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Comparing JSON-XS/README (file contents):
Revision 1.6 by root, Sat Mar 24 19:42:14 2007 UTC vs.
Revision 1.34 by root, Thu Mar 11 17:36:09 2010 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, the deserialised data structure is identical on
42	the Perl level. (e.g. the string "2.0" doesn't suddenly become "2").	62	the Perl level. (e.g. the string "2.0" doesn't suddenly become "2"
		63	just because it looks like a number). There minor are exceptions
		64	to this, read the MAPPING section below to learn about those.
43		65
44	* strict checking of JSON correctness	66	* strict checking of JSON correctness
		67
45	There is no guessing, no generating of illegal JSON texts by	68	There is no guessing, no generating of illegal JSON texts by
46	default, and only JSON is accepted as input by default (the latter	69	default, and only JSON is accepted as input by default (the latter
47	is a security feature).	70	is a security feature).
48		71
49	* fast	72	* fast
50	Compared to other JSON modules, this module compares favourably in
51	terms of speed, too.
52		73
		74	Compared to other JSON modules and other serialisers such as
		75	Storable, this module usually compares favourably in terms of speed,
		76	too.
		77
53	* simple to use	78	* simple to use
		79
54	This module has both a simple functional interface as well as an OO	80	This module has both a simple functional interface as well as an
55	interface.	81	object oriented interface interface.
56		82
57	* reasonably versatile output formats	83	* reasonably versatile output formats
		84
58	You can choose between the most compact guarenteed single-line	85	You can choose between the most compact guaranteed-single-line
59	format possible (nice for simple line-based protocols), a pure-ascii	86	format possible (nice for simple line-based protocols), a pure-ASCII
60	format (for when your transport is not 8-bit clean), or a	87	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	88	the whole Unicode range), or a pretty-printed format (for when you
62	can combine those features in whatever way you like.	89	want to read that stuff). Or you can combine those features in
		90	whatever way you like.
63		91
64	FUNCTIONAL INTERFACE	92	FUNCTIONAL INTERFACE
65	The following convinience methods are provided by this module. They are	93	The following convenience methods are provided by this module. They are
66	exported by default:	94	exported by default:
67		95
68	$json_text = to_json $perl_scalar	96	$json_text = encode_json $perl_scalar
69	Converts the given Perl data structure (a simple scalar or a	97	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.	98	string (that is, the string contains octets only). Croaks on error.
72		99
73	This function call is functionally identical to:	100	This function call is functionally identical to:
74		101
75	$json_text = JSON::XS->new->utf8->encode ($perl_scalar)	102	$json_text = JSON::XS->new->utf8->encode ($perl_scalar)
76		103
77	except being faster.	104	Except being faster.
78		105
79	$perl_scalar = from_json $json_text	106	$perl_scalar = decode_json $json_text
80	The opposite of "to_json": expects an UTF-8 (binary) string and	107	The opposite of "encode_json": expects an UTF-8 (binary) string and
81	tries to parse that as an UTF-8 encoded JSON text, returning the	108	tries to parse that as an UTF-8 encoded JSON text, returning the
82	resulting simple scalar or reference. Croaks on error.	109	resulting reference. Croaks on error.
83		110
84	This function call is functionally identical to:	111	This function call is functionally identical to:
85		112
86	$perl_scalar = JSON::XS->new->utf8->decode ($json_text)	113	$perl_scalar = JSON::XS->new->utf8->decode ($json_text)
87		114
88	except being faster.	115	Except being faster.
		116
		117	$is_boolean = JSON::XS::is_bool $scalar
		118	Returns true if the passed scalar represents either JSON::XS::true
		119	or JSON::XS::false, two constants that act like 1 and 0,
		120	respectively and are used to represent JSON "true" and "false"
		121	values in Perl.
		122
		123	See MAPPING, below, for more information on how JSON values are
		124	mapped to Perl.
		125
		126	A FEW NOTES ON UNICODE AND PERL
		127	Since this often leads to confusion, here are a few very clear words on
		128	how Unicode works in Perl, modulo bugs.
		129
		130	1. Perl strings can store characters with ordinal values > 255.
		131	This enables you to store Unicode characters as single characters in
		132	a Perl string - very natural.
		133
		134	2. Perl does not associate an encoding with your strings.
		135	... until you force it to, e.g. when matching it against a regex, or
		136	printing the scalar to a file, in which case Perl either interprets
		137	your string as locale-encoded text, octets/binary, or as Unicode,
		138	depending on various settings. In no case is an encoding stored
		139	together with your data, it is use that decides encoding, not any
		140	magical meta data.
		141
		142	3. The internal utf-8 flag has no meaning with regards to the encoding
		143	of your string.
		144	Just ignore that flag unless you debug a Perl bug, a module written
		145	in XS or want to dive into the internals of perl. Otherwise it will
		146	only confuse you, as, despite the name, it says nothing about how
		147	your string is encoded. You can have Unicode strings with that flag
		148	set, with that flag clear, and you can have binary data with that
		149	flag set and that flag clear. Other possibilities exist, too.
		150
		151	If you didn't know about that flag, just the better, pretend it
		152	doesn't exist.
		153
		154	4. A "Unicode String" is simply a string where each character can be
		155	validly interpreted as a Unicode code point.
		156	If you have UTF-8 encoded data, it is no longer a Unicode string,
		157	but a Unicode string encoded in UTF-8, giving you a binary string.
		158
		159	5. A string containing "high" (> 255) character values is not a UTF-8
		160	string.
		161	It's a fact. Learn to live with it.
		162
		163	I hope this helps :)
89		164
90	OBJECT-ORIENTED INTERFACE	165	OBJECT-ORIENTED INTERFACE
91	The object oriented interface lets you configure your own encoding or	166	The object oriented interface lets you configure your own encoding or
92	decoding style, within the limits of supported formats.	167	decoding style, within the limits of supported formats.
93		168
101		176
102	my $json = JSON::XS->new->utf8->space_after->encode ({a => [1,2]})	177	my $json = JSON::XS->new->utf8->space_after->encode ({a => [1,2]})
103	=> {"a": [1, 2]}	178	=> {"a": [1, 2]}
104		179
105	$json = $json->ascii ([$enable])	180	$json = $json->ascii ([$enable])
		181	$enabled = $json->get_ascii
106	If $enable is true (or missing), then the "encode" method will not	182	If $enable is true (or missing), then the "encode" method will not
107	generate characters outside the code range 0..127 (which is ASCII).	183	generate characters outside the code range 0..127 (which is ASCII).
108	Any unicode characters outside that range will be escaped using	184	Any Unicode characters outside that range will be escaped using
109	either a single \uXXXX (BMP characters) or a double \uHHHH\uLLLLL	185	either a single \uXXXX (BMP characters) or a double \uHHHH\uLLLLL
110	escape sequence, as per RFC4627.	186	escape sequence, as per RFC4627. The resulting encoded JSON text can
		187	be treated as a native Unicode string, an ascii-encoded,
		188	latin1-encoded or UTF-8 encoded string, or any other superset of
		189	ASCII.
111		190
112	If $enable is false, then the "encode" method will not escape	191	If $enable is false, then the "encode" method will not escape
113	Unicode characters unless required by the JSON syntax. This results	192	Unicode characters unless required by the JSON syntax or other
114	in a faster and more compact format.	193	flags. This results in a faster and more compact format.
		194
		195	See also the section ENCODING/CODESET FLAG NOTES later in this
		196	document.
		197
		198	The main use for this flag is to produce JSON texts that can be
		199	transmitted over a 7-bit channel, as the encoded JSON texts will not
		200	contain any 8 bit characters.
115		201
116	JSON::XS->new->ascii (1)->encode ([chr 0x10401])	202	JSON::XS->new->ascii (1)->encode ([chr 0x10401])
117	=> ["\ud801\udc01"]	203	=> ["\ud801\udc01"]
118		204
		205	$json = $json->latin1 ([$enable])
		206	$enabled = $json->get_latin1
		207	If $enable is true (or missing), then the "encode" method will
		208	encode the resulting JSON text as latin1 (or iso-8859-1), escaping
		209	any characters outside the code range 0..255. The resulting string
		210	can be treated as a latin1-encoded JSON text or a native Unicode
		211	string. The "decode" method will not be affected in any way by this
		212	flag, as "decode" by default expects Unicode, which is a strict
		213	superset of latin1.
		214
		215	If $enable is false, then the "encode" method will not escape
		216	Unicode characters unless required by the JSON syntax or other
		217	flags.
		218
		219	See also the section ENCODING/CODESET FLAG NOTES later in this
		220	document.
		221
		222	The main use for this flag is efficiently encoding binary data as
		223	JSON text, as most octets will not be escaped, resulting in a
		224	smaller encoded size. The disadvantage is that the resulting JSON
		225	text is encoded in latin1 (and must correctly be treated as such
		226	when storing and transferring), a rare encoding for JSON. It is
		227	therefore most useful when you want to store data structures known
		228	to contain binary data efficiently in files or databases, not when
		229	talking to other JSON encoders/decoders.
		230
		231	JSON::XS->new->latin1->encode (["\x{89}\x{abc}"]
		232	=> ["\x{89}\\u0abc"] # (perl syntax, U+abc escaped, U+89 not)
		233
119	$json = $json->utf8 ([$enable])	234	$json = $json->utf8 ([$enable])
		235	$enabled = $json->get_utf8
120	If $enable is true (or missing), then the "encode" method will	236	If $enable is true (or missing), then the "encode" method will
121	encode the JSON result into UTF-8, as required by many protocols,	237	encode the JSON result into UTF-8, as required by many protocols,
122	while the "decode" method expects to be handled an UTF-8-encoded	238	while the "decode" method expects to be handled an UTF-8-encoded
123	string. Please note that UTF-8-encoded strings do not contain any	239	string. Please note that UTF-8-encoded strings do not contain any
124	characters outside the range 0..255, they are thus useful for	240	characters outside the range 0..255, they are thus useful for
125	bytewise/binary I/O. In future versions, enabling this option might	241	bytewise/binary I/O. In future versions, enabling this option might
126	enable autodetection of the UTF-16 and UTF-32 encoding families, as	242	enable autodetection of the UTF-16 and UTF-32 encoding families, as
127	described in RFC4627.	243	described in RFC4627.
128		244
129	If $enable is false, then the "encode" method will return the JSON	245	If $enable is false, then the "encode" method will return the JSON
130	string as a (non-encoded) unicode string, while "decode" expects	246	string as a (non-encoded) Unicode string, while "decode" expects
131	thus a unicode string. Any decoding or encoding (e.g. to UTF-8 or	247	thus a Unicode string. Any decoding or encoding (e.g. to UTF-8 or
132	UTF-16) needs to be done yourself, e.g. using the Encode module.	248	UTF-16) needs to be done yourself, e.g. using the Encode module.
		249
		250	See also the section ENCODING/CODESET FLAG NOTES later in this
		251	document.
133		252
134	Example, output UTF-16BE-encoded JSON:	253	Example, output UTF-16BE-encoded JSON:
135		254
136	use Encode;	255	use Encode;
137	$jsontext = encode "UTF-16BE", JSON::XS->new->encode ($object);	256	$jsontext = encode "UTF-16BE", JSON::XS->new->encode ($object);
…		…
156	2	275	2
157	]	276	]
158	}	277	}
159		278
160	$json = $json->indent ([$enable])	279	$json = $json->indent ([$enable])
		280	$enabled = $json->get_indent
161	If $enable is true (or missing), then the "encode" method will use a	281	If $enable is true (or missing), then the "encode" method will use a
162	multiline format as output, putting every array member or	282	multiline format as output, putting every array member or
163	object/hash key-value pair into its own line, identing them	283	object/hash key-value pair into its own line, indenting them
164	properly.	284	properly.
165		285
166	If $enable is false, no newlines or indenting will be produced, and	286	If $enable is false, no newlines or indenting will be produced, and
167	the resulting JSON text is guarenteed not to contain any "newlines".	287	the resulting JSON text is guaranteed not to contain any "newlines".
168		288
169	This setting has no effect when decoding JSON texts.	289	This setting has no effect when decoding JSON texts.
170		290
171	$json = $json->space_before ([$enable])	291	$json = $json->space_before ([$enable])
		292	$enabled = $json->get_space_before
172	If $enable is true (or missing), then the "encode" method will add	293	If $enable is true (or missing), then the "encode" method will add
173	an extra optional space before the ":" separating keys from values	294	an extra optional space before the ":" separating keys from values
174	in JSON objects.	295	in JSON objects.
175		296
176	If $enable is false, then the "encode" method will not add any extra	297	If $enable is false, then the "encode" method will not add any extra
…		…
182	Example, space_before enabled, space_after and indent disabled:	303	Example, space_before enabled, space_after and indent disabled:
183		304
184	{"key" :"value"}	305	{"key" :"value"}
185		306
186	$json = $json->space_after ([$enable])	307	$json = $json->space_after ([$enable])
		308	$enabled = $json->get_space_after
187	If $enable is true (or missing), then the "encode" method will add	309	If $enable is true (or missing), then the "encode" method will add
188	an extra optional space after the ":" separating keys from values in	310	an extra optional space after the ":" separating keys from values in
189	JSON objects and extra whitespace after the "," separating key-value	311	JSON objects and extra whitespace after the "," separating key-value
190	pairs and array members.	312	pairs and array members.
191		313
…		…
196		318
197	Example, space_before and indent disabled, space_after enabled:	319	Example, space_before and indent disabled, space_after enabled:
198		320
199	{"key": "value"}	321	{"key": "value"}
200		322
		323	$json = $json->relaxed ([$enable])
		324	$enabled = $json->get_relaxed
		325	If $enable is true (or missing), then "decode" will accept some
		326	extensions to normal JSON syntax (see below). "encode" will not be
		327	affected in anyway. *Be aware that this option makes you accept
		328	invalid JSON texts as if they were valid!*. I suggest only to use
		329	this option to parse application-specific files written by humans
		330	(configuration files, resource files etc.)
		331
		332	If $enable is false (the default), then "decode" will only accept
		333	valid JSON texts.
		334
		335	Currently accepted extensions are:
		336
		337	* list items can have an end-comma
		338
		339	JSON separates array elements and key-value pairs with commas.
		340	This can be annoying if you write JSON texts manually and want
		341	to be able to quickly append elements, so this extension accepts
		342	comma at the end of such items not just between them:
		343
		344	[
		345	1,
		346	2, <- this comma not normally allowed
		347	]
		348	{
		349	"k1": "v1",
		350	"k2": "v2", <- this comma not normally allowed
		351	}
		352
		353	* shell-style '#'-comments
		354
		355	Whenever JSON allows whitespace, shell-style comments are
		356	additionally allowed. They are terminated by the first
		357	carriage-return or line-feed character, after which more
		358	white-space and comments are allowed.
		359
		360	[
		361	1, # this comment not allowed in JSON
		362	# neither this one...
		363	]
		364
201	$json = $json->canonical ([$enable])	365	$json = $json->canonical ([$enable])
		366	$enabled = $json->get_canonical
202	If $enable is true (or missing), then the "encode" method will	367	If $enable is true (or missing), then the "encode" method will
203	output JSON objects by sorting their keys. This is adding a	368	output JSON objects by sorting their keys. This is adding a
204	comparatively high overhead.	369	comparatively high overhead.
205		370
206	If $enable is false, then the "encode" method will output key-value	371	If $enable is false, then the "encode" method will output key-value
207	pairs in the order Perl stores them (which will likely change	372	pairs in the order Perl stores them (which will likely change
208	between runs of the same script).	373	between runs of the same script).
209		374
210	This option is useful if you want the same data structure to be	375	This option is useful if you want the same data structure to be
211	encoded as the same JSON text (given the same overall settings). If	376	encoded as the same JSON text (given the same overall settings). If
212	it is disabled, the same hash migh be encoded differently even if	377	it is disabled, the same hash might be encoded differently even if
213	contains the same data, as key-value pairs have no inherent ordering	378	contains the same data, as key-value pairs have no inherent ordering
214	in Perl.	379	in Perl.
215		380
216	This setting has no effect when decoding JSON texts.	381	This setting has no effect when decoding JSON texts.
217		382
		383	This setting has currently no effect on tied hashes.
		384
218	$json = $json->allow_nonref ([$enable])	385	$json = $json->allow_nonref ([$enable])
		386	$enabled = $json->get_allow_nonref
219	If $enable is true (or missing), then the "encode" method can	387	If $enable is true (or missing), then the "encode" method can
220	convert a non-reference into its corresponding string, number or	388	convert a non-reference into its corresponding string, number or
221	null JSON value, which is an extension to RFC4627. Likewise,	389	null JSON value, which is an extension to RFC4627. Likewise,
222	"decode" will accept those JSON values instead of croaking.	390	"decode" will accept those JSON values instead of croaking.
223		391
…		…
230	"allow_nonref", resulting in an invalid JSON text:	398	"allow_nonref", resulting in an invalid JSON text:
231		399
232	JSON::XS->new->allow_nonref->encode ("Hello, World!")	400	JSON::XS->new->allow_nonref->encode ("Hello, World!")
233	=> "Hello, World!"	401	=> "Hello, World!"
234		402
		403	$json = $json->allow_unknown ([$enable])
		404	$enabled = $json->get_allow_unknown
		405	If $enable is true (or missing), then "encode" will not throw an
		406	exception when it encounters values it cannot represent in JSON (for
		407	example, filehandles) but instead will encode a JSON "null" value.
		408	Note that blessed objects are not included here and are handled
		409	separately by c<allow_nonref>.
		410
		411	If $enable is false (the default), then "encode" will throw an
		412	exception when it encounters anything it cannot encode as JSON.
		413
		414	This option does not affect "decode" in any way, and it is
		415	recommended to leave it off unless you know your communications
		416	partner.
		417
		418	$json = $json->allow_blessed ([$enable])
		419	$enabled = $json->get_allow_blessed
		420	If $enable is true (or missing), then the "encode" method will not
		421	barf when it encounters a blessed reference. Instead, the value of
		422	the convert_blessed option will decide whether "null"
		423	("convert_blessed" disabled or no "TO_JSON" method found) or a
		424	representation of the object ("convert_blessed" enabled and
		425	"TO_JSON" method found) is being encoded. Has no effect on "decode".
		426
		427	If $enable is false (the default), then "encode" will throw an
		428	exception when it encounters a blessed object.
		429
		430	$json = $json->convert_blessed ([$enable])
		431	$enabled = $json->get_convert_blessed
		432	If $enable is true (or missing), then "encode", upon encountering a
		433	blessed object, will check for the availability of the "TO_JSON"
		434	method on the object's class. If found, it will be called in scalar
		435	context and the resulting scalar will be encoded instead of the
		436	object. If no "TO_JSON" method is found, the value of
		437	"allow_blessed" will decide what to do.
		438
		439	The "TO_JSON" method may safely call die if it wants. If "TO_JSON"
		440	returns other blessed objects, those will be handled in the same
		441	way. "TO_JSON" must take care of not causing an endless recursion
		442	cycle (== crash) in this case. The name of "TO_JSON" was chosen
		443	because other methods called by the Perl core (== not by the user of
		444	the object) are usually in upper case letters and to avoid
		445	collisions with any "to_json" function or method.
		446
		447	This setting does not yet influence "decode" in any way, but in the
		448	future, global hooks might get installed that influence "decode" and
		449	are enabled by this setting.
		450
		451	If $enable is false, then the "allow_blessed" setting will decide
		452	what to do when a blessed object is found.
		453
		454	$json = $json->filter_json_object ([$coderef->($hashref)])
		455	When $coderef is specified, it will be called from "decode" each
		456	time it decodes a JSON object. The only argument is a reference to
		457	the newly-created hash. If the code references returns a single
		458	scalar (which need not be a reference), this value (i.e. a copy of
		459	that scalar to avoid aliasing) is inserted into the deserialised
		460	data structure. If it returns an empty list (NOTE: not "undef",
		461	which is a valid scalar), the original deserialised hash will be
		462	inserted. This setting can slow down decoding considerably.
		463
		464	When $coderef is omitted or undefined, any existing callback will be
		465	removed and "decode" will not change the deserialised hash in any
		466	way.
		467
		468	Example, convert all JSON objects into the integer 5:
		469
		470	my $js = JSON::XS->new->filter_json_object (sub { 5 });
		471	# returns [5]
		472	$js->decode ('[{}]')
		473	# throw an exception because allow_nonref is not enabled
		474	# so a lone 5 is not allowed.
		475	$js->decode ('{"a":1, "b":2}');
		476
		477	$json = $json->filter_json_single_key_object ($key [=>
		478	$coderef->($value)])
		479	Works remotely similar to "filter_json_object", but is only called
		480	for JSON objects having a single key named $key.
		481
		482	This $coderef is called before the one specified via
		483	"filter_json_object", if any. It gets passed the single value in the
		484	JSON object. If it returns a single value, it will be inserted into
		485	the data structure. If it returns nothing (not even "undef" but the
		486	empty list), the callback from "filter_json_object" will be called
		487	next, as if no single-key callback were specified.
		488
		489	If $coderef is omitted or undefined, the corresponding callback will
		490	be disabled. There can only ever be one callback for a given key.
		491
		492	As this callback gets called less often then the
		493	"filter_json_object" one, decoding speed will not usually suffer as
		494	much. Therefore, single-key objects make excellent targets to
		495	serialise Perl objects into, especially as single-key JSON objects
		496	are as close to the type-tagged value concept as JSON gets (it's
		497	basically an ID/VALUE tuple). Of course, JSON does not support this
		498	in any way, so you need to make sure your data never looks like a
		499	serialised Perl hash.
		500
		501	Typical names for the single object key are "__class_whatever__", or
		502	"$__dollars_are_rarely_used__$" or "}ugly_brace_placement", or even
		503	things like "__class_md5sum(classname)__", to reduce the risk of
		504	clashing with real hashes.
		505
		506	Example, decode JSON objects of the form "{ "__widget__" => <id> }"
		507	into the corresponding $WIDGET{<id>} object:
		508
		509	# return whatever is in $WIDGET{5}:
		510	JSON::XS
		511	->new
		512	->filter_json_single_key_object (__widget__ => sub {
		513	$WIDGET{ $_[0] }
		514	})
		515	->decode ('{"__widget__": 5')
		516
		517	# this can be used with a TO_JSON method in some "widget" class
		518	# for serialisation to json:
		519	sub WidgetBase::TO_JSON {
		520	my ($self) = @_;
		521
		522	unless ($self->{id}) {
		523	$self->{id} = ..get..some..id..;
		524	$WIDGET{$self->{id}} = $self;
		525	}
		526
		527	{ __widget__ => $self->{id} }
		528	}
		529
235	$json = $json->shrink ([$enable])	530	$json = $json->shrink ([$enable])
		531	$enabled = $json->get_shrink
236	Perl usually over-allocates memory a bit when allocating space for	532	Perl usually over-allocates memory a bit when allocating space for
237	strings. This flag optionally resizes strings generated by either	533	strings. This flag optionally resizes strings generated by either
238	"encode" or "decode" to their minimum size possible. This can save	534	"encode" or "decode" to their minimum size possible. This can save
239	memory when your JSON texts are either very very long or you have	535	memory when your JSON texts are either very very long or you have
240	many short strings. It will also try to downgrade any strings to	536	many short strings. It will also try to downgrade any strings to
241	octet-form if possible: perl stores strings internally either in an	537	octet-form if possible: perl stores strings internally either in an
242	encoding called UTF-X or in octet-form. The latter cannot store	538	encoding called UTF-X or in octet-form. The latter cannot store
243	everything but uses less space in general.	539	everything but uses less space in general (and some buggy Perl or C
		540	code might even rely on that internal representation being used).
		541
		542	The actual definition of what shrink does might change in future
		543	versions, but it will always try to save space at the expense of
		544	time.
244		545
245	If $enable is true (or missing), the string returned by "encode"	546	If $enable is true (or missing), the string returned by "encode"
246	will be shrunk-to-fit, while all strings generated by "decode" will	547	will be shrunk-to-fit, while all strings generated by "decode" will
247	also be shrunk-to-fit.	548	also be shrunk-to-fit.
248		549
…		…
251		552
252	In the future, this setting might control other things, such as	553	In the future, this setting might control other things, such as
253	converting strings that look like integers or floats into integers	554	converting strings that look like integers or floats into integers
254	or floats internally (there is no difference on the Perl level),	555	or floats internally (there is no difference on the Perl level),
255	saving space.	556	saving space.
		557
		558	$json = $json->max_depth ([$maximum_nesting_depth])
		559	$max_depth = $json->get_max_depth
		560	Sets the maximum nesting level (default 512) accepted while encoding
		561	or decoding. If a higher nesting level is detected in JSON text or a
		562	Perl data structure, then the encoder and decoder will stop and
		563	croak at that point.
		564
		565	Nesting level is defined by number of hash- or arrayrefs that the
		566	encoder needs to traverse to reach a given point or the number of
		567	"{" or "[" characters without their matching closing parenthesis
		568	crossed to reach a given character in a string.
		569
		570	Setting the maximum depth to one disallows any nesting, so that
		571	ensures that the object is only a single hash/object or array.
		572
		573	If no argument is given, the highest possible setting will be used,
		574	which is rarely useful.
		575
		576	Note that nesting is implemented by recursion in C. The default
		577	value has been chosen to be as large as typical operating systems
		578	allow without crashing.
		579
		580	See SECURITY CONSIDERATIONS, below, for more info on why this is
		581	useful.
		582
		583	$json = $json->max_size ([$maximum_string_size])
		584	$max_size = $json->get_max_size
		585	Set the maximum length a JSON text may have (in bytes) where
		586	decoding is being attempted. The default is 0, meaning no limit.
		587	When "decode" is called on a string that is longer then this many
		588	bytes, it will not attempt to decode the string but throw an
		589	exception. This setting has no effect on "encode" (yet).
		590
		591	If no argument is given, the limit check will be deactivated (same
		592	as when 0 is specified).
		593
		594	See SECURITY CONSIDERATIONS, below, for more info on why this is
		595	useful.
256		596
257	$json_text = $json->encode ($perl_scalar)	597	$json_text = $json->encode ($perl_scalar)
258	Converts the given Perl data structure (a simple scalar or a	598	Converts the given Perl data structure (a simple scalar or a
259	reference to a hash or array) to its JSON representation. Simple	599	reference to a hash or array) to its JSON representation. Simple
260	scalars will be converted into JSON string or number sequences,	600	scalars will be converted into JSON string or number sequences,
…		…
269		609
270	JSON numbers and strings become simple Perl scalars. JSON arrays	610	JSON numbers and strings become simple Perl scalars. JSON arrays
271	become Perl arrayrefs and JSON objects become Perl hashrefs. "true"	611	become Perl arrayrefs and JSON objects become Perl hashrefs. "true"
272	becomes 1, "false" becomes 0 and "null" becomes "undef".	612	becomes 1, "false" becomes 0 and "null" becomes "undef".
273		613
		614	($perl_scalar, $characters) = $json->decode_prefix ($json_text)
		615	This works like the "decode" method, but instead of raising an
		616	exception when there is trailing garbage after the first JSON
		617	object, it will silently stop parsing there and return the number of
		618	characters consumed so far.
		619
		620	This is useful if your JSON texts are not delimited by an outer
		621	protocol (which is not the brightest thing to do in the first place)
		622	and you need to know where the JSON text ends.
		623
		624	JSON::XS->new->decode_prefix ("[1] the tail")
		625	=> ([], 3)
		626
		627	INCREMENTAL PARSING
		628	In some cases, there is the need for incremental parsing of JSON texts.
		629	While this module always has to keep both JSON text and resulting Perl
		630	data structure in memory at one time, it does allow you to parse a JSON
		631	stream incrementally. It does so by accumulating text until it has a
		632	full JSON object, which it then can decode. This process is similar to
		633	using "decode_prefix" to see if a full JSON object is available, but is
		634	much more efficient (and can be implemented with a minimum of method
		635	calls).
		636
		637	JSON::XS will only attempt to parse the JSON text once it is sure it has
		638	enough text to get a decisive result, using a very simple but truly
		639	incremental parser. This means that it sometimes won't stop as early as
		640	the full parser, for example, it doesn't detect parenthese mismatches.
		641	The only thing it guarantees is that it starts decoding as soon as a
		642	syntactically valid JSON text has been seen. This means you need to set
		643	resource limits (e.g. "max_size") to ensure the parser will stop parsing
		644	in the presence if syntax errors.
		645
		646	The following methods implement this incremental parser.
		647
		648	[void, scalar or list context] = $json->incr_parse ([$string])
		649	This is the central parsing function. It can both append new text
		650	and extract objects from the stream accumulated so far (both of
		651	these functions are optional).
		652
		653	If $string is given, then this string is appended to the already
		654	existing JSON fragment stored in the $json object.
		655
		656	After that, if the function is called in void context, it will
		657	simply return without doing anything further. This can be used to
		658	add more text in as many chunks as you want.
		659
		660	If the method is called in scalar context, then it will try to
		661	extract exactly one JSON object. If that is successful, it will
		662	return this object, otherwise it will return "undef". If there is a
		663	parse error, this method will croak just as "decode" would do (one
		664	can then use "incr_skip" to skip the errornous part). This is the
		665	most common way of using the method.
		666
		667	And finally, in list context, it will try to extract as many objects
		668	from the stream as it can find and return them, or the empty list
		669	otherwise. For this to work, there must be no separators between the
		670	JSON objects or arrays, instead they must be concatenated
		671	back-to-back. If an error occurs, an exception will be raised as in
		672	the scalar context case. Note that in this case, any
		673	previously-parsed JSON texts will be lost.
		674
		675	$lvalue_string = $json->incr_text
		676	This method returns the currently stored JSON fragment as an lvalue,
		677	that is, you can manipulate it. This only works when a preceding
		678	call to "incr_parse" in scalar context successfully returned an
		679	object. Under all other circumstances you must not call this
		680	function (I mean it. although in simple tests it might actually
		681	work, it will fail under real world conditions). As a special
		682	exception, you can also call this method before having parsed
		683	anything.
		684
		685	This function is useful in two cases: a) finding the trailing text
		686	after a JSON object or b) parsing multiple JSON objects separated by
		687	non-JSON text (such as commas).
		688
		689	$json->incr_skip
		690	This will reset the state of the incremental parser and will remove
		691	the parsed text from the input buffer so far. This is useful after
		692	"incr_parse" died, in which case the input buffer and incremental
		693	parser state is left unchanged, to skip the text parsed so far and
		694	to reset the parse state.
		695
		696	The difference to "incr_reset" is that only text until the parse
		697	error occured is removed.
		698
		699	$json->incr_reset
		700	This completely resets the incremental parser, that is, after this
		701	call, it will be as if the parser had never parsed anything.
		702
		703	This is useful if you want to repeatedly parse JSON objects and want
		704	to ignore any trailing data, which means you have to reset the
		705	parser after each successful decode.
		706
		707	LIMITATIONS
		708	All options that affect decoding are supported, except "allow_nonref".
		709	The reason for this is that it cannot be made to work sensibly: JSON
		710	objects and arrays are self-delimited, i.e. you can concatenate them
		711	back to back and still decode them perfectly. This does not hold true
		712	for JSON numbers, however.
		713
		714	For example, is the string 1 a single JSON number, or is it simply the
		715	start of 12? Or is 12 a single JSON number, or the concatenation of 1
		716	and 2? In neither case you can tell, and this is why JSON::XS takes the
		717	conservative route and disallows this case.
		718
		719	EXAMPLES
		720	Some examples will make all this clearer. First, a simple example that
		721	works similarly to "decode_prefix": We want to decode the JSON object at
		722	the start of a string and identify the portion after the JSON object:
		723
		724	my $text = "[1,2,3] hello";
		725
		726	my $json = new JSON::XS;
		727
		728	my $obj = $json->incr_parse ($text)
		729	or die "expected JSON object or array at beginning of string";
		730
		731	my $tail = $json->incr_text;
		732	# $tail now contains " hello"
		733
		734	Easy, isn't it?
		735
		736	Now for a more complicated example: Imagine a hypothetical protocol
		737	where you read some requests from a TCP stream, and each request is a
		738	JSON array, without any separation between them (in fact, it is often
		739	useful to use newlines as "separators", as these get interpreted as
		740	whitespace at the start of the JSON text, which makes it possible to
		741	test said protocol with "telnet"...).
		742
		743	Here is how you'd do it (it is trivial to write this in an event-based
		744	manner):
		745
		746	my $json = new JSON::XS;
		747
		748	# read some data from the socket
		749	while (sysread $socket, my $buf, 4096) {
		750
		751	# split and decode as many requests as possible
		752	for my $request ($json->incr_parse ($buf)) {
		753	# act on the $request
		754	}
		755	}
		756
		757	Another complicated example: Assume you have a string with JSON objects
		758	or arrays, all separated by (optional) comma characters (e.g. "[1],[2],
		759	[3]"). To parse them, we have to skip the commas between the JSON texts,
		760	and here is where the lvalue-ness of "incr_text" comes in useful:
		761
		762	my $text = "[1],[2], [3]";
		763	my $json = new JSON::XS;
		764
		765	# void context, so no parsing done
		766	$json->incr_parse ($text);
		767
		768	# now extract as many objects as possible. note the
		769	# use of scalar context so incr_text can be called.
		770	while (my $obj = $json->incr_parse) {
		771	# do something with $obj
		772
		773	# now skip the optional comma
		774	$json->incr_text =~ s/^ \s* , //x;
		775	}
		776
		777	Now lets go for a very complex example: Assume that you have a gigantic
		778	JSON array-of-objects, many gigabytes in size, and you want to parse it,
		779	but you cannot load it into memory fully (this has actually happened in
		780	the real world :).
		781
		782	Well, you lost, you have to implement your own JSON parser. But JSON::XS
		783	can still help you: You implement a (very simple) array parser and let
		784	JSON decode the array elements, which are all full JSON objects on their
		785	own (this wouldn't work if the array elements could be JSON numbers, for
		786	example):
		787
		788	my $json = new JSON::XS;
		789
		790	# open the monster
		791	open my $fh, "<bigfile.json"
		792	or die "bigfile: $!";
		793
		794	# first parse the initial "["
		795	for (;;) {
		796	sysread $fh, my $buf, 65536
		797	or die "read error: $!";
		798	$json->incr_parse ($buf); # void context, so no parsing
		799
		800	# Exit the loop once we found and removed(!) the initial "[".
		801	# In essence, we are (ab-)using the $json object as a simple scalar
		802	# we append data to.
		803	last if $json->incr_text =~ s/^ \s* \[ //x;
		804	}
		805
		806	# now we have the skipped the initial "[", so continue
		807	# parsing all the elements.
		808	for (;;) {
		809	# in this loop we read data until we got a single JSON object
		810	for (;;) {
		811	if (my $obj = $json->incr_parse) {
		812	# do something with $obj
		813	last;
		814	}
		815
		816	# add more data
		817	sysread $fh, my $buf, 65536
		818	or die "read error: $!";
		819	$json->incr_parse ($buf); # void context, so no parsing
		820	}
		821
		822	# in this loop we read data until we either found and parsed the
		823	# separating "," between elements, or the final "]"
		824	for (;;) {
		825	# first skip whitespace
		826	$json->incr_text =~ s/^\s*//;
		827
		828	# if we find "]", we are done
		829	if ($json->incr_text =~ s/^\]//) {
		830	print "finished.\n";
		831	exit;
		832	}
		833
		834	# if we find ",", we can continue with the next element
		835	if ($json->incr_text =~ s/^,//) {
		836	last;
		837	}
		838
		839	# if we find anything else, we have a parse error!
		840	if (length $json->incr_text) {
		841	die "parse error near ", $json->incr_text;
		842	}
		843
		844	# else add more data
		845	sysread $fh, my $buf, 65536
		846	or die "read error: $!";
		847	$json->incr_parse ($buf); # void context, so no parsing
		848	}
		849
		850	This is a complex example, but most of the complexity comes from the
		851	fact that we are trying to be correct (bear with me if I am wrong, I
		852	never ran the above example :).
		853
274	MAPPING	854	MAPPING
275	This section describes how JSON::XS maps Perl values to JSON values and	855	This section describes how JSON::XS maps Perl values to JSON values and
276	vice versa. These mappings are designed to "do the right thing" in most	856	vice versa. These mappings are designed to "do the right thing" in most
277	circumstances automatically, preserving round-tripping characteristics	857	circumstances automatically, preserving round-tripping characteristics
278	(what you put in comes out as something equivalent).	858	(what you put in comes out as something equivalent).
279		859
280	For the more enlightened: note that in the following descriptions,	860	For the more enlightened: note that in the following descriptions,
281	lowercase perl refers to the Perl interpreter, while uppcercase Perl	861	lowercase perl refers to the Perl interpreter, while uppercase Perl
282	refers to the abstract Perl language itself.	862	refers to the abstract Perl language itself.
283		863
284	JSON -> PERL	864	JSON -> PERL
285	object	865	object
286	A JSON object becomes a reference to a hash in Perl. No ordering of	866	A JSON object becomes a reference to a hash in Perl. No ordering of
287	object keys is preserved (JSON does not preserver object key	867	object keys is preserved (JSON does not preserve object key ordering
288	ordering itself).	868	itself).
289		869
290	array	870	array
291	A JSON array becomes a reference to an array in Perl.	871	A JSON array becomes a reference to an array in Perl.
292		872
293	string	873	string
294	A JSON string becomes a string scalar in Perl - Unicode codepoints	874	A JSON string becomes a string scalar in Perl - Unicode codepoints
295	in JSON are represented by the same codepoints in the Perl string,	875	in JSON are represented by the same codepoints in the Perl string,
296	so no manual decoding is necessary.	876	so no manual decoding is necessary.
297		877
298	number	878	number
299	A JSON number becomes either an integer or numeric (floating point)	879	A JSON number becomes either an integer, numeric (floating point) or
300	scalar in perl, depending on its range and any fractional parts. On	880	string scalar in perl, depending on its range and any fractional
301	the Perl level, there is no difference between those as Perl handles	881	parts. On the Perl level, there is no difference between those as
302	all the conversion details, but an integer may take slightly less	882	Perl handles all the conversion details, but an integer may take
303	memory and might represent more values exactly than (floating point)	883	slightly less memory and might represent more values exactly than
		884	floating point numbers.
		885
		886	If the number consists of digits only, JSON::XS will try to
		887	represent it as an integer value. If that fails, it will try to
		888	represent it as a numeric (floating point) value if that is possible
		889	without loss of precision. Otherwise it will preserve the number as
		890	a string value (in which case you lose roundtripping ability, as the
		891	JSON number will be re-encoded toa JSON string).
		892
		893	Numbers containing a fractional or exponential part will always be
		894	represented as numeric (floating point) values, possibly at a loss
		895	of precision (in which case you might lose perfect roundtripping
		896	ability, but the JSON number will still be re-encoded as a JSON
304	numbers.	897	number).
305		898
306	true, false	899	true, false
307	These JSON atoms become 0, 1, respectively. Information is lost in	900	These JSON atoms become "JSON::XS::true" and "JSON::XS::false",
308	this process. Future versions might represent those values	901	respectively. They are overloaded to act almost exactly like the
309	differently, but they will be guarenteed to act like these integers	902	numbers 1 and 0. You can check whether a scalar is a JSON boolean by
310	would normally in Perl.	903	using the "JSON::XS::is_bool" function.
311		904
312	null	905	null
313	A JSON null atom becomes "undef" in Perl.	906	A JSON null atom becomes "undef" in Perl.
314		907
315	PERL -> JSON	908	PERL -> JSON
…		…
317	truly typeless language, so we can only guess which JSON type is meant	910	truly typeless language, so we can only guess which JSON type is meant
318	by a Perl value.	911	by a Perl value.
319		912
320	hash references	913	hash references
321	Perl hash references become JSON objects. As there is no inherent	914	Perl hash references become JSON objects. As there is no inherent
322	ordering in hash keys, they will usually be encoded in a	915	ordering in hash keys (or JSON objects), they will usually be
323	pseudo-random order that can change between runs of the same program	916	encoded in a pseudo-random order that can change between runs of the
324	but stays generally the same within a single run of a program.	917	same program but stays generally the same within a single run of a
325	JSON::XS can optionally sort the hash keys (determined by the	918	program. JSON::XS can optionally sort the hash keys (determined by
326	canonical flag), so the same datastructure will serialise to the	919	the canonical flag), so the same datastructure will serialise to
327	same JSON text (given same settings and version of JSON::XS), but	920	the same JSON text (given same settings and version of JSON::XS),
328	this incurs a runtime overhead.	921	but this incurs a runtime overhead and is only rarely useful, e.g.
		922	when you want to compare some JSON text against another for
		923	equality.
329		924
330	array references	925	array references
331	Perl array references become JSON arrays.	926	Perl array references become JSON arrays.
332		927
		928	other references
		929	Other unblessed references are generally not allowed and will cause
		930	an exception to be thrown, except for references to the integers 0
		931	and 1, which get turned into "false" and "true" atoms in JSON. You
		932	can also use "JSON::XS::false" and "JSON::XS::true" to improve
		933	readability.
		934
		935	encode_json [\0, JSON::XS::true] # yields [false,true]
		936
		937	JSON::XS::true, JSON::XS::false
		938	These special values become JSON true and JSON false values,
		939	respectively. You can also use "\1" and "\0" directly if you want.
		940
333	blessed objects	941	blessed objects
334	Blessed objects are not allowed. JSON::XS currently tries to encode	942	Blessed objects are not directly representable in JSON. See the
335	their underlying representation (hash- or arrayref), but this	943	"allow_blessed" and "convert_blessed" methods on various options on
336	behaviour might change in future versions.	944	how to deal with this: basically, you can choose between throwing an
		945	exception, encoding the reference as if it weren't blessed, or
		946	provide your own serialiser method.
337		947
338	simple scalars	948	simple scalars
339	Simple Perl scalars (any scalar that is not a reference) are the	949	Simple Perl scalars (any scalar that is not a reference) are the
340	most difficult objects to encode: JSON::XS will encode undefined	950	most difficult objects to encode: JSON::XS will encode undefined
341	scalars as JSON null value, scalars that have last been used in a	951	scalars as JSON "null" values, scalars that have last been used in a
342	string context before encoding as JSON strings and anything else as	952	string context before encoding as JSON strings, and anything else as
343	number value:	953	number value:
344		954
345	# dump as number	955	# dump as number
346	to_json [2] # yields [2]	956	encode_json [2] # yields [2]
347	to_json [-3.0e17] # yields [-3e+17]	957	encode_json [-3.0e17] # yields [-3e+17]
348	my $value = 5; to_json [$value] # yields [5]	958	my $value = 5; encode_json [$value] # yields [5]
349		959
350	# used as string, so dump as string	960	# used as string, so dump as string
351	print $value;	961	print $value;
352	to_json [$value] # yields ["5"]	962	encode_json [$value] # yields ["5"]
353		963
354	# undef becomes null	964	# undef becomes null
355	to_json [undef] # yields [null]	965	encode_json [undef] # yields [null]
356		966
357	You can force the type to be a string by stringifying it:	967	You can force the type to be a JSON string by stringifying it:
358		968
359	my $x = 3.1; # some variable containing a number	969	my $x = 3.1; # some variable containing a number
360	"$x"; # stringified	970	"$x"; # stringified
361	$x .= ""; # another, more awkward way to stringify	971	$x .= ""; # another, more awkward way to stringify
362	print $x; # perl does it for you, too, quite often	972	print $x; # perl does it for you, too, quite often
363		973
364	You can force the type to be a number by numifying it:	974	You can force the type to be a JSON number by numifying it:
365		975
366	my $x = "3"; # some variable containing a string	976	my $x = "3"; # some variable containing a string
367	$x += 0; # numify it, ensuring it will be dumped as a number	977	$x += 0; # numify it, ensuring it will be dumped as a number
368	$x *= 1; # same thing, the choise is yours.	978	$x *= 1; # same thing, the choice is yours.
369		979
370	You can not currently output JSON booleans or force the type in	980	You can not currently force the type in other, less obscure, ways.
371	other, less obscure, ways. Tell me if you need this capability.	981	Tell me if you need this capability (but don't forget to explain why
		982	it's needed :).
372		983
373	circular data structures	984	ENCODING/CODESET FLAG NOTES
374	Those will be encoded until memory or stackspace runs out.	985	The interested reader might have seen a number of flags that signify
		986	encodings or codesets - "utf8", "latin1" and "ascii". There seems to be
		987	some confusion on what these do, so here is a short comparison:
375		988
376	COMPARISON	989	"utf8" controls whether the JSON text created by "encode" (and expected
377	As already mentioned, this module was created because none of the	990	by "decode") is UTF-8 encoded or not, while "latin1" and "ascii" only
378	existing JSON modules could be made to work correctly. First I will	991	control whether "encode" escapes character values outside their
379	describe the problems (or pleasures) I encountered with various existing	992	respective codeset range. Neither of these flags conflict with each
380	JSON modules, followed by some benchmark values. JSON::XS was designed	993	other, although some combinations make less sense than others.
381	not to suffer from any of these problems or limitations.
382		994
383	JSON 1.07	995	Care has been taken to make all flags symmetrical with respect to
384	Slow (but very portable, as it is written in pure Perl).	996	"encode" and "decode", that is, texts encoded with any combination of
		997	these flag values will be correctly decoded when the same flags are used
		998	- in general, if you use different flag settings while encoding vs. when
		999	decoding you likely have a bug somewhere.
385		1000
386	Undocumented/buggy Unicode handling (how JSON handles unicode values	1001	Below comes a verbose discussion of these flags. Note that a "codeset"
387	is undocumented. One can get far by feeding it unicode strings and	1002	is simply an abstract set of character-codepoint pairs, while an
388	doing en-/decoding oneself, but unicode escapes are not working	1003	encoding takes those codepoint numbers and encodes them, in our case
		1004	into octets. Unicode is (among other things) a codeset, UTF-8 is an
		1005	encoding, and ISO-8859-1 (= latin 1) and ASCII are both codesets and
		1006	encodings at the same time, which can be confusing.
		1007
		1008	"utf8" flag disabled
		1009	When "utf8" is disabled (the default), then "encode"/"decode"
		1010	generate and expect Unicode strings, that is, characters with high
		1011	ordinal Unicode values (> 255) will be encoded as such characters,
		1012	and likewise such characters are decoded as-is, no canges to them
		1013	will be done, except "(re-)interpreting" them as Unicode codepoints
		1014	or Unicode characters, respectively (to Perl, these are the same
		1015	thing in strings unless you do funny/weird/dumb stuff).
		1016
		1017	This is useful when you want to do the encoding yourself (e.g. when
		1018	you want to have UTF-16 encoded JSON texts) or when some other layer
		1019	does the encoding for you (for example, when printing to a terminal
		1020	using a filehandle that transparently encodes to UTF-8 you certainly
		1021	do NOT want to UTF-8 encode your data first and have Perl encode it
		1022	another time).
		1023
		1024	"utf8" flag enabled
		1025	If the "utf8"-flag is enabled, "encode"/"decode" will encode all
		1026	characters using the corresponding UTF-8 multi-byte sequence, and
		1027	will expect your input strings to be encoded as UTF-8, that is, no
		1028	"character" of the input string must have any value > 255, as UTF-8
		1029	does not allow that.
		1030
		1031	The "utf8" flag therefore switches between two modes: disabled means
		1032	you will get a Unicode string in Perl, enabled means you get an
		1033	UTF-8 encoded octet/binary string in Perl.
		1034
		1035	"latin1" or "ascii" flags enabled
		1036	With "latin1" (or "ascii") enabled, "encode" will escape characters
		1037	with ordinal values > 255 (> 127 with "ascii") and encode the
		1038	remaining characters as specified by the "utf8" flag.
		1039
		1040	If "utf8" is disabled, then the result is also correctly encoded in
		1041	those character sets (as both are proper subsets of Unicode, meaning
		1042	that a Unicode string with all character values < 256 is the same
		1043	thing as a ISO-8859-1 string, and a Unicode string with all
		1044	character values < 128 is the same thing as an ASCII string in
389	properly).	1045	Perl).
390		1046
391	No roundtripping (strings get clobbered if they look like numbers,	1047	If "utf8" is enabled, you still get a correct UTF-8-encoded string,
392	e.g. the string 2.0 will encode to 2.0 instead of "2.0", and that	1048	regardless of these flags, just some more characters will be escaped
393	will decode into the number 2.	1049	using "\uXXXX" then before.
394		1050
395	JSON::PC 0.01	1051	Note that ISO-8859-1-encoded strings are not compatible with UTF-8
396	Very fast.	1052	encoding, while ASCII-encoded strings are. That is because the
		1053	ISO-8859-1 encoding is NOT a subset of UTF-8 (despite the ISO-8859-1
		1054	codeset being a subset of Unicode), while ASCII is.
397		1055
398	Undocumented/buggy Unicode handling.	1056	Surprisingly, "decode" will ignore these flags and so treat all
		1057	input values as governed by the "utf8" flag. If it is disabled, this
		1058	allows you to decode ISO-8859-1- and ASCII-encoded strings, as both
		1059	strict subsets of Unicode. If it is enabled, you can correctly
		1060	decode UTF-8 encoded strings.
399		1061
400	No roundtripping.	1062	So neither "latin1" nor "ascii" are incompatible with the "utf8"
		1063	flag - they only govern when the JSON output engine escapes a
		1064	character or not.
401		1065
402	Has problems handling many Perl values (e.g. regex results and other	1066	The main use for "latin1" is to relatively efficiently store binary
403	magic values will make it croak).	1067	data as JSON, at the expense of breaking compatibility with most
		1068	JSON decoders.
404		1069
405	Does not even generate valid JSON ("{1,2}" gets converted to "{1:2}"	1070	The main use for "ascii" is to force the output to not contain
406	which is not a valid JSON text.	1071	characters with values > 127, which means you can interpret the
		1072	resulting string as UTF-8, ISO-8859-1, ASCII, KOI8-R or most about
		1073	any character set and 8-bit-encoding, and still get the same data
		1074	structure back. This is useful when your channel for JSON transfer
		1075	is not 8-bit clean or the encoding might be mangled in between (e.g.
		1076	in mail), and works because ASCII is a proper subset of most 8-bit
		1077	and multibyte encodings in use in the world.
407		1078
408	Unmaintained (maintainer unresponsive for many months, bugs are not	1079	JSON and ECMAscript
409	getting fixed).	1080	JSON syntax is based on how literals are represented in javascript (the
		1081	not-standardised predecessor of ECMAscript) which is presumably why it
		1082	is called "JavaScript Object Notation".
410		1083
411	JSON::Syck 0.21	1084	However, JSON is not a subset (and also not a superset of course) of
412	Very buggy (often crashes).	1085	ECMAscript (the standard) or javascript (whatever browsers actually
		1086	implement).
413		1087
414	Very inflexible (no human-readable format supported, format pretty	1088	If you want to use javascript's "eval" function to "parse" JSON, you
415	much undocumented. I need at least a format for easy reading by	1089	might run into parse errors for valid JSON texts, or the resulting data
416	humans and a single-line compact format for use in a protocol, and	1090	structure might not be queryable:
417	preferably a way to generate ASCII-only JSON texts).
418		1091
419	Completely broken (and confusingly documented) Unicode handling	1092	One of the problems is that U+2028 and U+2029 are valid characters
420	(unicode escapes are not working properly, you need to set	1093	inside JSON strings, but are not allowed in ECMAscript string literals,
421	ImplicitUnicode to different values on en- and decoding to get	1094	so the following Perl fragment will not output something that can be
422	symmetric behaviour).	1095	guaranteed to be parsable by javascript's "eval":
423		1096
424	No roundtripping (simple cases work, but this depends on wether the	1097	use JSON::XS;
425	scalar value was used in a numeric context or not).
426		1098
427	Dumping hashes may skip hash values depending on iterator state.	1099	print encode_json [chr 0x2028];
428		1100
429	Unmaintained (maintainer unresponsive for many months, bugs are not	1101	The right fix for this is to use a proper JSON parser in your javascript
430	getting fixed).	1102	programs, and not rely on "eval" (see for example Douglas Crockford's
		1103	json2.js parser).
431		1104
432	Does not check input for validity (i.e. will accept non-JSON input	1105	If this is not an option, you can, as a stop-gap measure, simply encode
433	and return "something" instead of raising an exception. This is a	1106	to ASCII-only JSON:
434	security issue: imagine two banks transfering money between each
435	other using JSON. One bank might parse a given non-JSON request and
436	deduct money, while the other might reject the transaction with a
437	syntax error. While a good protocol will at least recover, that is
438	extra unnecessary work and the transaction will still not succeed).
439		1107
440	JSON::DWIW 0.04	1108	use JSON::XS;
441	Very fast. Very natural. Very nice.
442		1109
443	Undocumented unicode handling (but the best of the pack. Unicode	1110	print JSON::XS->new->ascii->encode ([chr 0x2028]);
444	escapes still don't get parsed properly).
445		1111
446	Very inflexible.	1112	Note that this will enlarge the resulting JSON text quite a bit if you
		1113	have many non-ASCII characters. You might be tempted to run some regexes
		1114	to only escape U+2028 and U+2029, e.g.:
447		1115
448	No roundtripping.	1116	# DO NOT USE THIS!
		1117	my $json = JSON::XS->new->utf8->encode ([chr 0x2028]);
		1118	$json =~ s/\xe2\x80\xa8/\\u2028/g; # escape U+2028
		1119	$json =~ s/\xe2\x80\xa9/\\u2029/g; # escape U+2029
		1120	print $json;
449		1121
450	Does not generate valid JSON texts (key strings are often unquoted,	1122	Note that this is a bad idea: the above only works for U+2028 and
451	empty keys result in nothing being output)	1123	U+2029 and thus only for fully ECMAscript-compliant parsers. Many
		1124	existing javascript implementations, however, have issues with other
		1125	characters as well - using "eval" naively simply will cause problems.
452		1126
453	Does not check input for validity.	1127	Another problem is that some javascript implementations reserve some
		1128	property names for their own purposes (which probably makes them
		1129	non-ECMAscript-compliant). For example, Iceweasel reserves the
		1130	"__proto__" property name for it's own purposes.
		1131
		1132	If that is a problem, you could parse try to filter the resulting JSON
		1133	output for these property strings, e.g.:
		1134
		1135	$json =~ s/"__proto__"\s*:/"__proto__renamed":/g;
		1136
		1137	This works because "__proto__" is not valid outside of strings, so every
		1138	occurence of ""__proto__"\s*:" must be a string used as property name.
		1139
		1140	If you know of other incompatibilities, please let me know.
		1141
		1142	JSON and YAML
		1143	You often hear that JSON is a subset of YAML. This is, however, a mass
		1144	hysteria(*) and very far from the truth (as of the time of this
		1145	writing), so let me state it clearly: *in general, there is no way to
		1146	configure JSON::XS to output a data structure as valid YAML* that works
		1147	in all cases.
		1148
		1149	If you really must use JSON::XS to generate YAML, you should use this
		1150	algorithm (subject to change in future versions):
		1151
		1152	my $to_yaml = JSON::XS->new->utf8->space_after (1);
		1153	my $yaml = $to_yaml->encode ($ref) . "\n";
		1154
		1155	This will usually generate JSON texts that also parse as valid YAML.
		1156	Please note that YAML has hardcoded limits on (simple) object key
		1157	lengths that JSON doesn't have and also has different and incompatible
		1158	unicode character escape syntax, so you should make sure that your hash
		1159	keys are noticeably shorter than the 1024 "stream characters" YAML
		1160	allows and that you do not have characters with codepoint values outside
		1161	the Unicode BMP (basic multilingual page). YAML also does not allow "\/"
		1162	sequences in strings (which JSON::XS does not currently generate, but
		1163	other JSON generators might).
		1164
		1165	There might be other incompatibilities that I am not aware of (or the
		1166	YAML specification has been changed yet again - it does so quite often).
		1167	In general you should not try to generate YAML with a JSON generator or
		1168	vice versa, or try to parse JSON with a YAML parser or vice versa:
		1169	chances are high that you will run into severe interoperability problems
		1170	when you least expect it.
		1171
		1172	(*) I have been pressured multiple times by Brian Ingerson (one of the
		1173	authors of the YAML specification) to remove this paragraph, despite
		1174	him acknowledging that the actual incompatibilities exist. As I was
		1175	personally bitten by this "JSON is YAML" lie, I refused and said I
		1176	will continue to educate people about these issues, so others do not
		1177	run into the same problem again and again. After this, Brian called
		1178	me a (quote)complete and worthless idiot(unquote).
		1179
		1180	In my opinion, instead of pressuring and insulting people who
		1181	actually clarify issues with YAML and the wrong statements of some
		1182	of its proponents, I would kindly suggest reading the JSON spec
		1183	(which is not that difficult or long) and finally make YAML
		1184	compatible to it, and educating users about the changes, instead of
		1185	spreading lies about the real compatibility for many years and
		1186	trying to silence people who point out that it isn't true.
		1187
		1188	Addendum/2009: the YAML 1.2 spec is still incomaptible with JSON,
		1189	even though the incompatibilities have been documented (and are
		1190	known to Brian) for many years and the spec makes explicit claims
		1191	that YAML is a superset of JSON. It would be so easy to fix, but
		1192	apparently, bullying and corrupting userdata is so much easier.
454		1193
455	SPEED	1194	SPEED
456	It seems that JSON::XS is surprisingly fast, as shown in the following	1195	It seems that JSON::XS is surprisingly fast, as shown in the following
457	tables. They have been generated with the help of the "eg/bench" program	1196	tables. They have been generated with the help of the "eg/bench" program
458	in the JSON::XS distribution, to make it easy to compare on your own	1197	in the JSON::XS distribution, to make it easy to compare on your own
459	system.	1198	system.
460		1199
461	First comes a comparison between various modules using a very short JSON	1200	First comes a comparison between various modules using a very short
462	string (83 bytes), showing the number of encodes/decodes per second	1201	single-line JSON string (also available at
463	(JSON::XS is the functional interface, while JSON::XS/2 is the OO	1202	<http://dist.schmorp.de/misc/json/short.json>).
464	interface with pretty-printing and hashkey sorting enabled). Higher is
465	better:
466		1203
		1204	{"method": "handleMessage", "params": ["user1",
		1205	"we were just talking"], "id": null, "array":[1,11,234,-5,1e5,1e7,
		1206	1, 0]}
		1207
		1208	It shows the number of encodes/decodes per second (JSON::XS uses the
		1209	functional interface, while JSON::XS/2 uses the OO interface with
		1210	pretty-printing and hashkey sorting enabled, JSON::XS/3 enables shrink.
		1211	JSON::DWIW/DS uses the deserialise function, while JSON::DWIW::FJ uses
		1212	the from_json method). Higher is better:
		1213
467	module \| encode \| decode \|	1214	module \| encode \| decode \|
468	-----------\|------------\|------------\|	1215	--------------\|------------\|------------\|
469	JSON \| 14006 \| 6820 \|	1216	JSON::DWIW/DS \| 86302.551 \| 102300.098 \|
470	JSON::DWIW \| 200937 \| 120386 \|	1217	JSON::DWIW/FJ \| 86302.551 \| 75983.768 \|
471	JSON::PC \| 85065 \| 129366 \|	1218	JSON::PP \| 15827.562 \| 6638.658 \|
472	JSON::Syck \| 59898 \| 44232 \|	1219	JSON::Syck \| 63358.066 \| 47662.545 \|
473	JSON::XS \| 1171478 \| 342435 \|	1220	JSON::XS \| 511500.488 \| 511500.488 \|
474	JSON::XS/2 \| 730760 \| 328714 \|	1221	JSON::XS/2 \| 291271.111 \| 388361.481 \|
		1222	JSON::XS/3 \| 361577.931 \| 361577.931 \|
		1223	Storable \| 66788.280 \| 265462.278 \|
475	-----------+------------+------------+	1224	--------------+------------+------------+
476		1225
477	That is, JSON::XS is 6 times faster than than JSON::DWIW and about 80	1226	That is, JSON::XS is almost six times faster than JSON::DWIW on
478	times faster than JSON, even with pretty-printing and key sorting.	1227	encoding, about five times faster on decoding, and over thirty to
		1228	seventy times faster than JSON's pure perl implementation. It also
		1229	compares favourably to Storable for small amounts of data.
479		1230
480	Using a longer test string (roughly 18KB, generated from Yahoo! Locals	1231	Using a longer test string (roughly 18KB, generated from Yahoo! Locals
481	search API (http://nanoref.com/yahooapis/mgPdGg):	1232	search API (<http://dist.schmorp.de/misc/json/long.json>).
482		1233
483	module \| encode \| decode \|	1234	module \| encode \| decode \|
484	-----------\|------------\|------------\|	1235	--------------\|------------\|------------\|
485	JSON \| 673 \| 38 \|	1236	JSON::DWIW/DS \| 1647.927 \| 2673.916 \|
486	JSON::DWIW \| 5271 \| 770 \|	1237	JSON::DWIW/FJ \| 1630.249 \| 2596.128 \|
487	JSON::PC \| 9901 \| 2491 \|	1238	JSON::PP \| 400.640 \| 62.311 \|
488	JSON::Syck \| 2360 \| 786 \|	1239	JSON::Syck \| 1481.040 \| 1524.869 \|
489	JSON::XS \| 37398 \| 3202 \|	1240	JSON::XS \| 20661.596 \| 9541.183 \|
490	JSON::XS/2 \| 13765 \| 3153 \|	1241	JSON::XS/2 \| 10683.403 \| 9416.938 \|
		1242	JSON::XS/3 \| 20661.596 \| 9400.054 \|
		1243	Storable \| 19765.806 \| 10000.725 \|
491	-----------+------------+------------+	1244	--------------+------------+------------+
492		1245
493	Again, JSON::XS leads by far in the encoding case, while still beating	1246	Again, JSON::XS leads by far (except for Storable which non-surprisingly
494	every other module in the decoding case.	1247	decodes a bit faster).
495		1248
496	On large strings containing lots of unicode characters, some modules	1249	On large strings containing lots of high Unicode characters, some
497	(such as JSON::PC) decode faster than JSON::XS, but the result will be	1250	modules (such as JSON::PC) seem to decode faster than JSON::XS, but the
498	broken due to missing unicode handling. Others refuse to decode or	1251	result will be broken due to missing (or wrong) Unicode handling. Others
499	encode properly, so it was impossible to prepare a fair comparison table	1252	refuse to decode or encode properly, so it was impossible to prepare a
500	for that case.	1253	fair comparison table for that case.
501		1254
502	RESOURCE LIMITS	1255	SECURITY CONSIDERATIONS
503	JSON::XS does not impose any limits on the size of JSON texts or Perl	1256	When you are using JSON in a protocol, talking to untrusted potentially
504	values they represent - if your machine can handle it, JSON::XS will	1257	hostile creatures requires relatively few measures.
505	encode or decode it. Future versions might optionally impose structure	1258
506	depth and memory use resource limits.	1259	First of all, your JSON decoder should be secure, that is, should not
		1260	have any buffer overflows. Obviously, this module should ensure that and
		1261	I am trying hard on making that true, but you never know.
		1262
		1263	Second, you need to avoid resource-starving attacks. That means you
		1264	should limit the size of JSON texts you accept, or make sure then when
		1265	your resources run out, that's just fine (e.g. by using a separate
		1266	process that can crash safely). The size of a JSON text in octets or
		1267	characters is usually a good indication of the size of the resources
		1268	required to decode it into a Perl structure. While JSON::XS can check
		1269	the size of the JSON text, it might be too late when you already have it
		1270	in memory, so you might want to check the size before you accept the
		1271	string.
		1272
		1273	Third, JSON::XS recurses using the C stack when decoding objects and
		1274	arrays. The C stack is a limited resource: for instance, on my amd64
		1275	machine with 8MB of stack size I can decode around 180k nested arrays
		1276	but only 14k nested JSON objects (due to perl itself recursing deeply on
		1277	croak to free the temporary). If that is exceeded, the program crashes.
		1278	To be conservative, the default nesting limit is set to 512. If your
		1279	process has a smaller stack, you should adjust this setting accordingly
		1280	with the "max_depth" method.
		1281
		1282	Something else could bomb you, too, that I forgot to think of. In that
		1283	case, you get to keep the pieces. I am always open for hints, though...
		1284
		1285	Also keep in mind that JSON::XS might leak contents of your Perl data
		1286	structures in its error messages, so when you serialise sensitive
		1287	information you might want to make sure that exceptions thrown by
		1288	JSON::XS will not end up in front of untrusted eyes.
		1289
		1290	If you are using JSON::XS to return packets to consumption by JavaScript
		1291	scripts in a browser you should have a look at
		1292	<http://blog.archive.jpsykes.com/47/practical-csrf-and-json-security/>
		1293	to see whether you are vulnerable to some common attack vectors (which
		1294	really are browser design bugs, but it is still you who will have to
		1295	deal with it, as major browser developers care only for features, not
		1296	about getting security right).
		1297
		1298	THREADS
		1299	This module is not guaranteed to be thread safe and there are no plans
		1300	to change this until Perl gets thread support (as opposed to the
		1301	horribly slow so-called "threads" which are simply slow and bloated
		1302	process simulations - use fork, it's much faster, cheaper, better).
		1303
		1304	(It might actually work, but you have been warned).
507		1305
508	BUGS	1306	BUGS
509	While the goal of this module is to be correct, that unfortunately does	1307	While the goal of this module is to be correct, that unfortunately does
510	not mean its bug-free, only that I think its design is bug-free. It is	1308	not mean it's bug-free, only that I think its design is bug-free. If you
511	still very young and not well-tested. If you keep reporting bugs they	1309	keep reporting bugs they will be fixed swiftly, though.
512	will be fixed swiftly, though.	1310
		1311	Please refrain from using rt.cpan.org or any other bug reporting
		1312	service. I put the contact address into my modules for a reason.
		1313
		1314	SEE ALSO
		1315	The json_xs command line utility for quick experiments.
513		1316
514	AUTHOR	1317	AUTHOR
515	Marc Lehmann <schmorp@schmorp.de>	1318	Marc Lehmann <schmorp@schmorp.de>
516	http://home.schmorp.de/	1319	http://home.schmorp.de/
517		1320

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Comparing JSON-XS/README (file contents): Revision 1.6 by root, Sat Mar 24 19:42:14 2007 UTC vs. Revision 1.34 by root, Thu Mar 11 17:36:09 2010 UTC

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Comparing JSON-XS/README (file contents):
Revision 1.6 by root, Sat Mar 24 19:42:14 2007 UTC vs.
Revision 1.34 by root, Thu Mar 11 17:36:09 2010 UTC