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Revision 1.36 by root, Wed Jul 27 15:53:40 2011 UTC

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

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