
193	it, returning the resulting simple scalar or reference. Croaks on	615	returning the resulting simple scalar or reference. Croaks on error.
194	error.
195		616
196	JSON numbers and strings become simple Perl scalars. JSON arrays	617	($perl_scalar, $characters) = $json->decode_prefix ($json_text)
197	become Perl arrayrefs and JSON objects become Perl hashrefs. "true"	618	This works like the "decode" method, but instead of raising an
198	becomes 1, "false" becomes 0 and "null" becomes "undef".	619	exception when there is trailing garbage after the first JSON
		620	object, it will silently stop parsing there and return the number of
		621	characters consumed so far.
199		622
200	COMPARISON	623	This is useful if your JSON texts are not delimited by an outer
201	As already mentioned, this module was created because none of the	624	protocol and you need to know where the JSON text ends.
202	existing JSON modules could be made to work correctly. First I will
203	describe the problems (or pleasures) I encountered with various existing
204	JSON modules, followed by some benchmark values. JSON::XS was designed
205	not to suffer from any of these problems or limitations.
206		625
207	JSON 1.07	626	JSON::XS->new->decode_prefix ("[1] the tail")
208	Slow (but very portable, as it is written in pure Perl).	627	=> ([], 3)
209		628
210	Undocumented/buggy Unicode handling (how JSON handles unicode values	629	INCREMENTAL PARSING
211	is undocumented. One can get far by feeding it unicode strings and	630	In some cases, there is the need for incremental parsing of JSON texts.
212	doing en-/decoding oneself, but unicode escapes are not working	631	While this module always has to keep both JSON text and resulting Perl
		632	data structure in memory at one time, it does allow you to parse a JSON
		633	stream incrementally. It does so by accumulating text until it has a
		634	full JSON object, which it then can decode. This process is similar to
		635	using "decode_prefix" to see if a full JSON object is available, but is
		636	much more efficient (and can be implemented with a minimum of method
		637	calls).
		638
		639	JSON::XS will only attempt to parse the JSON text once it is sure it has
		640	enough text to get a decisive result, using a very simple but truly
		641	incremental parser. This means that it sometimes won't stop as early as
		642	the full parser, for example, it doesn't detect mismatched parentheses.
		643	The only thing it guarantees is that it starts decoding as soon as a
		644	syntactically valid JSON text has been seen. This means you need to set
		645	resource limits (e.g. "max_size") to ensure the parser will stop parsing
		646	in the presence if syntax errors.
		647
		648	The following methods implement this incremental parser.
		649
		650	[void, scalar or list context] = $json->incr_parse ([$string])
		651	This is the central parsing function. It can both append new text
		652	and extract objects from the stream accumulated so far (both of
		653	these functions are optional).
		654
		655	If $string is given, then this string is appended to the already
		656	existing JSON fragment stored in the $json object.
		657
		658	After that, if the function is called in void context, it will
		659	simply return without doing anything further. This can be used to
		660	add more text in as many chunks as you want.
		661
		662	If the method is called in scalar context, then it will try to
		663	extract exactly *one* JSON object. If that is successful, it will
		664	return this object, otherwise it will return "undef". If there is a
		665	parse error, this method will croak just as "decode" would do (one
		666	can then use "incr_skip" to skip the erroneous part). This is the
		667	most common way of using the method.
		668
		669	And finally, in list context, it will try to extract as many objects
		670	from the stream as it can find and return them, or the empty list
		671	otherwise. For this to work, there must be no separators between the
		672	JSON objects or arrays, instead they must be concatenated
		673	back-to-back. If an error occurs, an exception will be raised as in
		674	the scalar context case. Note that in this case, any
		675	previously-parsed JSON texts will be lost.
		676
		677	Example: Parse some JSON arrays/objects in a given string and return
		678	them.
		679
		680	my @objs = JSON::XS->new->incr_parse ("[5][7][1,2]");
		681
		682	$lvalue_string = $json->incr_text
		683	This method returns the currently stored JSON fragment as an lvalue,
		684	that is, you can manipulate it. This *only* works when a preceding
		685	call to "incr_parse" in *scalar context* successfully returned an
		686	object. Under all other circumstances you must not call this
		687	function (I mean it. although in simple tests it might actually
		688	work, it *will* fail under real world conditions). As a special
		689	exception, you can also call this method before having parsed
		690	anything.
		691
		692	This function is useful in two cases: a) finding the trailing text
		693	after a JSON object or b) parsing multiple JSON objects separated by
		694	non-JSON text (such as commas).
		695
		696	$json->incr_skip
		697	This will reset the state of the incremental parser and will remove
		698	the parsed text from the input buffer so far. This is useful after
		699	"incr_parse" died, in which case the input buffer and incremental
		700	parser state is left unchanged, to skip the text parsed so far and
		701	to reset the parse state.
		702
		703	The difference to "incr_reset" is that only text until the parse
		704	error occurred is removed.
		705
		706	$json->incr_reset
		707	This completely resets the incremental parser, that is, after this
		708	call, it will be as if the parser had never parsed anything.
		709
		710	This is useful if you want to repeatedly parse JSON objects and want
		711	to ignore any trailing data, which means you have to reset the
		712	parser after each successful decode.
		713
		714	LIMITATIONS
		715	All options that affect decoding are supported, except "allow_nonref".
		716	The reason for this is that it cannot be made to work sensibly: JSON
		717	objects and arrays are self-delimited, i.e. you can concatenate them
		718	back to back and still decode them perfectly. This does not hold true
		719	for JSON numbers, however.
		720
		721	For example, is the string 1 a single JSON number, or is it simply the
		722	start of 12? Or is 12 a single JSON number, or the concatenation of 1
		723	and 2? In neither case you can tell, and this is why JSON::XS takes the
		724	conservative route and disallows this case.
		725
		726	EXAMPLES
		727	Some examples will make all this clearer. First, a simple example that
		728	works similarly to "decode_prefix": We want to decode the JSON object at
		729	the start of a string and identify the portion after the JSON object:
		730
		731	my $text = "[1,2,3] hello";
		732
		733	my $json = new JSON::XS;
		734
		735	my $obj = $json->incr_parse ($text)
		736	or die "expected JSON object or array at beginning of string";
		737
		738	my $tail = $json->incr_text;
		739	# $tail now contains " hello"
		740
		741	Easy, isn't it?
		742
		743	Now for a more complicated example: Imagine a hypothetical protocol
		744	where you read some requests from a TCP stream, and each request is a
		745	JSON array, without any separation between them (in fact, it is often
		746	useful to use newlines as "separators", as these get interpreted as
		747	whitespace at the start of the JSON text, which makes it possible to
		748	test said protocol with "telnet"...).
		749
		750	Here is how you'd do it (it is trivial to write this in an event-based
		751	manner):
		752
		753	my $json = new JSON::XS;
		754
		755	# read some data from the socket
		756	while (sysread $socket, my $buf, 4096) {
		757
		758	# split and decode as many requests as possible
		759	for my $request ($json->incr_parse ($buf)) {
		760	# act on the $request
		761	}
		762	}
		763
		764	Another complicated example: Assume you have a string with JSON objects
		765	or arrays, all separated by (optional) comma characters (e.g. "[1],[2],
		766	[3]"). To parse them, we have to skip the commas between the JSON texts,
		767	and here is where the lvalue-ness of "incr_text" comes in useful:
		768
		769	my $text = "[1],[2], [3]";
		770	my $json = new JSON::XS;
		771
		772	# void context, so no parsing done
		773	$json->incr_parse ($text);
		774
		775	# now extract as many objects as possible. note the
		776	# use of scalar context so incr_text can be called.
		777	while (my $obj = $json->incr_parse) {
		778	# do something with $obj
		779
		780	# now skip the optional comma
		781	$json->incr_text =~ s/^ \s* , //x;
		782	}
		783
		784	Now lets go for a very complex example: Assume that you have a gigantic
		785	JSON array-of-objects, many gigabytes in size, and you want to parse it,
		786	but you cannot load it into memory fully (this has actually happened in
		787	the real world :).
		788
		789	Well, you lost, you have to implement your own JSON parser. But JSON::XS
		790	can still help you: You implement a (very simple) array parser and let
		791	JSON decode the array elements, which are all full JSON objects on their
		792	own (this wouldn't work if the array elements could be JSON numbers, for
		793	example):
		794
		795	my $json = new JSON::XS;
		796
		797	# open the monster
		798	open my $fh, "<bigfile.json"
		799	or die "bigfile: $!";
		800
		801	# first parse the initial "["
		802	for (;;) {
		803	sysread $fh, my $buf, 65536
		804	or die "read error: $!";
		805	$json->incr_parse ($buf); # void context, so no parsing
		806
		807	# Exit the loop once we found and removed(!) the initial "[".
		808	# In essence, we are (ab-)using the $json object as a simple scalar
		809	# we append data to.
		810	last if $json->incr_text =~ s/^ \s* \[ //x;
		811	}
		812
		813	# now we have the skipped the initial "[", so continue
		814	# parsing all the elements.
		815	for (;;) {
		816	# in this loop we read data until we got a single JSON object
		817	for (;;) {
		818	if (my $obj = $json->incr_parse) {
		819	# do something with $obj
		820	last;
		821	}
		822
		823	# add more data
		824	sysread $fh, my $buf, 65536
		825	or die "read error: $!";
		826	$json->incr_parse ($buf); # void context, so no parsing
		827	}
		828
		829	# in this loop we read data until we either found and parsed the
		830	# separating "," between elements, or the final "]"
		831	for (;;) {
		832	# first skip whitespace
		833	$json->incr_text =~ s/^\s*//;
		834
		835	# if we find "]", we are done
		836	if ($json->incr_text =~ s/^\]//) {
		837	print "finished.\n";
		838	exit;
		839	}
		840
		841	# if we find ",", we can continue with the next element
		842	if ($json->incr_text =~ s/^,//) {
		843	last;
		844	}
		845
		846	# if we find anything else, we have a parse error!
		847	if (length $json->incr_text) {
		848	die "parse error near ", $json->incr_text;
		849	}
		850
		851	# else add more data
		852	sysread $fh, my $buf, 65536
		853	or die "read error: $!";
		854	$json->incr_parse ($buf); # void context, so no parsing
		855	}
		856
		857	This is a complex example, but most of the complexity comes from the
		858	fact that we are trying to be correct (bear with me if I am wrong, I
		859	never ran the above example :).
		860
		861	MAPPING
		862	This section describes how JSON::XS maps Perl values to JSON values and
		863	vice versa. These mappings are designed to "do the right thing" in most
		864	circumstances automatically, preserving round-tripping characteristics
		865	(what you put in comes out as something equivalent).
		866
		867	For the more enlightened: note that in the following descriptions,
		868	lowercase *perl* refers to the Perl interpreter, while uppercase *Perl*
		869	refers to the abstract Perl language itself.
		870
		871	JSON -> PERL
		872	object
		873	A JSON object becomes a reference to a hash in Perl. No ordering of
		874	object keys is preserved (JSON does not preserve object key ordering
		875	itself).
		876
		877	array
		878	A JSON array becomes a reference to an array in Perl.
		879
		880	string
		881	A JSON string becomes a string scalar in Perl - Unicode codepoints
		882	in JSON are represented by the same codepoints in the Perl string,
		883	so no manual decoding is necessary.
		884
		885	number
		886	A JSON number becomes either an integer, numeric (floating point) or
		887	string scalar in perl, depending on its range and any fractional
		888	parts. On the Perl level, there is no difference between those as
		889	Perl handles all the conversion details, but an integer may take
		890	slightly less memory and might represent more values exactly than
		891	floating point numbers.
		892
		893	If the number consists of digits only, JSON::XS will try to
		894	represent it as an integer value. If that fails, it will try to
		895	represent it as a numeric (floating point) value if that is possible
		896	without loss of precision. Otherwise it will preserve the number as
		897	a string value (in which case you lose roundtripping ability, as the
		898	JSON number will be re-encoded to a JSON string).
		899
		900	Numbers containing a fractional or exponential part will always be
		901	represented as numeric (floating point) values, possibly at a loss
		902	of precision (in which case you might lose perfect roundtripping
		903	ability, but the JSON number will still be re-encoded as a JSON
		904	number).
		905
		906	Note that precision is not accuracy - binary floating point values
		907	cannot represent most decimal fractions exactly, and when converting
		908	from and to floating point, JSON::XS only guarantees precision up to
		909	but not including the least significant bit.
		910
		911	true, false
		912	These JSON atoms become "Types::Serialiser::true" and
		913	"Types::Serialiser::false", respectively. They are overloaded to act
		914	almost exactly like the numbers 1 and 0. You can check whether a
		915	scalar is a JSON boolean by using the "Types::Serialiser::is_bool"
		916	function (after "use Types::Serialier", of course).
		917
		918	null
		919	A JSON null atom becomes "undef" in Perl.
		920
		921	shell-style comments ("# *text*")
		922	As a nonstandard extension to the JSON syntax that is enabled by the
		923	"relaxed" setting, shell-style comments are allowed. They can start
		924	anywhere outside strings and go till the end of the line.
		925
		926	tagged values ("(*tag*)*value*").
		927	Another nonstandard extension to the JSON syntax, enabled with the
		928	"allow_tags" setting, are tagged values. In this implementation, the
		929	*tag* must be a perl package/class name encoded as a JSON string,
		930	and the *value* must be a JSON array encoding optional constructor
		931	arguments.
		932
		933	See "OBJECT SERIALISATION", below, for details.
		934
		935	PERL -> JSON
		936	The mapping from Perl to JSON is slightly more difficult, as Perl is a
		937	truly typeless language, so we can only guess which JSON type is meant
		938	by a Perl value.
		939
		940	hash references
		941	Perl hash references become JSON objects. As there is no inherent
		942	ordering in hash keys (or JSON objects), they will usually be
		943	encoded in a pseudo-random order. JSON::XS can optionally sort the
		944	hash keys (determined by the *canonical* flag), so the same
		945	datastructure will serialise to the same JSON text (given same
		946	settings and version of JSON::XS), but this incurs a runtime
		947	overhead and is only rarely useful, e.g. when you want to compare
		948	some JSON text against another for equality.
		949
		950	array references
		951	Perl array references become JSON arrays.
		952
		953	other references
		954	Other unblessed references are generally not allowed and will cause
		955	an exception to be thrown, except for references to the integers 0
		956	and 1, which get turned into "false" and "true" atoms in JSON.
		957
		958	Since "JSON::XS" uses the boolean model from Types::Serialiser, you
		959	can also "use Types::Serialiser" and then use
		960	"Types::Serialiser::false" and "Types::Serialiser::true" to improve
		961	readability.
		962
		963	use Types::Serialiser;
		964	encode_json [\0, Types::Serialiser::true] # yields [false,true]
		965
		966	Types::Serialiser::true, Types::Serialiser::false
		967	These special values from the Types::Serialiser module become JSON
		968	true and JSON false values, respectively. You can also use "\1" and
		969	"\0" directly if you want.
		970
		971	blessed objects
		972	Blessed objects are not directly representable in JSON, but
		973	"JSON::XS" allows various ways of handling objects. See "OBJECT
		974	SERIALISATION", below, for details.
		975
		976	simple scalars
		977	Simple Perl scalars (any scalar that is not a reference) are the
		978	most difficult objects to encode: JSON::XS will encode undefined
		979	scalars as JSON "null" values, scalars that have last been used in a
		980	string context before encoding as JSON strings, and anything else as
		981	number value:
		982
		983	# dump as number
		984	encode_json [2] # yields [2]
		985	encode_json [-3.0e17] # yields [-3e+17]
		986	my $value = 5; encode_json [$value] # yields [5]
		987
		988	# used as string, so dump as string
		989	print $value;
		990	encode_json [$value] # yields ["5"]
		991
		992	# undef becomes null
		993	encode_json [undef] # yields [null]
		994
		995	You can force the type to be a JSON string by stringifying it:
		996
		997	my $x = 3.1; # some variable containing a number
		998	"$x"; # stringified
		999	$x .= ""; # another, more awkward way to stringify
		1000	print $x; # perl does it for you, too, quite often
		1001
		1002	You can force the type to be a JSON number by numifying it:
		1003
		1004	my $x = "3"; # some variable containing a string
		1005	$x += 0; # numify it, ensuring it will be dumped as a number
		1006	$x *= 1; # same thing, the choice is yours.
		1007
		1008	You can not currently force the type in other, less obscure, ways.
		1009	Tell me if you need this capability (but don't forget to explain why
		1010	it's needed :).
		1011
		1012	Note that numerical precision has the same meaning as under Perl (so
		1013	binary to decimal conversion follows the same rules as in Perl,
		1014	which can differ to other languages). Also, your perl interpreter
		1015	might expose extensions to the floating point numbers of your
		1016	platform, such as infinities or NaN's - these cannot be represented
		1017	in JSON, and it is an error to pass those in.
		1018
		1019	OBJECT SERIALISATION
		1020	As JSON cannot directly represent Perl objects, you have to choose
		1021	between a pure JSON representation (without the ability to deserialise
		1022	the object automatically again), and a nonstandard extension to the JSON
		1023	syntax, tagged values.
		1024
		1025	SERIALISATION
		1026	What happens when "JSON::XS" encounters a Perl object depends on the
		1027	"allow_blessed", "convert_blessed" and "allow_tags" settings, which are
		1028	used in this order:
		1029
		1030	1. "allow_tags" is enabled and the object has a "FREEZE" method.
		1031	In this case, "JSON::XS" uses the Types::Serialiser object
		1032	serialisation protocol to create a tagged JSON value, using a
		1033	nonstandard extension to the JSON syntax.
		1034
		1035	This works by invoking the "FREEZE" method on the object, with the
		1036	first argument being the object to serialise, and the second
		1037	argument being the constant string "JSON" to distinguish it from
		1038	other serialisers.
		1039
		1040	The "FREEZE" method can return any number of values (i.e. zero or
		1041	more). These values and the paclkage/classname of the object will
		1042	then be encoded as a tagged JSON value in the following format:
		1043
		1044	("classname")[FREEZE return values...]
		1045
		1046	e.g.:
		1047
		1048	("URI")["http://www.google.com/"]
		1049	("MyDate")[2013,10,29]
		1050	("ImageData::JPEG")["Z3...VlCg=="]
		1051
		1052	For example, the hypothetical "My::Object" "FREEZE" method might use
		1053	the objects "type" and "id" members to encode the object:
		1054
		1055	sub My::Object::FREEZE {
		1056	my ($self, $serialiser) = @_;
		1057
		1058	($self->{type}, $self->{id})
		1059	}
		1060
		1061	2. "convert_blessed" is enabled and the object has a "TO_JSON" method.
		1062	In this case, the "TO_JSON" method of the object is invoked in
		1063	scalar context. It must return a single scalar that can be directly
		1064	encoded into JSON. This scalar replaces the object in the JSON text.
		1065
		1066	For example, the following "TO_JSON" method will convert all URI
		1067	objects to JSON strings when serialised. The fatc that these values
		1068	originally were URI objects is lost.
		1069
		1070	sub URI::TO_JSON {
		1071	my ($uri) = @_;
		1072	$uri->as_string
		1073	}
		1074
		1075	3. "allow_blessed" is enabled.
		1076	The object will be serialised as a JSON null value.
		1077
		1078	4. none of the above
		1079	If none of the settings are enabled or the respective methods are
		1080	missing, "JSON::XS" throws an exception.
		1081
		1082	DESERIALISATION
		1083	For deserialisation there are only two cases to consider: either
		1084	nonstandard tagging was used, in which case "allow_tags" decides, or
		1085	objects cannot be automatically be deserialised, in which case you can
		1086	use postprocessing or the "filter_json_object" or
		1087	"filter_json_single_key_object" callbacks to get some real objects our
		1088	of your JSON.
		1089
		1090	This section only considers the tagged value case: I a tagged JSON
		1091	object is encountered during decoding and "allow_tags" is disabled, a
		1092	parse error will result (as if tagged values were not part of the
		1093	grammar).
		1094
		1095	If "allow_tags" is enabled, "JSON::XS" will look up the "THAW" method of
		1096	the package/classname used during serialisation (it will not attempt to
		1097	load the package as a Perl module). If there is no such method, the
		1098	decoding will fail with an error.
		1099
		1100	Otherwise, the "THAW" method is invoked with the classname as first
		1101	argument, the constant string "JSON" as second argument, and all the
		1102	values from the JSON array (the values originally returned by the
		1103	"FREEZE" method) as remaining arguments.
		1104
		1105	The method must then return the object. While technically you can return
		1106	any Perl scalar, you might have to enable the "enable_nonref" setting to
		1107	make that work in all cases, so better return an actual blessed
		1108	reference.
		1109
		1110	As an example, let's implement a "THAW" function that regenerates the
		1111	"My::Object" from the "FREEZE" example earlier:
		1112
		1113	sub My::Object::THAW {
		1114	my ($class, $serialiser, $type, $id) = @_;
		1115
		1116	$class->new (type => $type, id => $id)
		1117	}
		1118
		1119	ENCODING/CODESET FLAG NOTES
		1120	The interested reader might have seen a number of flags that signify
		1121	encodings or codesets - "utf8", "latin1" and "ascii". There seems to be
		1122	some confusion on what these do, so here is a short comparison:
		1123
		1124	"utf8" controls whether the JSON text created by "encode" (and expected
		1125	by "decode") is UTF-8 encoded or not, while "latin1" and "ascii" only
		1126	control whether "encode" escapes character values outside their
		1127	respective codeset range. Neither of these flags conflict with each
		1128	other, although some combinations make less sense than others.
		1129
		1130	Care has been taken to make all flags symmetrical with respect to
		1131	"encode" and "decode", that is, texts encoded with any combination of
		1132	these flag values will be correctly decoded when the same flags are used
		1133	- in general, if you use different flag settings while encoding vs. when
		1134	decoding you likely have a bug somewhere.
		1135
		1136	Below comes a verbose discussion of these flags. Note that a "codeset"
		1137	is simply an abstract set of character-codepoint pairs, while an
		1138	encoding takes those codepoint numbers and *encodes* them, in our case
		1139	into octets. Unicode is (among other things) a codeset, UTF-8 is an
		1140	encoding, and ISO-8859-1 (= latin 1) and ASCII are both codesets *and*
		1141	encodings at the same time, which can be confusing.
		1142
		1143	"utf8" flag disabled
		1144	When "utf8" is disabled (the default), then "encode"/"decode"
		1145	generate and expect Unicode strings, that is, characters with high
		1146	ordinal Unicode values (> 255) will be encoded as such characters,
		1147	and likewise such characters are decoded as-is, no changes to them
		1148	will be done, except "(re-)interpreting" them as Unicode codepoints
		1149	or Unicode characters, respectively (to Perl, these are the same
		1150	thing in strings unless you do funny/weird/dumb stuff).
		1151
		1152	This is useful when you want to do the encoding yourself (e.g. when
		1153	you want to have UTF-16 encoded JSON texts) or when some other layer
		1154	does the encoding for you (for example, when printing to a terminal
		1155	using a filehandle that transparently encodes to UTF-8 you certainly
		1156	do NOT want to UTF-8 encode your data first and have Perl encode it
		1157	another time).
		1158
		1159	"utf8" flag enabled
		1160	If the "utf8"-flag is enabled, "encode"/"decode" will encode all
		1161	characters using the corresponding UTF-8 multi-byte sequence, and
		1162	will expect your input strings to be encoded as UTF-8, that is, no
		1163	"character" of the input string must have any value > 255, as UTF-8
		1164	does not allow that.
		1165
		1166	The "utf8" flag therefore switches between two modes: disabled means
		1167	you will get a Unicode string in Perl, enabled means you get an
		1168	UTF-8 encoded octet/binary string in Perl.
		1169
		1170	"latin1" or "ascii" flags enabled
		1171	With "latin1" (or "ascii") enabled, "encode" will escape characters
		1172	with ordinal values > 255 (> 127 with "ascii") and encode the
		1173	remaining characters as specified by the "utf8" flag.
		1174
		1175	If "utf8" is disabled, then the result is also correctly encoded in
		1176	those character sets (as both are proper subsets of Unicode, meaning
		1177	that a Unicode string with all character values < 256 is the same
		1178	thing as a ISO-8859-1 string, and a Unicode string with all
		1179	character values < 128 is the same thing as an ASCII string in
213	properly).	1180	Perl).
214		1181
215	No roundtripping (strings get clobbered if they look like numbers,	1182	If "utf8" is enabled, you still get a correct UTF-8-encoded string,
216	e.g. the string 2.0 will encode to 2.0 instead of "2.0", and that	1183	regardless of these flags, just some more characters will be escaped
217	will decode into the number 2.	1184	using "\uXXXX" then before.
218		1185
219	JSON::PC 0.01	1186	Note that ISO-8859-1-*encoded* strings are not compatible with UTF-8
220	Very fast.	1187	encoding, while ASCII-encoded strings are. That is because the
		1188	ISO-8859-1 encoding is NOT a subset of UTF-8 (despite the ISO-8859-1
		1189	*codeset* being a subset of Unicode), while ASCII is.
221		1190
222	Undocumented/buggy Unicode handling.	1191	Surprisingly, "decode" will ignore these flags and so treat all
		1192	input values as governed by the "utf8" flag. If it is disabled, this
		1193	allows you to decode ISO-8859-1- and ASCII-encoded strings, as both
		1194	strict subsets of Unicode. If it is enabled, you can correctly
		1195	decode UTF-8 encoded strings.
223		1196
224	No roundtripping.	1197	So neither "latin1" nor "ascii" are incompatible with the "utf8"
		1198	flag - they only govern when the JSON output engine escapes a
		1199	character or not.
225		1200
226	Has problems handling many Perl values (e.g. regex results and other	1201	The main use for "latin1" is to relatively efficiently store binary
227	magic values will make it croak).	1202	data as JSON, at the expense of breaking compatibility with most
		1203	JSON decoders.
228		1204
229	Does not even generate valid JSON ("{1,2}" gets converted to "{1:2}"	1205	The main use for "ascii" is to force the output to not contain
230	which is not a valid JSON string.	1206	characters with values > 127, which means you can interpret the
		1207	resulting string as UTF-8, ISO-8859-1, ASCII, KOI8-R or most about
		1208	any character set and 8-bit-encoding, and still get the same data
		1209	structure back. This is useful when your channel for JSON transfer
		1210	is not 8-bit clean or the encoding might be mangled in between (e.g.
		1211	in mail), and works because ASCII is a proper subset of most 8-bit
		1212	and multibyte encodings in use in the world.
231		1213
232	Unmaintained (maintainer unresponsive for many months, bugs are not	1214	JSON and ECMAscript
233	getting fixed).	1215	JSON syntax is based on how literals are represented in javascript (the
		1216	not-standardised predecessor of ECMAscript) which is presumably why it
		1217	is called "JavaScript Object Notation".
234		1218
235	JSON::Syck 0.21	1219	However, JSON is not a subset (and also not a superset of course) of
236	Very buggy (often crashes).	1220	ECMAscript (the standard) or javascript (whatever browsers actually
		1221	implement).
237		1222
238	Very inflexible (no human-readable format supported, format pretty	1223	If you want to use javascript's "eval" function to "parse" JSON, you
239	much undocumented. I need at least a format for easy reading by	1224	might run into parse errors for valid JSON texts, or the resulting data
240	humans and a single-line compact format for use in a protocol, and	1225	structure might not be queryable:
241	preferably a way to generate ASCII-only JSON strings).
242		1226
243	Completely broken (and confusingly documented) Unicode handling	1227	One of the problems is that U+2028 and U+2029 are valid characters
244	(unicode escapes are not working properly, you need to set	1228	inside JSON strings, but are not allowed in ECMAscript string literals,
245	ImplicitUnicode to *different* values on en- and decoding to get	1229	so the following Perl fragment will not output something that can be
246	symmetric behaviour).	1230	guaranteed to be parsable by javascript's "eval":
247		1231
248	No roundtripping (simple cases work, but this depends on wether the	1232	use JSON::XS;
249	scalar value was used in a numeric context or not).
250		1233
251	Dumping hashes may skip hash values depending on iterator state.	1234	print encode_json [chr 0x2028];
252		1235
253	Unmaintained (maintainer unresponsive for many months, bugs are not	1236	The right fix for this is to use a proper JSON parser in your javascript
254	getting fixed).	1237	programs, and not rely on "eval" (see for example Douglas Crockford's
		1238	json2.js parser).
255		1239
256	Does not check input for validity (i.e. will accept non-JSON input	1240	If this is not an option, you can, as a stop-gap measure, simply encode
257	and return "something" instead of raising an exception. This is a	1241	to ASCII-only JSON:
258	security issue: imagine two banks transfering money between each
259	other using JSON. One bank might parse a given non-JSON request and
260	deduct money, while the other might reject the transaction with a
261	syntax error. While a good protocol will at least recover, that is
262	extra unnecessary work and the transaction will still not succeed).
263		1242
264	JSON::DWIW 0.04	1243	use JSON::XS;
265	Very fast. Very natural. Very nice.
266		1244
267	Undocumented unicode handling (but the best of the pack. Unicode	1245	print JSON::XS->new->ascii->encode ([chr 0x2028]);
268	escapes still don't get parsed properly).
269		1246
270	Very inflexible.	1247	Note that this will enlarge the resulting JSON text quite a bit if you
		1248	have many non-ASCII characters. You might be tempted to run some regexes
		1249	to only escape U+2028 and U+2029, e.g.:
271		1250
272	No roundtripping.	1251	# DO NOT USE THIS!
		1252	my $json = JSON::XS->new->utf8->encode ([chr 0x2028]);
		1253	$json =~ s/\xe2\x80\xa8/\\u2028/g; # escape U+2028
		1254	$json =~ s/\xe2\x80\xa9/\\u2029/g; # escape U+2029
		1255	print $json;
273		1256
274	Does not generate valid JSON (key strings are often unquoted, empty	1257	Note that *this is a bad idea*: the above only works for U+2028 and
275	keys result in nothing being output)	1258	U+2029 and thus only for fully ECMAscript-compliant parsers. Many
		1259	existing javascript implementations, however, have issues with other
		1260	characters as well - using "eval" naively simply *will* cause problems.
276		1261
277	Does not check input for validity.	1262	Another problem is that some javascript implementations reserve some
		1263	property names for their own purposes (which probably makes them
		1264	non-ECMAscript-compliant). For example, Iceweasel reserves the
		1265	"__proto__" property name for its own purposes.
		1266
		1267	If that is a problem, you could parse try to filter the resulting JSON
		1268	output for these property strings, e.g.:
		1269
		1270	$json =~ s/"__proto__"\s*:/"__proto__renamed":/g;
		1271
		1272	This works because "__proto__" is not valid outside of strings, so every
		1273	occurrence of ""__proto__"\s*:" must be a string used as property name.
		1274
		1275	If you know of other incompatibilities, please let me know.
		1276
		1277	JSON and YAML
		1278	You often hear that JSON is a subset of YAML. This is, however, a mass
		1279	hysteria(*) and very far from the truth (as of the time of this
		1280	writing), so let me state it clearly: *in general, there is no way to
		1281	configure JSON::XS to output a data structure as valid YAML* that works
		1282	in all cases.
		1283
		1284	If you really must use JSON::XS to generate YAML, you should use this
		1285	algorithm (subject to change in future versions):
		1286
		1287	my $to_yaml = JSON::XS->new->utf8->space_after (1);
		1288	my $yaml = $to_yaml->encode ($ref) . "\n";
		1289
		1290	This will *usually* generate JSON texts that also parse as valid YAML.
		1291	Please note that YAML has hardcoded limits on (simple) object key
		1292	lengths that JSON doesn't have and also has different and incompatible
		1293	unicode character escape syntax, so you should make sure that your hash
		1294	keys are noticeably shorter than the 1024 "stream characters" YAML
		1295	allows and that you do not have characters with codepoint values outside
		1296	the Unicode BMP (basic multilingual page). YAML also does not allow "\/"
		1297	sequences in strings (which JSON::XS does not *currently* generate, but
		1298	other JSON generators might).
		1299
		1300	There might be other incompatibilities that I am not aware of (or the
		1301	YAML specification has been changed yet again - it does so quite often).
		1302	In general you should not try to generate YAML with a JSON generator or
		1303	vice versa, or try to parse JSON with a YAML parser or vice versa:
		1304	chances are high that you will run into severe interoperability problems
		1305	when you least expect it.
		1306
		1307	(*) I have been pressured multiple times by Brian Ingerson (one of the
		1308	authors of the YAML specification) to remove this paragraph, despite
		1309	him acknowledging that the actual incompatibilities exist. As I was
		1310	personally bitten by this "JSON is YAML" lie, I refused and said I
		1311	will continue to educate people about these issues, so others do not
		1312	run into the same problem again and again. After this, Brian called
		1313	me a (quote)*complete and worthless idiot*(unquote).
		1314
		1315	In my opinion, instead of pressuring and insulting people who
		1316	actually clarify issues with YAML and the wrong statements of some
		1317	of its proponents, I would kindly suggest reading the JSON spec
		1318	(which is not that difficult or long) and finally make YAML
		1319	compatible to it, and educating users about the changes, instead of
		1320	spreading lies about the real compatibility for many *years* and
		1321	trying to silence people who point out that it isn't true.
		1322
		1323	Addendum/2009: the YAML 1.2 spec is still incompatible with JSON,
		1324	even though the incompatibilities have been documented (and are
		1325	known to Brian) for many years and the spec makes explicit claims
		1326	that YAML is a superset of JSON. It would be so easy to fix, but
		1327	apparently, bullying people and corrupting userdata is so much
		1328	easier.
278		1329
279	SPEED	1330	SPEED
280	It seems that JSON::XS is surprisingly fast, as shown in the following	1331	It seems that JSON::XS is surprisingly fast, as shown in the following
281	tables. They have been generated with the help of the "eg/bench" program	1332	tables. They have been generated with the help of the "eg/bench" program
282	in the JSON::XS distribution, to make it easy to compare on your own	1333	in the JSON::XS distribution, to make it easy to compare on your own
283	system.	1334	system.
284		1335
285	First is a comparison between various modules using a very simple JSON	1336	First comes a comparison between various modules using a very short
		1337	single-line JSON string (also available at
		1338	<http://dist.schmorp.de/misc/json/short.json>).
		1339
		1340	{"method": "handleMessage", "params": ["user1",
		1341	"we were just talking"], "id": null, "array":[1,11,234,-5,1e5,1e7,
		1342	1, 0]}
		1343
286	string, showing the number of encodes/decodes per second (JSON::XS is	1344	It shows the number of encodes/decodes per second (JSON::XS uses the
287	the functional interface, while JSON::XS/2 is the OO interface with	1345	functional interface, while JSON::XS/2 uses the OO interface with
288	pretty-printing and hashkey sorting enabled).	1346	pretty-printing and hashkey sorting enabled, JSON::XS/3 enables shrink.
		1347	JSON::DWIW/DS uses the deserialise function, while JSON::DWIW::FJ uses
		1348	the from_json method). Higher is better:
289		1349
290	module | encode | decode |	1350	module | encode | decode |
291	-----------|------------|------------|	1351	--------------|------------|------------|
292	JSON | 14006 | 6820 |	1352	JSON::DWIW/DS | 86302.551 | 102300.098 |
293	JSON::DWIW | 200937 | 120386 |	1353	JSON::DWIW/FJ | 86302.551 | 75983.768 |
294	JSON::PC | 85065 | 129366 |	1354	JSON::PP | 15827.562 | 6638.658 |
295	JSON::Syck | 59898 | 44232 |	1355	JSON::Syck | 63358.066 | 47662.545 |
296	JSON::XS | 1171478 | 342435 |	1356	JSON::XS | 511500.488 | 511500.488 |
297	JSON::XS/2 | 730760 | 328714 |	1357	JSON::XS/2 | 291271.111 | 388361.481 |
		1358	JSON::XS/3 | 361577.931 | 361577.931 |
		1359	Storable | 66788.280 | 265462.278 |
298	-----------+------------+------------+	1360	--------------+------------+------------+
299		1361
300	That is, JSON::XS is 6 times faster than than JSON::DWIW and about 80	1362	That is, JSON::XS is almost six times faster than JSON::DWIW on
301	times faster than JSON, even with pretty-printing and key sorting.	1363	encoding, about five times faster on decoding, and over thirty to
		1364	seventy times faster than JSON's pure perl implementation. It also
		1365	compares favourably to Storable for small amounts of data.
302		1366
303	Using a longer test string (roughly 8KB, generated from Yahoo! Locals	1367	Using a longer test string (roughly 18KB, generated from Yahoo! Locals
304	search API (http://nanoref.com/yahooapis/mgPdGg):	1368	search API (<http://dist.schmorp.de/misc/json/long.json>).
305		1369
306	module | encode | decode |	1370	module | encode | decode |
307	-----------|------------|------------|	1371	--------------|------------|------------|
308	JSON | 673 | 38 |	1372	JSON::DWIW/DS | 1647.927 | 2673.916 |
309	JSON::DWIW | 5271 | 770 |	1373	JSON::DWIW/FJ | 1630.249 | 2596.128 |
310	JSON::PC | 9901 | 2491 |	1374	JSON::PP | 400.640 | 62.311 |
311	JSON::Syck | 2360 | 786 |	1375	JSON::Syck | 1481.040 | 1524.869 |
312	JSON::XS | 37398 | 3202 |	1376	JSON::XS | 20661.596 | 9541.183 |
313	JSON::XS/2 | 13765 | 3153 |	1377	JSON::XS/2 | 10683.403 | 9416.938 |
		1378	JSON::XS/3 | 20661.596 | 9400.054 |
		1379	Storable | 19765.806 | 10000.725 |
314	-----------+------------+------------+	1380	--------------+------------+------------+
315		1381
316	Again, JSON::XS leads by far in the encoding case, while still beating	1382	Again, JSON::XS leads by far (except for Storable which non-surprisingly
317	every other module in the decoding case.	1383	decodes a bit faster).
318		1384
319	Last example is an almost 8MB large hash with many large binary values	1385	On large strings containing lots of high Unicode characters, some
320	(PNG files), resulting in a lot of escaping:	1386	modules (such as JSON::PC) seem to decode faster than JSON::XS, but the
		1387	result will be broken due to missing (or wrong) Unicode handling. Others
		1388	refuse to decode or encode properly, so it was impossible to prepare a
		1389	fair comparison table for that case.
		1390
		1391	SECURITY CONSIDERATIONS
		1392	When you are using JSON in a protocol, talking to untrusted potentially
		1393	hostile creatures requires relatively few measures.
		1394
		1395	First of all, your JSON decoder should be secure, that is, should not
		1396	have any buffer overflows. Obviously, this module should ensure that and
		1397	I am trying hard on making that true, but you never know.
		1398
		1399	Second, you need to avoid resource-starving attacks. That means you
		1400	should limit the size of JSON texts you accept, or make sure then when
		1401	your resources run out, that's just fine (e.g. by using a separate
		1402	process that can crash safely). The size of a JSON text in octets or
		1403	characters is usually a good indication of the size of the resources
		1404	required to decode it into a Perl structure. While JSON::XS can check
		1405	the size of the JSON text, it might be too late when you already have it
		1406	in memory, so you might want to check the size before you accept the
		1407	string.
		1408
		1409	Third, JSON::XS recurses using the C stack when decoding objects and
		1410	arrays. The C stack is a limited resource: for instance, on my amd64
		1411	machine with 8MB of stack size I can decode around 180k nested arrays
		1412	but only 14k nested JSON objects (due to perl itself recursing deeply on
		1413	croak to free the temporary). If that is exceeded, the program crashes.
		1414	To be conservative, the default nesting limit is set to 512. If your
		1415	process has a smaller stack, you should adjust this setting accordingly
		1416	with the "max_depth" method.
		1417
		1418	Something else could bomb you, too, that I forgot to think of. In that
		1419	case, you get to keep the pieces. I am always open for hints, though...
		1420
		1421	Also keep in mind that JSON::XS might leak contents of your Perl data
		1422	structures in its error messages, so when you serialise sensitive
		1423	information you might want to make sure that exceptions thrown by
		1424	JSON::XS will not end up in front of untrusted eyes.
		1425
		1426	If you are using JSON::XS to return packets to consumption by JavaScript
		1427	scripts in a browser you should have a look at
		1428	<http://blog.archive.jpsykes.com/47/practical-csrf-and-json-security/>
		1429	to see whether you are vulnerable to some common attack vectors (which
		1430	really are browser design bugs, but it is still you who will have to
		1431	deal with it, as major browser developers care only for features, not
		1432	about getting security right).
		1433
		1434	INTEROPERABILITY WITH OTHER MODULES
		1435	"JSON::XS" uses the Types::Serialiser module to provide boolean
		1436	constants. That means that the JSON true and false values will be
		1437	comaptible to true and false values of iother modules that do the same,
		1438	such as JSON::PP and CBOR::XS.
		1439
		1440	THREADS
		1441	This module is *not* guaranteed to be thread safe and there are no plans
		1442	to change this until Perl gets thread support (as opposed to the
		1443	horribly slow so-called "threads" which are simply slow and bloated
		1444	process simulations - use fork, it's *much* faster, cheaper, better).
		1445
		1446	(It might actually work, but you have been warned).
		1447
		1448	THE PERILS OF SETLOCALE
		1449	Sometimes people avoid the Perl locale support and directly call the
		1450	system's setlocale function with "LC_ALL".
		1451
		1452	This breaks both perl and modules such as JSON::XS, as stringification
		1453	of numbers no longer works correctly (e.g. "$x = 0.1; print "$x"+1"
		1454	might print 1, and JSON::XS might output illegal JSON as JSON::XS relies
		1455	on perl to stringify numbers).
		1456
		1457	The solution is simple: don't call "setlocale", or use it for only those
		1458	categories you need, such as "LC_MESSAGES" or "LC_CTYPE".
		1459
		1460	If you need "LC_NUMERIC", you should enable it only around the code that
		1461	actually needs it (avoiding stringification of numbers), and restore it
		1462	afterwards.
321		1463
322	BUGS	1464	BUGS
323	While the goal of this module is to be correct, that unfortunately does	1465	While the goal of this module is to be correct, that unfortunately does
324	not mean its bug-free, only that I think its design is bug-free. It is	1466	not mean it's bug-free, only that I think its design is bug-free. If you
325	still very young and not well-tested. If you keep reporting bugs they	1467	keep reporting bugs they will be fixed swiftly, though.
326	will be fixed swiftly, though.	1468
		1469	Please refrain from using rt.cpan.org or any other bug reporting
		1470	service. I put the contact address into my modules for a reason.
		1471
		1472	SEE ALSO
		1473	The json_xs command line utility for quick experiments.
327		1474
328	AUTHOR	1475	AUTHOR
329	Marc Lehmann <schmorp@schmorp.de>	1476	Marc Lehmann <schmorp@schmorp.de>
330	http://home.schmorp.de/	1477	http://home.schmorp.de/
331		1478

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