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Revision 1.2 by root, Thu Mar 22 17:28:50 2007 UTC vs.
Revision 1.144 by root, Mon Oct 28 23:19:54 2013 UTC

1	=head1 NAME	1	=head1 NAME
2		2
3	JSON::XS - JSON serialising/deserialising, done correctly and fast	3	JSON::XS - JSON serialising/deserialising, done correctly and fast
4		4
		5	=encoding utf-8
		6
		7	JSON::XS - 正しくて高速な JSON シリアライザ/デシリアライザ
		8	(http://fleur.hio.jp/perldoc/mix/lib/JSON/XS.html)
		9
5	=head1 SYNOPSIS	10	=head1 SYNOPSIS
6		11
7	use JSON::XS;	12	use JSON::XS;
		13
		14	# exported functions, they croak on error
		15	# and expect/generate UTF-8
		16
		17	$utf8_encoded_json_text = encode_json $perl_hash_or_arrayref;
		18	$perl_hash_or_arrayref = decode_json $utf8_encoded_json_text;
		19
		20	# OO-interface
		21
		22	$coder = JSON::XS->new->ascii->pretty->allow_nonref;
		23	$pretty_printed_unencoded = $coder->encode ($perl_scalar);
		24	$perl_scalar = $coder->decode ($unicode_json_text);
		25
		26	# Note that JSON version 2.0 and above will automatically use JSON::XS
		27	# if available, at virtually no speed overhead either, so you should
		28	# be able to just:
		29
		30	use JSON;
		31
		32	# and do the same things, except that you have a pure-perl fallback now.
8		33
9	=head1 DESCRIPTION	34	=head1 DESCRIPTION
10		35
11	This module converts Perl data structures to JSON and vice versa. Its	36	This module converts Perl data structures to JSON and vice versa. Its
12	primary goal is to be I<correct> and its secondary goal is to be	37	primary goal is to be I<correct> and its secondary goal is to be
13	I<fast>. To reach the latter goal it was written in C.	38	I<fast>. To reach the latter goal it was written in C.
		39
		40	Beginning with version 2.0 of the JSON module, when both JSON and
		41	JSON::XS are installed, then JSON will fall back on JSON::XS (this can be
		42	overridden) with no overhead due to emulation (by inheriting constructor
		43	and methods). If JSON::XS is not available, it will fall back to the
		44	compatible JSON::PP module as backend, so using JSON instead of JSON::XS
		45	gives you a portable JSON API that can be fast when you need and doesn't
		46	require a C compiler when that is a problem.
14		47
15	As this is the n-th-something JSON module on CPAN, what was the reason	48	As this is the n-th-something JSON module on CPAN, what was the reason
16	to write yet another JSON module? While it seems there are many JSON	49	to write yet another JSON module? While it seems there are many JSON
17	modules, none of them correctly handle all corner cases, and in most cases	50	modules, none of them correctly handle all corner cases, and in most cases
18	their maintainers are unresponsive, gone missing, or not listening to bug	51	their maintainers are unresponsive, gone missing, or not listening to bug
19	reports for other reasons.	52	reports for other reasons.
20		53
21	See COMPARISON, below, for a comparison to some other JSON modules.	54	See MAPPING, below, on how JSON::XS maps perl values to JSON values and
		55	vice versa.
22		56
23	=head2 FEATURES	57	=head2 FEATURES
24		58
25	=over 4	59	=over 4
26		60
27	=item * correct handling of unicode issues	61	=item * correct Unicode handling
28		62
29	This module knows how to handle Unicode, and even documents how it does so.	63	This module knows how to handle Unicode, documents how and when it does
		64	so, and even documents what "correct" means.
30		65
31	=item * round-trip integrity	66	=item * round-trip integrity
32		67
33	When you serialise a perl data structure using only datatypes supported	68	When you serialise a perl data structure using only data types supported
34	by JSON, the deserialised data structure is identical on the Perl level.	69	by JSON and Perl, the deserialised data structure is identical on the Perl
35	(e.g. the string "2.0" doesn't suddenly become "2").	70	level. (e.g. the string "2.0" doesn't suddenly become "2" just because
		71	it looks like a number). There I<are> minor exceptions to this, read the
		72	MAPPING section below to learn about those.
36		73
37	=item * strict checking of JSON correctness	74	=item * strict checking of JSON correctness
38		75
39	There is no guessing, no generating of illegal JSON strings by default,	76	There is no guessing, no generating of illegal JSON texts by default,
40	and only JSON is accepted as input (the latter is a security feature).	77	and only JSON is accepted as input by default (the latter is a security
		78	feature).
41		79
42	=item * fast	80	=item * fast
43		81
44	compared to other JSON modules, this module compares favourably.	82	Compared to other JSON modules and other serialisers such as Storable,
		83	this module usually compares favourably in terms of speed, too.
45		84
46	=item * simple to use	85	=item * simple to use
47		86
48	This module has both a simple functional interface as well as an OO	87	This module has both a simple functional interface as well as an object
49	interface.	88	oriented interface.
50		89
51	=item * reasonably versatile output formats	90	=item * reasonably versatile output formats
52		91
53	You can choose between the most compact format possible, a pure-ascii	92	You can choose between the most compact guaranteed-single-line format
54	format, or a pretty-printed format. Or you can combine those features in	93	possible (nice for simple line-based protocols), a pure-ASCII format
55	whatever way you like.	94	(for when your transport is not 8-bit clean, still supports the whole
		95	Unicode range), or a pretty-printed format (for when you want to read that
		96	stuff). Or you can combine those features in whatever way you like.
56		97
57	=back	98	=back
58		99
59	=cut	100	=cut
60		101
61	package JSON::XS;	102	package JSON::XS;
62		103
63	BEGIN {	104	use common::sense;
64	$VERSION = '0.1';	105
		106	our $VERSION = 2.34;
65	@ISA = qw(Exporter);	107	our @ISA = qw(Exporter);
66		108
67	@EXPORT = qw(to_json from_json);	109	our @EXPORT = qw(encode_json decode_json);
68	require Exporter;
69		110
70	require XSLoader;	111	use Exporter;
71	XSLoader::load JSON::XS::, $VERSION;	112	use XSLoader;
72	}	113
		114	use Types::Serialiser ();
73		115
74	=head1 FUNCTIONAL INTERFACE	116	=head1 FUNCTIONAL INTERFACE
75		117
76	The following convinience methods are provided by this module. They are	118	The following convenience methods are provided by this module. They are
77	exported by default:	119	exported by default:
78		120
79	=over 4	121	=over 4
80		122
81	=item $json_string = to_json $perl_scalar	123	=item $json_text = encode_json $perl_scalar
82		124
83	Converts the given Perl data structure (a simple scalar or a reference to	125	Converts the given Perl data structure to a UTF-8 encoded, binary string
84	a hash or array) to a UTF-8 encoded, binary string (that is, the string contains	126	(that is, the string contains octets only). Croaks on error.
85	octets only). Croaks on error.
86		127
87	This function call is functionally identical to C<< JSON::XS->new->utf8	128	This function call is functionally identical to:
88	(1)->encode ($perl_scalar) >>.
89		129
		130	$json_text = JSON::XS->new->utf8->encode ($perl_scalar)
		131
		132	Except being faster.
		133
90	=item $perl_scalar = from_json $json_string	134	=item $perl_scalar = decode_json $json_text
91		135
92	The opposite of C<to_json>: expects an UTF-8 (binary) string and tries to	136	The opposite of C<encode_json>: expects an UTF-8 (binary) string and tries
93	parse that as an UTF-8 encoded JSON string, returning the resulting simple	137	to parse that as an UTF-8 encoded JSON text, returning the resulting
94	scalar or reference. Croaks on error.	138	reference. Croaks on error.
95		139
96	This function call is functionally identical to C<< JSON::XS->new->utf8	140	This function call is functionally identical to:
97	(1)->decode ($json_string) >>.	141
		142	$perl_scalar = JSON::XS->new->utf8->decode ($json_text)
		143
		144	Except being faster.
98		145
99	=back	146	=back
		147
		148
		149	=head1 A FEW NOTES ON UNICODE AND PERL
		150
		151	Since this often leads to confusion, here are a few very clear words on
		152	how Unicode works in Perl, modulo bugs.
		153
		154	=over 4
		155
		156	=item 1. Perl strings can store characters with ordinal values > 255.
		157
		158	This enables you to store Unicode characters as single characters in a
		159	Perl string - very natural.
		160
		161	=item 2. Perl does I<not> associate an encoding with your strings.
		162
		163	... until you force it to, e.g. when matching it against a regex, or
		164	printing the scalar to a file, in which case Perl either interprets your
		165	string as locale-encoded text, octets/binary, or as Unicode, depending
		166	on various settings. In no case is an encoding stored together with your
		167	data, it is I<use> that decides encoding, not any magical meta data.
		168
		169	=item 3. The internal utf-8 flag has no meaning with regards to the
		170	encoding of your string.
		171
		172	Just ignore that flag unless you debug a Perl bug, a module written in
		173	XS or want to dive into the internals of perl. Otherwise it will only
		174	confuse you, as, despite the name, it says nothing about how your string
		175	is encoded. You can have Unicode strings with that flag set, with that
		176	flag clear, and you can have binary data with that flag set and that flag
		177	clear. Other possibilities exist, too.
		178
		179	If you didn't know about that flag, just the better, pretend it doesn't
		180	exist.
		181
		182	=item 4. A "Unicode String" is simply a string where each character can be
		183	validly interpreted as a Unicode code point.
		184
		185	If you have UTF-8 encoded data, it is no longer a Unicode string, but a
		186	Unicode string encoded in UTF-8, giving you a binary string.
		187
		188	=item 5. A string containing "high" (> 255) character values is I<not> a UTF-8 string.
		189
		190	It's a fact. Learn to live with it.
		191
		192	=back
		193
		194	I hope this helps :)
		195
100		196
101	=head1 OBJECT-ORIENTED INTERFACE	197	=head1 OBJECT-ORIENTED INTERFACE
102		198
103	The object oriented interface lets you configure your own encoding or	199	The object oriented interface lets you configure your own encoding or
104	decoding style, within the limits of supported formats.	200	decoding style, within the limits of supported formats.
…		…
111	strings. All boolean flags described below are by default I<disabled>.	207	strings. All boolean flags described below are by default I<disabled>.
112		208
113	The mutators for flags all return the JSON object again and thus calls can	209	The mutators for flags all return the JSON object again and thus calls can
114	be chained:	210	be chained:
115		211
116	my $json = JSON::XS->new->utf8(1)->pretty(1)->encode ({a => [1,2]})	212	my $json = JSON::XS->new->utf8->space_after->encode ({a => [1,2]})
117	=> {"a" : [1, 2]}	213	=> {"a": [1, 2]}
118		214
119	=item $json = $json->ascii ($enable)	215	=item $json = $json->ascii ([$enable])
120		216
		217	=item $enabled = $json->get_ascii
		218
121	If C<$enable> is true, then the C<encode> method will not generate	219	If C<$enable> is true (or missing), then the C<encode> method will not
122	characters outside the code range C<0..127>. Any unicode characters	220	generate characters outside the code range C<0..127> (which is ASCII). Any
123	outside that range will be escaped using either a single \uXXXX (BMP	221	Unicode characters outside that range will be escaped using either a
124	characters) or a double \uHHHH\uLLLLL escape sequence, as per RFC4627.	222	single \uXXXX (BMP characters) or a double \uHHHH\uLLLLL escape sequence,
		223	as per RFC4627. The resulting encoded JSON text can be treated as a native
		224	Unicode string, an ascii-encoded, latin1-encoded or UTF-8 encoded string,
		225	or any other superset of ASCII.
125		226
126	If C<$enable> is false, then the C<encode> method will not escape Unicode	227	If C<$enable> is false, then the C<encode> method will not escape Unicode
127	characters unless necessary.	228	characters unless required by the JSON syntax or other flags. This results
		229	in a faster and more compact format.
128		230
		231	See also the section I<ENCODING/CODESET FLAG NOTES> later in this
		232	document.
		233
		234	The main use for this flag is to produce JSON texts that can be
		235	transmitted over a 7-bit channel, as the encoded JSON texts will not
		236	contain any 8 bit characters.
		237
		238	JSON::XS->new->ascii (1)->encode ([chr 0x10401])
		239	=> ["\ud801\udc01"]
		240
		241	=item $json = $json->latin1 ([$enable])
		242
		243	=item $enabled = $json->get_latin1
		244
		245	If C<$enable> is true (or missing), then the C<encode> method will encode
		246	the resulting JSON text as latin1 (or iso-8859-1), escaping any characters
		247	outside the code range C<0..255>. The resulting string can be treated as a
		248	latin1-encoded JSON text or a native Unicode string. The C<decode> method
		249	will not be affected in any way by this flag, as C<decode> by default
		250	expects Unicode, which is a strict superset of latin1.
		251
		252	If C<$enable> is false, then the C<encode> method will not escape Unicode
		253	characters unless required by the JSON syntax or other flags.
		254
		255	See also the section I<ENCODING/CODESET FLAG NOTES> later in this
		256	document.
		257
		258	The main use for this flag is efficiently encoding binary data as JSON
		259	text, as most octets will not be escaped, resulting in a smaller encoded
		260	size. The disadvantage is that the resulting JSON text is encoded
		261	in latin1 (and must correctly be treated as such when storing and
		262	transferring), a rare encoding for JSON. It is therefore most useful when
		263	you want to store data structures known to contain binary data efficiently
		264	in files or databases, not when talking to other JSON encoders/decoders.
		265
		266	JSON::XS->new->latin1->encode (["\x{89}\x{abc}"]
		267	=> ["\x{89}\\u0abc"] # (perl syntax, U+abc escaped, U+89 not)
		268
129	=item $json = $json->utf8 ($enable)	269	=item $json = $json->utf8 ([$enable])
130		270
		271	=item $enabled = $json->get_utf8
		272
131	If C<$enable> is true, then the C<encode> method will encode the JSON	273	If C<$enable> is true (or missing), then the C<encode> method will encode
132	string into UTF-8, as required by many protocols, while the C<decode>	274	the JSON result into UTF-8, as required by many protocols, while the
133	method expects to be handled an UTF-8-encoded string. Please note that	275	C<decode> method expects to be handled an UTF-8-encoded string. Please
134	UTF-8-encoded strings do not contain any characters outside the range	276	note that UTF-8-encoded strings do not contain any characters outside the
135	C<0..255>, they are thus useful for bytewise/binary I/O.	277	range C<0..255>, they are thus useful for bytewise/binary I/O. In future
		278	versions, enabling this option might enable autodetection of the UTF-16
		279	and UTF-32 encoding families, as described in RFC4627.
136		280
137	If C<$enable> is false, then the C<encode> method will return the JSON	281	If C<$enable> is false, then the C<encode> method will return the JSON
138	string as a (non-encoded) unicode string, while C<decode> expects thus a	282	string as a (non-encoded) Unicode string, while C<decode> expects thus a
139	unicode string. Any decoding or encoding (e.g. to UTF-8 or UTF-16) needs	283	Unicode string. Any decoding or encoding (e.g. to UTF-8 or UTF-16) needs
140	to be done yourself, e.g. using the Encode module.	284	to be done yourself, e.g. using the Encode module.
141		285
		286	See also the section I<ENCODING/CODESET FLAG NOTES> later in this
		287	document.
		288
		289	Example, output UTF-16BE-encoded JSON:
		290
		291	use Encode;
		292	$jsontext = encode "UTF-16BE", JSON::XS->new->encode ($object);
		293
		294	Example, decode UTF-32LE-encoded JSON:
		295
		296	use Encode;
		297	$object = JSON::XS->new->decode (decode "UTF-32LE", $jsontext);
		298
142	=item $json = $json->pretty ($enabla)	299	=item $json = $json->pretty ([$enable])
143		300
144	This enables (or disables) all of the C<indent>, C<space_before> and	301	This enables (or disables) all of the C<indent>, C<space_before> and
145	C<space_after> (and in the future possibly more) settings in one call to	302	C<space_after> (and in the future possibly more) flags in one call to
146	generate the most readable (or most compact) form possible.	303	generate the most readable (or most compact) form possible.
147		304
		305	Example, pretty-print some simple structure:
		306
		307	my $json = JSON::XS->new->pretty(1)->encode ({a => [1,2]})
		308	=>
		309	{
		310	"a" : [
		311	1,
		312	2
		313	]
		314	}
		315
148	=item $json = $json->indent ($enable)	316	=item $json = $json->indent ([$enable])
149		317
		318	=item $enabled = $json->get_indent
		319
150	If C<$enable> is true, then the C<encode> method will use a multiline	320	If C<$enable> is true (or missing), then the C<encode> method will use a multiline
151	format as output, putting every array member or object/hash key-value pair	321	format as output, putting every array member or object/hash key-value pair
152	into its own line, identing them properly.	322	into its own line, indenting them properly.
153		323
154	If C<$enable> is false, no newlines or indenting will be produced, and the	324	If C<$enable> is false, no newlines or indenting will be produced, and the
155	resulting JSON strings is guarenteed not to contain any C<newlines>.	325	resulting JSON text is guaranteed not to contain any C<newlines>.
156		326
157	This setting has no effect when decoding JSON strings.	327	This setting has no effect when decoding JSON texts.
158		328
159	=item $json = $json->space_before ($enable)	329	=item $json = $json->space_before ([$enable])
160		330
		331	=item $enabled = $json->get_space_before
		332
161	If C<$enable> is true, then the C<encode> method will add an extra	333	If C<$enable> is true (or missing), then the C<encode> method will add an extra
162	optional space before the C<:> separating keys from values in JSON objects.	334	optional space before the C<:> separating keys from values in JSON objects.
163		335
164	If C<$enable> is false, then the C<encode> method will not add any extra	336	If C<$enable> is false, then the C<encode> method will not add any extra
165	space at those places.	337	space at those places.
166		338
167	This setting has no effect when decoding JSON strings. You will also most	339	This setting has no effect when decoding JSON texts. You will also
168	likely combine this setting with C<space_after>.	340	most likely combine this setting with C<space_after>.
169		341
		342	Example, space_before enabled, space_after and indent disabled:
		343
		344	{"key" :"value"}
		345
170	=item $json = $json->space_after ($enable)	346	=item $json = $json->space_after ([$enable])
171		347
		348	=item $enabled = $json->get_space_after
		349
172	If C<$enable> is true, then the C<encode> method will add an extra	350	If C<$enable> is true (or missing), then the C<encode> method will add an extra
173	optional space after the C<:> separating keys from values in JSON objects	351	optional space after the C<:> separating keys from values in JSON objects
174	and extra whitespace after the C<,> separating key-value pairs and array	352	and extra whitespace after the C<,> separating key-value pairs and array
175	members.	353	members.
176		354
177	If C<$enable> is false, then the C<encode> method will not add any extra	355	If C<$enable> is false, then the C<encode> method will not add any extra
178	space at those places.	356	space at those places.
179		357
180	This setting has no effect when decoding JSON strings.	358	This setting has no effect when decoding JSON texts.
181		359
		360	Example, space_before and indent disabled, space_after enabled:
		361
		362	{"key": "value"}
		363
		364	=item $json = $json->relaxed ([$enable])
		365
		366	=item $enabled = $json->get_relaxed
		367
		368	If C<$enable> is true (or missing), then C<decode> will accept some
		369	extensions to normal JSON syntax (see below). C<encode> will not be
		370	affected in anyway. I<Be aware that this option makes you accept invalid
		371	JSON texts as if they were valid!>. I suggest only to use this option to
		372	parse application-specific files written by humans (configuration files,
		373	resource files etc.)
		374
		375	If C<$enable> is false (the default), then C<decode> will only accept
		376	valid JSON texts.
		377
		378	Currently accepted extensions are:
		379
		380	=over 4
		381
		382	=item * list items can have an end-comma
		383
		384	JSON I<separates> array elements and key-value pairs with commas. This
		385	can be annoying if you write JSON texts manually and want to be able to
		386	quickly append elements, so this extension accepts comma at the end of
		387	such items not just between them:
		388
		389	[
		390	1,
		391	2, <- this comma not normally allowed
		392	]
		393	{
		394	"k1": "v1",
		395	"k2": "v2", <- this comma not normally allowed
		396	}
		397
		398	=item * shell-style '#'-comments
		399
		400	Whenever JSON allows whitespace, shell-style comments are additionally
		401	allowed. They are terminated by the first carriage-return or line-feed
		402	character, after which more white-space and comments are allowed.
		403
		404	[
		405	1, # this comment not allowed in JSON
		406	# neither this one...
		407	]
		408
		409	=back
		410
182	=item $json = $json->canonical ($enable)	411	=item $json = $json->canonical ([$enable])
183		412
		413	=item $enabled = $json->get_canonical
		414
184	If C<$enable> is true, then the C<encode> method will output JSON objects	415	If C<$enable> is true (or missing), then the C<encode> method will output JSON objects
185	by sorting their keys. This is adding a comparatively high overhead.	416	by sorting their keys. This is adding a comparatively high overhead.
186		417
187	If C<$enable> is false, then the C<encode> method will output key-value	418	If C<$enable> is false, then the C<encode> method will output key-value
188	pairs in the order Perl stores them (which will likely change between runs	419	pairs in the order Perl stores them (which will likely change between runs
189	of the same script).	420	of the same script, and can change even within the same run from 5.18
		421	onwards).
190		422
191	This option is useful if you want the same data structure to be encoded as	423	This option is useful if you want the same data structure to be encoded as
192	the same JSON string (given the same overall settings). If it is disabled,	424	the same JSON text (given the same overall settings). If it is disabled,
193	the same hash migh be encoded differently even if contains the same data,	425	the same hash might be encoded differently even if contains the same data,
194	as key-value pairs have no inherent ordering in Perl.	426	as key-value pairs have no inherent ordering in Perl.
195		427
196	This setting has no effect when decoding JSON strings.	428	This setting has no effect when decoding JSON texts.
197		429
		430	This setting has currently no effect on tied hashes.
		431
		432	=item $json = $json->allow_nonref ([$enable])
		433
		434	=item $enabled = $json->get_allow_nonref
		435
		436	If C<$enable> is true (or missing), then the C<encode> method can convert a
		437	non-reference into its corresponding string, number or null JSON value,
		438	which is an extension to RFC4627. Likewise, C<decode> will accept those JSON
		439	values instead of croaking.
		440
		441	If C<$enable> is false, then the C<encode> method will croak if it isn't
		442	passed an arrayref or hashref, as JSON texts must either be an object
		443	or array. Likewise, C<decode> will croak if given something that is not a
		444	JSON object or array.
		445
		446	Example, encode a Perl scalar as JSON value with enabled C<allow_nonref>,
		447	resulting in an invalid JSON text:
		448
		449	JSON::XS->new->allow_nonref->encode ("Hello, World!")
		450	=> "Hello, World!"
		451
		452	=item $json = $json->allow_unknown ([$enable])
		453
		454	=item $enabled = $json->get_allow_unknown
		455
		456	If C<$enable> is true (or missing), then C<encode> will I<not> throw an
		457	exception when it encounters values it cannot represent in JSON (for
		458	example, filehandles) but instead will encode a JSON C<null> value. Note
		459	that blessed objects are not included here and are handled separately by
		460	c<allow_nonref>.
		461
		462	If C<$enable> is false (the default), then C<encode> will throw an
		463	exception when it encounters anything it cannot encode as JSON.
		464
		465	This option does not affect C<decode> in any way, and it is recommended to
		466	leave it off unless you know your communications partner.
		467
		468	=item $json = $json->allow_blessed ([$enable])
		469
		470	=item $enabled = $json->get_allow_blessed
		471
		472	If C<$enable> is true (or missing), then the C<encode> method will not
		473	barf when it encounters a blessed reference. Instead, the value of the
		474	B<convert_blessed> option will decide whether C<null> (C<convert_blessed>
		475	disabled or no C<TO_JSON> method found) or a representation of the
		476	object (C<convert_blessed> enabled and C<TO_JSON> method found) is being
		477	encoded. Has no effect on C<decode>.
		478
		479	If C<$enable> is false (the default), then C<encode> will throw an
		480	exception when it encounters a blessed object.
		481
		482	=item $json = $json->convert_blessed ([$enable])
		483
		484	=item $enabled = $json->get_convert_blessed
		485
		486	If C<$enable> is true (or missing), then C<encode>, upon encountering a
		487	blessed object, will check for the availability of the C<TO_JSON> method
		488	on the object's class. If found, it will be called in scalar context
		489	and the resulting scalar will be encoded instead of the object. If no
		490	C<TO_JSON> method is found, the value of C<allow_blessed> will decide what
		491	to do.
		492
		493	The C<TO_JSON> method may safely call die if it wants. If C<TO_JSON>
		494	returns other blessed objects, those will be handled in the same
		495	way. C<TO_JSON> must take care of not causing an endless recursion cycle
		496	(== crash) in this case. The name of C<TO_JSON> was chosen because other
		497	methods called by the Perl core (== not by the user of the object) are
		498	usually in upper case letters and to avoid collisions with any C<to_json>
		499	function or method.
		500
		501	This setting does not yet influence C<decode> in any way, but in the
		502	future, global hooks might get installed that influence C<decode> and are
		503	enabled by this setting.
		504
		505	If C<$enable> is false, then the C<allow_blessed> setting will decide what
		506	to do when a blessed object is found.
		507
		508	=item $json = $json->filter_json_object ([$coderef->($hashref)])
		509
		510	When C<$coderef> is specified, it will be called from C<decode> each
		511	time it decodes a JSON object. The only argument is a reference to the
		512	newly-created hash. If the code references returns a single scalar (which
		513	need not be a reference), this value (i.e. a copy of that scalar to avoid
		514	aliasing) is inserted into the deserialised data structure. If it returns
		515	an empty list (NOTE: I<not> C<undef>, which is a valid scalar), the
		516	original deserialised hash will be inserted. This setting can slow down
		517	decoding considerably.
		518
		519	When C<$coderef> is omitted or undefined, any existing callback will
		520	be removed and C<decode> will not change the deserialised hash in any
		521	way.
		522
		523	Example, convert all JSON objects into the integer 5:
		524
		525	my $js = JSON::XS->new->filter_json_object (sub { 5 });
		526	# returns [5]
		527	$js->decode ('[{}]')
		528	# throw an exception because allow_nonref is not enabled
		529	# so a lone 5 is not allowed.
		530	$js->decode ('{"a":1, "b":2}');
		531
		532	=item $json = $json->filter_json_single_key_object ($key [=> $coderef->($value)])
		533
		534	Works remotely similar to C<filter_json_object>, but is only called for
		535	JSON objects having a single key named C<$key>.
		536
		537	This C<$coderef> is called before the one specified via
		538	C<filter_json_object>, if any. It gets passed the single value in the JSON
		539	object. If it returns a single value, it will be inserted into the data
		540	structure. If it returns nothing (not even C<undef> but the empty list),
		541	the callback from C<filter_json_object> will be called next, as if no
		542	single-key callback were specified.
		543
		544	If C<$coderef> is omitted or undefined, the corresponding callback will be
		545	disabled. There can only ever be one callback for a given key.
		546
		547	As this callback gets called less often then the C<filter_json_object>
		548	one, decoding speed will not usually suffer as much. Therefore, single-key
		549	objects make excellent targets to serialise Perl objects into, especially
		550	as single-key JSON objects are as close to the type-tagged value concept
		551	as JSON gets (it's basically an ID/VALUE tuple). Of course, JSON does not
		552	support this in any way, so you need to make sure your data never looks
		553	like a serialised Perl hash.
		554
		555	Typical names for the single object key are C<__class_whatever__>, or
		556	C<$__dollars_are_rarely_used__$> or C<}ugly_brace_placement>, or even
		557	things like C<__class_md5sum(classname)__>, to reduce the risk of clashing
		558	with real hashes.
		559
		560	Example, decode JSON objects of the form C<< { "__widget__" => <id> } >>
		561	into the corresponding C<< $WIDGET{<id>} >> object:
		562
		563	# return whatever is in $WIDGET{5}:
		564	JSON::XS
		565	->new
		566	->filter_json_single_key_object (__widget__ => sub {
		567	$WIDGET{ $_[0] }
		568	})
		569	->decode ('{"__widget__": 5')
		570
		571	# this can be used with a TO_JSON method in some "widget" class
		572	# for serialisation to json:
		573	sub WidgetBase::TO_JSON {
		574	my ($self) = @_;
		575
		576	unless ($self->{id}) {
		577	$self->{id} = ..get..some..id..;
		578	$WIDGET{$self->{id}} = $self;
		579	}
		580
		581	{ __widget__ => $self->{id} }
		582	}
		583
		584	=item $json = $json->shrink ([$enable])
		585
		586	=item $enabled = $json->get_shrink
		587
		588	Perl usually over-allocates memory a bit when allocating space for
		589	strings. This flag optionally resizes strings generated by either
		590	C<encode> or C<decode> to their minimum size possible. This can save
		591	memory when your JSON texts are either very very long or you have many
		592	short strings. It will also try to downgrade any strings to octet-form
		593	if possible: perl stores strings internally either in an encoding called
		594	UTF-X or in octet-form. The latter cannot store everything but uses less
		595	space in general (and some buggy Perl or C code might even rely on that
		596	internal representation being used).
		597
		598	The actual definition of what shrink does might change in future versions,
		599	but it will always try to save space at the expense of time.
		600
		601	If C<$enable> is true (or missing), the string returned by C<encode> will
		602	be shrunk-to-fit, while all strings generated by C<decode> will also be
		603	shrunk-to-fit.
		604
		605	If C<$enable> is false, then the normal perl allocation algorithms are used.
		606	If you work with your data, then this is likely to be faster.
		607
		608	In the future, this setting might control other things, such as converting
		609	strings that look like integers or floats into integers or floats
		610	internally (there is no difference on the Perl level), saving space.
		611
		612	=item $json = $json->max_depth ([$maximum_nesting_depth])
		613
		614	=item $max_depth = $json->get_max_depth
		615
		616	Sets the maximum nesting level (default C<512>) accepted while encoding
		617	or decoding. If a higher nesting level is detected in JSON text or a Perl
		618	data structure, then the encoder and decoder will stop and croak at that
		619	point.
		620
		621	Nesting level is defined by number of hash- or arrayrefs that the encoder
		622	needs to traverse to reach a given point or the number of C<{> or C<[>
		623	characters without their matching closing parenthesis crossed to reach a
		624	given character in a string.
		625
		626	Setting the maximum depth to one disallows any nesting, so that ensures
		627	that the object is only a single hash/object or array.
		628
		629	If no argument is given, the highest possible setting will be used, which
		630	is rarely useful.
		631
		632	Note that nesting is implemented by recursion in C. The default value has
		633	been chosen to be as large as typical operating systems allow without
		634	crashing.
		635
		636	See SECURITY CONSIDERATIONS, below, for more info on why this is useful.
		637
		638	=item $json = $json->max_size ([$maximum_string_size])
		639
		640	=item $max_size = $json->get_max_size
		641
		642	Set the maximum length a JSON text may have (in bytes) where decoding is
		643	being attempted. The default is C<0>, meaning no limit. When C<decode>
		644	is called on a string that is longer then this many bytes, it will not
		645	attempt to decode the string but throw an exception. This setting has no
		646	effect on C<encode> (yet).
		647
		648	If no argument is given, the limit check will be deactivated (same as when
		649	C<0> is specified).
		650
		651	See SECURITY CONSIDERATIONS, below, for more info on why this is useful.
		652
198	=item $json_string = $json->encode ($perl_scalar)	653	=item $json_text = $json->encode ($perl_scalar)
199		654
200	Converts the given Perl data structure (a simple scalar or a reference	655	Converts the given Perl value or data structure to its JSON
201	to a hash or array) to its JSON representation. Simple scalars will be	656	representation. Croaks on error.
202	converted into JSON string or number sequences, while references to arrays
203	become JSON arrays and references to hashes become JSON objects. Undefined
204	Perl values (e.g. C<undef>) become JSON C<null> values. Neither C<true>
205	nor C<false> values will be generated.
206		657
207	=item $perl_scalar = $json->decode ($json_string)	658	=item $perl_scalar = $json->decode ($json_text)
208		659
209	The opposite of C<encode>: expects a JSON string and tries to parse it,	660	The opposite of C<encode>: expects a JSON text and tries to parse it,
210	returning the resulting simple scalar or reference. Croaks on error.	661	returning the resulting simple scalar or reference. Croaks on error.
211		662
212	JSON numbers and strings become simple Perl scalars. JSON arrays become	663	=item ($perl_scalar, $characters) = $json->decode_prefix ($json_text)
213	Perl arrayrefs and JSON objects become Perl hashrefs. C<true> becomes	664
214	C<1>, C<false> becomes C<0> and C<null> becomes C<undef>.	665	This works like the C<decode> method, but instead of raising an exception
		666	when there is trailing garbage after the first JSON object, it will
		667	silently stop parsing there and return the number of characters consumed
		668	so far.
		669
		670	This is useful if your JSON texts are not delimited by an outer protocol
		671	and you need to know where the JSON text ends.
		672
		673	JSON::XS->new->decode_prefix ("[1] the tail")
		674	=> ([], 3)
215		675
216	=back	676	=back
217		677
		678
		679	=head1 INCREMENTAL PARSING
		680
		681	In some cases, there is the need for incremental parsing of JSON
		682	texts. While this module always has to keep both JSON text and resulting
		683	Perl data structure in memory at one time, it does allow you to parse a
		684	JSON stream incrementally. It does so by accumulating text until it has
		685	a full JSON object, which it then can decode. This process is similar to
		686	using C<decode_prefix> to see if a full JSON object is available, but
		687	is much more efficient (and can be implemented with a minimum of method
		688	calls).
		689
		690	JSON::XS will only attempt to parse the JSON text once it is sure it
		691	has enough text to get a decisive result, using a very simple but
		692	truly incremental parser. This means that it sometimes won't stop as
		693	early as the full parser, for example, it doesn't detect mismatched
		694	parentheses. The only thing it guarantees is that it starts decoding as
		695	soon as a syntactically valid JSON text has been seen. This means you need
		696	to set resource limits (e.g. C<max_size>) to ensure the parser will stop
		697	parsing in the presence if syntax errors.
		698
		699	The following methods implement this incremental parser.
		700
		701	=over 4
		702
		703	=item [void, scalar or list context] = $json->incr_parse ([$string])
		704
		705	This is the central parsing function. It can both append new text and
		706	extract objects from the stream accumulated so far (both of these
		707	functions are optional).
		708
		709	If C<$string> is given, then this string is appended to the already
		710	existing JSON fragment stored in the C<$json> object.
		711
		712	After that, if the function is called in void context, it will simply
		713	return without doing anything further. This can be used to add more text
		714	in as many chunks as you want.
		715
		716	If the method is called in scalar context, then it will try to extract
		717	exactly I<one> JSON object. If that is successful, it will return this
		718	object, otherwise it will return C<undef>. If there is a parse error,
		719	this method will croak just as C<decode> would do (one can then use
		720	C<incr_skip> to skip the erroneous part). This is the most common way of
		721	using the method.
		722
		723	And finally, in list context, it will try to extract as many objects
		724	from the stream as it can find and return them, or the empty list
		725	otherwise. For this to work, there must be no separators between the JSON
		726	objects or arrays, instead they must be concatenated back-to-back. If
		727	an error occurs, an exception will be raised as in the scalar context
		728	case. Note that in this case, any previously-parsed JSON texts will be
		729	lost.
		730
		731	Example: Parse some JSON arrays/objects in a given string and return
		732	them.
		733
		734	my @objs = JSON::XS->new->incr_parse ("[5][7][1,2]");
		735
		736	=item $lvalue_string = $json->incr_text
		737
		738	This method returns the currently stored JSON fragment as an lvalue, that
		739	is, you can manipulate it. This I<only> works when a preceding call to
		740	C<incr_parse> in I<scalar context> successfully returned an object. Under
		741	all other circumstances you must not call this function (I mean it.
		742	although in simple tests it might actually work, it I<will> fail under
		743	real world conditions). As a special exception, you can also call this
		744	method before having parsed anything.
		745
		746	This function is useful in two cases: a) finding the trailing text after a
		747	JSON object or b) parsing multiple JSON objects separated by non-JSON text
		748	(such as commas).
		749
		750	=item $json->incr_skip
		751
		752	This will reset the state of the incremental parser and will remove
		753	the parsed text from the input buffer so far. This is useful after
		754	C<incr_parse> died, in which case the input buffer and incremental parser
		755	state is left unchanged, to skip the text parsed so far and to reset the
		756	parse state.
		757
		758	The difference to C<incr_reset> is that only text until the parse error
		759	occurred is removed.
		760
		761	=item $json->incr_reset
		762
		763	This completely resets the incremental parser, that is, after this call,
		764	it will be as if the parser had never parsed anything.
		765
		766	This is useful if you want to repeatedly parse JSON objects and want to
		767	ignore any trailing data, which means you have to reset the parser after
		768	each successful decode.
		769
		770	=back
		771
		772	=head2 LIMITATIONS
		773
		774	All options that affect decoding are supported, except
		775	C<allow_nonref>. The reason for this is that it cannot be made to work
		776	sensibly: JSON objects and arrays are self-delimited, i.e. you can
		777	concatenate them back to back and still decode them perfectly. This does
		778	not hold true for JSON numbers, however.
		779
		780	For example, is the string C<1> a single JSON number, or is it simply the
		781	start of C<12>? Or is C<12> a single JSON number, or the concatenation
		782	of C<1> and C<2>? In neither case you can tell, and this is why JSON::XS
		783	takes the conservative route and disallows this case.
		784
		785	=head2 EXAMPLES
		786
		787	Some examples will make all this clearer. First, a simple example that
		788	works similarly to C<decode_prefix>: We want to decode the JSON object at
		789	the start of a string and identify the portion after the JSON object:
		790
		791	my $text = "[1,2,3] hello";
		792
		793	my $json = new JSON::XS;
		794
		795	my $obj = $json->incr_parse ($text)
		796	or die "expected JSON object or array at beginning of string";
		797
		798	my $tail = $json->incr_text;
		799	# $tail now contains " hello"
		800
		801	Easy, isn't it?
		802
		803	Now for a more complicated example: Imagine a hypothetical protocol where
		804	you read some requests from a TCP stream, and each request is a JSON
		805	array, without any separation between them (in fact, it is often useful to
		806	use newlines as "separators", as these get interpreted as whitespace at
		807	the start of the JSON text, which makes it possible to test said protocol
		808	with C<telnet>...).
		809
		810	Here is how you'd do it (it is trivial to write this in an event-based
		811	manner):
		812
		813	my $json = new JSON::XS;
		814
		815	# read some data from the socket
		816	while (sysread $socket, my $buf, 4096) {
		817
		818	# split and decode as many requests as possible
		819	for my $request ($json->incr_parse ($buf)) {
		820	# act on the $request
		821	}
		822	}
		823
		824	Another complicated example: Assume you have a string with JSON objects
		825	or arrays, all separated by (optional) comma characters (e.g. C<[1],[2],
		826	[3]>). To parse them, we have to skip the commas between the JSON texts,
		827	and here is where the lvalue-ness of C<incr_text> comes in useful:
		828
		829	my $text = "[1],[2], [3]";
		830	my $json = new JSON::XS;
		831
		832	# void context, so no parsing done
		833	$json->incr_parse ($text);
		834
		835	# now extract as many objects as possible. note the
		836	# use of scalar context so incr_text can be called.
		837	while (my $obj = $json->incr_parse) {
		838	# do something with $obj
		839
		840	# now skip the optional comma
		841	$json->incr_text =~ s/^ \s* , //x;
		842	}
		843
		844	Now lets go for a very complex example: Assume that you have a gigantic
		845	JSON array-of-objects, many gigabytes in size, and you want to parse it,
		846	but you cannot load it into memory fully (this has actually happened in
		847	the real world :).
		848
		849	Well, you lost, you have to implement your own JSON parser. But JSON::XS
		850	can still help you: You implement a (very simple) array parser and let
		851	JSON decode the array elements, which are all full JSON objects on their
		852	own (this wouldn't work if the array elements could be JSON numbers, for
		853	example):
		854
		855	my $json = new JSON::XS;
		856
		857	# open the monster
		858	open my $fh, "<bigfile.json"
		859	or die "bigfile: $!";
		860
		861	# first parse the initial "["
		862	for (;;) {
		863	sysread $fh, my $buf, 65536
		864	or die "read error: $!";
		865	$json->incr_parse ($buf); # void context, so no parsing
		866
		867	# Exit the loop once we found and removed(!) the initial "[".
		868	# In essence, we are (ab-)using the $json object as a simple scalar
		869	# we append data to.
		870	last if $json->incr_text =~ s/^ \s* \[ //x;
		871	}
		872
		873	# now we have the skipped the initial "[", so continue
		874	# parsing all the elements.
		875	for (;;) {
		876	# in this loop we read data until we got a single JSON object
		877	for (;;) {
		878	if (my $obj = $json->incr_parse) {
		879	# do something with $obj
		880	last;
		881	}
		882
		883	# add more data
		884	sysread $fh, my $buf, 65536
		885	or die "read error: $!";
		886	$json->incr_parse ($buf); # void context, so no parsing
		887	}
		888
		889	# in this loop we read data until we either found and parsed the
		890	# separating "," between elements, or the final "]"
		891	for (;;) {
		892	# first skip whitespace
		893	$json->incr_text =~ s/^\s*//;
		894
		895	# if we find "]", we are done
		896	if ($json->incr_text =~ s/^\]//) {
		897	print "finished.\n";
		898	exit;
		899	}
		900
		901	# if we find ",", we can continue with the next element
		902	if ($json->incr_text =~ s/^,//) {
		903	last;
		904	}
		905
		906	# if we find anything else, we have a parse error!
		907	if (length $json->incr_text) {
		908	die "parse error near ", $json->incr_text;
		909	}
		910
		911	# else add more data
		912	sysread $fh, my $buf, 65536
		913	or die "read error: $!";
		914	$json->incr_parse ($buf); # void context, so no parsing
		915	}
		916
		917	This is a complex example, but most of the complexity comes from the fact
		918	that we are trying to be correct (bear with me if I am wrong, I never ran
		919	the above example :).
		920
		921
		922
		923	=head1 MAPPING
		924
		925	This section describes how JSON::XS maps Perl values to JSON values and
		926	vice versa. These mappings are designed to "do the right thing" in most
		927	circumstances automatically, preserving round-tripping characteristics
		928	(what you put in comes out as something equivalent).
		929
		930	For the more enlightened: note that in the following descriptions,
		931	lowercase I<perl> refers to the Perl interpreter, while uppercase I<Perl>
		932	refers to the abstract Perl language itself.
		933
		934
		935	=head2 JSON -> PERL
		936
		937	=over 4
		938
		939	=item object
		940
		941	A JSON object becomes a reference to a hash in Perl. No ordering of object
		942	keys is preserved (JSON does not preserve object key ordering itself).
		943
		944	=item array
		945
		946	A JSON array becomes a reference to an array in Perl.
		947
		948	=item string
		949
		950	A JSON string becomes a string scalar in Perl - Unicode codepoints in JSON
		951	are represented by the same codepoints in the Perl string, so no manual
		952	decoding is necessary.
		953
		954	=item number
		955
		956	A JSON number becomes either an integer, numeric (floating point) or
		957	string scalar in perl, depending on its range and any fractional parts. On
		958	the Perl level, there is no difference between those as Perl handles all
		959	the conversion details, but an integer may take slightly less memory and
		960	might represent more values exactly than floating point numbers.
		961
		962	If the number consists of digits only, JSON::XS will try to represent
		963	it as an integer value. If that fails, it will try to represent it as
		964	a numeric (floating point) value if that is possible without loss of
		965	precision. Otherwise it will preserve the number as a string value (in
		966	which case you lose roundtripping ability, as the JSON number will be
		967	re-encoded to a JSON string).
		968
		969	Numbers containing a fractional or exponential part will always be
		970	represented as numeric (floating point) values, possibly at a loss of
		971	precision (in which case you might lose perfect roundtripping ability, but
		972	the JSON number will still be re-encoded as a JSON number).
		973
		974	Note that precision is not accuracy - binary floating point values cannot
		975	represent most decimal fractions exactly, and when converting from and to
		976	floating point, JSON::XS only guarantees precision up to but not including
		977	the least significant bit.
		978
		979	=item true, false
		980
		981	These JSON atoms become C<Types::Serialiser::true> and
		982	C<Types::Serialiser::false>, respectively. They are overloaded to act
		983	almost exactly like the numbers C<1> and C<0>. You can check whether
		984	a scalar is a JSON boolean by using the C<Types::Serialiser::is_bool>
		985	function (after C<use Types::Serialier>, of course).
		986
		987	=item null
		988
		989	A JSON null atom becomes C<undef> in Perl.
		990
		991	=back
		992
		993
		994	=head2 PERL -> JSON
		995
		996	The mapping from Perl to JSON is slightly more difficult, as Perl is a
		997	truly typeless language, so we can only guess which JSON type is meant by
		998	a Perl value.
		999
		1000	=over 4
		1001
		1002	=item hash references
		1003
		1004	Perl hash references become JSON objects. As there is no inherent
		1005	ordering in hash keys (or JSON objects), they will usually be encoded
		1006	in a pseudo-random order. JSON::XS can optionally sort the hash keys
		1007	(determined by the I<canonical> flag), so the same datastructure will
		1008	serialise to the same JSON text (given same settings and version of
		1009	JSON::XS), but this incurs a runtime overhead and is only rarely useful,
		1010	e.g. when you want to compare some JSON text against another for equality.
		1011
		1012	=item array references
		1013
		1014	Perl array references become JSON arrays.
		1015
		1016	=item other references
		1017
		1018	Other unblessed references are generally not allowed and will cause an
		1019	exception to be thrown, except for references to the integers C<0> and
		1020	C<1>, which get turned into C<false> and C<true> atoms in JSON.
		1021
		1022	Since C<JSON::XS> uses the boolean model from L<Types::Serialiser>, you
		1023	can also C<use Types::Serialiser> and then use C<Types::Serialiser::false>
		1024	and C<Types::Serialiser::true> to improve readability.
		1025
		1026	use Types::Serialiser;
		1027	encode_json [\0, Types::Serialiser::true] # yields [false,true]
		1028
		1029	=item Types::Serialiser::true, Types::Serialiser::false
		1030
		1031	These special values from the L<Types::Serialiser> module become JSON true
		1032	and JSON false values, respectively. You can also use C<\1> and C<\0>
		1033	directly if you want.
		1034
		1035	=item blessed objects
		1036
		1037	Blessed objects are not directly representable in JSON. See the
		1038	C<allow_blessed> and C<convert_blessed> methods on various options on
		1039	how to deal with this: basically, you can choose between throwing an
		1040	exception, encoding the reference as if it weren't blessed, or provide
		1041	your own serialiser method.
		1042
		1043	=item simple scalars
		1044
		1045	Simple Perl scalars (any scalar that is not a reference) are the most
		1046	difficult objects to encode: JSON::XS will encode undefined scalars as
		1047	JSON C<null> values, scalars that have last been used in a string context
		1048	before encoding as JSON strings, and anything else as number value:
		1049
		1050	# dump as number
		1051	encode_json [2] # yields [2]
		1052	encode_json [-3.0e17] # yields [-3e+17]
		1053	my $value = 5; encode_json [$value] # yields [5]
		1054
		1055	# used as string, so dump as string
		1056	print $value;
		1057	encode_json [$value] # yields ["5"]
		1058
		1059	# undef becomes null
		1060	encode_json [undef] # yields [null]
		1061
		1062	You can force the type to be a JSON string by stringifying it:
		1063
		1064	my $x = 3.1; # some variable containing a number
		1065	"$x"; # stringified
		1066	$x .= ""; # another, more awkward way to stringify
		1067	print $x; # perl does it for you, too, quite often
		1068
		1069	You can force the type to be a JSON number by numifying it:
		1070
		1071	my $x = "3"; # some variable containing a string
		1072	$x += 0; # numify it, ensuring it will be dumped as a number
		1073	$x *= 1; # same thing, the choice is yours.
		1074
		1075	You can not currently force the type in other, less obscure, ways. Tell me
		1076	if you need this capability (but don't forget to explain why it's needed
		1077	:).
		1078
		1079	Note that numerical precision has the same meaning as under Perl (so
		1080	binary to decimal conversion follows the same rules as in Perl, which
		1081	can differ to other languages). Also, your perl interpreter might expose
		1082	extensions to the floating point numbers of your platform, such as
		1083	infinities or NaN's - these cannot be represented in JSON, and it is an
		1084	error to pass those in.
		1085
		1086	=back
		1087
		1088
		1089	=head1 ENCODING/CODESET FLAG NOTES
		1090
		1091	The interested reader might have seen a number of flags that signify
		1092	encodings or codesets - C<utf8>, C<latin1> and C<ascii>. There seems to be
		1093	some confusion on what these do, so here is a short comparison:
		1094
		1095	C<utf8> controls whether the JSON text created by C<encode> (and expected
		1096	by C<decode>) is UTF-8 encoded or not, while C<latin1> and C<ascii> only
		1097	control whether C<encode> escapes character values outside their respective
		1098	codeset range. Neither of these flags conflict with each other, although
		1099	some combinations make less sense than others.
		1100
		1101	Care has been taken to make all flags symmetrical with respect to
		1102	C<encode> and C<decode>, that is, texts encoded with any combination of
		1103	these flag values will be correctly decoded when the same flags are used
		1104	- in general, if you use different flag settings while encoding vs. when
		1105	decoding you likely have a bug somewhere.
		1106
		1107	Below comes a verbose discussion of these flags. Note that a "codeset" is
		1108	simply an abstract set of character-codepoint pairs, while an encoding
		1109	takes those codepoint numbers and I<encodes> them, in our case into
		1110	octets. Unicode is (among other things) a codeset, UTF-8 is an encoding,
		1111	and ISO-8859-1 (= latin 1) and ASCII are both codesets I<and> encodings at
		1112	the same time, which can be confusing.
		1113
		1114	=over 4
		1115
		1116	=item C<utf8> flag disabled
		1117
		1118	When C<utf8> is disabled (the default), then C<encode>/C<decode> generate
		1119	and expect Unicode strings, that is, characters with high ordinal Unicode
		1120	values (> 255) will be encoded as such characters, and likewise such
		1121	characters are decoded as-is, no changes to them will be done, except
		1122	"(re-)interpreting" them as Unicode codepoints or Unicode characters,
		1123	respectively (to Perl, these are the same thing in strings unless you do
		1124	funny/weird/dumb stuff).
		1125
		1126	This is useful when you want to do the encoding yourself (e.g. when you
		1127	want to have UTF-16 encoded JSON texts) or when some other layer does
		1128	the encoding for you (for example, when printing to a terminal using a
		1129	filehandle that transparently encodes to UTF-8 you certainly do NOT want
		1130	to UTF-8 encode your data first and have Perl encode it another time).
		1131
		1132	=item C<utf8> flag enabled
		1133
		1134	If the C<utf8>-flag is enabled, C<encode>/C<decode> will encode all
		1135	characters using the corresponding UTF-8 multi-byte sequence, and will
		1136	expect your input strings to be encoded as UTF-8, that is, no "character"
		1137	of the input string must have any value > 255, as UTF-8 does not allow
		1138	that.
		1139
		1140	The C<utf8> flag therefore switches between two modes: disabled means you
		1141	will get a Unicode string in Perl, enabled means you get an UTF-8 encoded
		1142	octet/binary string in Perl.
		1143
		1144	=item C<latin1> or C<ascii> flags enabled
		1145
		1146	With C<latin1> (or C<ascii>) enabled, C<encode> will escape characters
		1147	with ordinal values > 255 (> 127 with C<ascii>) and encode the remaining
		1148	characters as specified by the C<utf8> flag.
		1149
		1150	If C<utf8> is disabled, then the result is also correctly encoded in those
		1151	character sets (as both are proper subsets of Unicode, meaning that a
		1152	Unicode string with all character values < 256 is the same thing as a
		1153	ISO-8859-1 string, and a Unicode string with all character values < 128 is
		1154	the same thing as an ASCII string in Perl).
		1155
		1156	If C<utf8> is enabled, you still get a correct UTF-8-encoded string,
		1157	regardless of these flags, just some more characters will be escaped using
		1158	C<\uXXXX> then before.
		1159
		1160	Note that ISO-8859-1-I<encoded> strings are not compatible with UTF-8
		1161	encoding, while ASCII-encoded strings are. That is because the ISO-8859-1
		1162	encoding is NOT a subset of UTF-8 (despite the ISO-8859-1 I<codeset> being
		1163	a subset of Unicode), while ASCII is.
		1164
		1165	Surprisingly, C<decode> will ignore these flags and so treat all input
		1166	values as governed by the C<utf8> flag. If it is disabled, this allows you
		1167	to decode ISO-8859-1- and ASCII-encoded strings, as both strict subsets of
		1168	Unicode. If it is enabled, you can correctly decode UTF-8 encoded strings.
		1169
		1170	So neither C<latin1> nor C<ascii> are incompatible with the C<utf8> flag -
		1171	they only govern when the JSON output engine escapes a character or not.
		1172
		1173	The main use for C<latin1> is to relatively efficiently store binary data
		1174	as JSON, at the expense of breaking compatibility with most JSON decoders.
		1175
		1176	The main use for C<ascii> is to force the output to not contain characters
		1177	with values > 127, which means you can interpret the resulting string
		1178	as UTF-8, ISO-8859-1, ASCII, KOI8-R or most about any character set and
		1179	8-bit-encoding, and still get the same data structure back. This is useful
		1180	when your channel for JSON transfer is not 8-bit clean or the encoding
		1181	might be mangled in between (e.g. in mail), and works because ASCII is a
		1182	proper subset of most 8-bit and multibyte encodings in use in the world.
		1183
		1184	=back
		1185
		1186
		1187	=head2 JSON and ECMAscript
		1188
		1189	JSON syntax is based on how literals are represented in javascript (the
		1190	not-standardised predecessor of ECMAscript) which is presumably why it is
		1191	called "JavaScript Object Notation".
		1192
		1193	However, JSON is not a subset (and also not a superset of course) of
		1194	ECMAscript (the standard) or javascript (whatever browsers actually
		1195	implement).
		1196
		1197	If you want to use javascript's C<eval> function to "parse" JSON, you
		1198	might run into parse errors for valid JSON texts, or the resulting data
		1199	structure might not be queryable:
		1200
		1201	One of the problems is that U+2028 and U+2029 are valid characters inside
		1202	JSON strings, but are not allowed in ECMAscript string literals, so the
		1203	following Perl fragment will not output something that can be guaranteed
		1204	to be parsable by javascript's C<eval>:
		1205
		1206	use JSON::XS;
		1207
		1208	print encode_json [chr 0x2028];
		1209
		1210	The right fix for this is to use a proper JSON parser in your javascript
		1211	programs, and not rely on C<eval> (see for example Douglas Crockford's
		1212	F<json2.js> parser).
		1213
		1214	If this is not an option, you can, as a stop-gap measure, simply encode to
		1215	ASCII-only JSON:
		1216
		1217	use JSON::XS;
		1218
		1219	print JSON::XS->new->ascii->encode ([chr 0x2028]);
		1220
		1221	Note that this will enlarge the resulting JSON text quite a bit if you
		1222	have many non-ASCII characters. You might be tempted to run some regexes
		1223	to only escape U+2028 and U+2029, e.g.:
		1224
		1225	# DO NOT USE THIS!
		1226	my $json = JSON::XS->new->utf8->encode ([chr 0x2028]);
		1227	$json =~ s/\xe2\x80\xa8/\\u2028/g; # escape U+2028
		1228	$json =~ s/\xe2\x80\xa9/\\u2029/g; # escape U+2029
		1229	print $json;
		1230
		1231	Note that I<this is a bad idea>: the above only works for U+2028 and
		1232	U+2029 and thus only for fully ECMAscript-compliant parsers. Many existing
		1233	javascript implementations, however, have issues with other characters as
		1234	well - using C<eval> naively simply I<will> cause problems.
		1235
		1236	Another problem is that some javascript implementations reserve
		1237	some property names for their own purposes (which probably makes
		1238	them non-ECMAscript-compliant). For example, Iceweasel reserves the
		1239	C<__proto__> property name for its own purposes.
		1240
		1241	If that is a problem, you could parse try to filter the resulting JSON
		1242	output for these property strings, e.g.:
		1243
		1244	$json =~ s/"__proto__"\s*:/"__proto__renamed":/g;
		1245
		1246	This works because C<__proto__> is not valid outside of strings, so every
		1247	occurrence of C<"__proto__"\s*:> must be a string used as property name.
		1248
		1249	If you know of other incompatibilities, please let me know.
		1250
		1251
		1252	=head2 JSON and YAML
		1253
		1254	You often hear that JSON is a subset of YAML. This is, however, a mass
		1255	hysteria(*) and very far from the truth (as of the time of this writing),
		1256	so let me state it clearly: I<in general, there is no way to configure
		1257	JSON::XS to output a data structure as valid YAML> that works in all
		1258	cases.
		1259
		1260	If you really must use JSON::XS to generate YAML, you should use this
		1261	algorithm (subject to change in future versions):
		1262
		1263	my $to_yaml = JSON::XS->new->utf8->space_after (1);
		1264	my $yaml = $to_yaml->encode ($ref) . "\n";
		1265
		1266	This will I<usually> generate JSON texts that also parse as valid
		1267	YAML. Please note that YAML has hardcoded limits on (simple) object key
		1268	lengths that JSON doesn't have and also has different and incompatible
		1269	unicode character escape syntax, so you should make sure that your hash
		1270	keys are noticeably shorter than the 1024 "stream characters" YAML allows
		1271	and that you do not have characters with codepoint values outside the
		1272	Unicode BMP (basic multilingual page). YAML also does not allow C<\/>
		1273	sequences in strings (which JSON::XS does not I<currently> generate, but
		1274	other JSON generators might).
		1275
		1276	There might be other incompatibilities that I am not aware of (or the YAML
		1277	specification has been changed yet again - it does so quite often). In
		1278	general you should not try to generate YAML with a JSON generator or vice
		1279	versa, or try to parse JSON with a YAML parser or vice versa: chances are
		1280	high that you will run into severe interoperability problems when you
		1281	least expect it.
		1282
		1283	=over 4
		1284
		1285	=item (*)
		1286
		1287	I have been pressured multiple times by Brian Ingerson (one of the
		1288	authors of the YAML specification) to remove this paragraph, despite him
		1289	acknowledging that the actual incompatibilities exist. As I was personally
		1290	bitten by this "JSON is YAML" lie, I refused and said I will continue to
		1291	educate people about these issues, so others do not run into the same
		1292	problem again and again. After this, Brian called me a (quote)I<complete
		1293	and worthless idiot>(unquote).
		1294
		1295	In my opinion, instead of pressuring and insulting people who actually
		1296	clarify issues with YAML and the wrong statements of some of its
		1297	proponents, I would kindly suggest reading the JSON spec (which is not
		1298	that difficult or long) and finally make YAML compatible to it, and
		1299	educating users about the changes, instead of spreading lies about the
		1300	real compatibility for many I<years> and trying to silence people who
		1301	point out that it isn't true.
		1302
		1303	Addendum/2009: the YAML 1.2 spec is still incompatible with JSON, even
		1304	though the incompatibilities have been documented (and are known to Brian)
		1305	for many years and the spec makes explicit claims that YAML is a superset
		1306	of JSON. It would be so easy to fix, but apparently, bullying people and
		1307	corrupting userdata is so much easier.
		1308
		1309	=back
		1310
		1311
		1312	=head2 SPEED
		1313
		1314	It seems that JSON::XS is surprisingly fast, as shown in the following
		1315	tables. They have been generated with the help of the C<eg/bench> program
		1316	in the JSON::XS distribution, to make it easy to compare on your own
		1317	system.
		1318
		1319	First comes a comparison between various modules using
		1320	a very short single-line JSON string (also available at
		1321	L<http://dist.schmorp.de/misc/json/short.json>).
		1322
		1323	{"method": "handleMessage", "params": ["user1",
		1324	"we were just talking"], "id": null, "array":[1,11,234,-5,1e5,1e7,
		1325	1, 0]}
		1326
		1327	It shows the number of encodes/decodes per second (JSON::XS uses
		1328	the functional interface, while JSON::XS/2 uses the OO interface
		1329	with pretty-printing and hashkey sorting enabled, JSON::XS/3 enables
		1330	shrink. JSON::DWIW/DS uses the deserialise function, while JSON::DWIW::FJ
		1331	uses the from_json method). Higher is better:
		1332
		1333	module | encode | decode |
		1334	--------------|------------|------------|
		1335	JSON::DWIW/DS | 86302.551 | 102300.098 |
		1336	JSON::DWIW/FJ | 86302.551 | 75983.768 |
		1337	JSON::PP | 15827.562 | 6638.658 |
		1338	JSON::Syck | 63358.066 | 47662.545 |
		1339	JSON::XS | 511500.488 | 511500.488 |
		1340	JSON::XS/2 | 291271.111 | 388361.481 |
		1341	JSON::XS/3 | 361577.931 | 361577.931 |
		1342	Storable | 66788.280 | 265462.278 |
		1343	--------------+------------+------------+
		1344
		1345	That is, JSON::XS is almost six times faster than JSON::DWIW on encoding,
		1346	about five times faster on decoding, and over thirty to seventy times
		1347	faster than JSON's pure perl implementation. It also compares favourably
		1348	to Storable for small amounts of data.
		1349
		1350	Using a longer test string (roughly 18KB, generated from Yahoo! Locals
		1351	search API (L<http://dist.schmorp.de/misc/json/long.json>).
		1352
		1353	module | encode | decode |
		1354	--------------|------------|------------|
		1355	JSON::DWIW/DS | 1647.927 | 2673.916 |
		1356	JSON::DWIW/FJ | 1630.249 | 2596.128 |
		1357	JSON::PP | 400.640 | 62.311 |
		1358	JSON::Syck | 1481.040 | 1524.869 |
		1359	JSON::XS | 20661.596 | 9541.183 |
		1360	JSON::XS/2 | 10683.403 | 9416.938 |
		1361	JSON::XS/3 | 20661.596 | 9400.054 |
		1362	Storable | 19765.806 | 10000.725 |
		1363	--------------+------------+------------+
		1364
		1365	Again, JSON::XS leads by far (except for Storable which non-surprisingly
		1366	decodes a bit faster).
		1367
		1368	On large strings containing lots of high Unicode characters, some modules
		1369	(such as JSON::PC) seem to decode faster than JSON::XS, but the result
		1370	will be broken due to missing (or wrong) Unicode handling. Others refuse
		1371	to decode or encode properly, so it was impossible to prepare a fair
		1372	comparison table for that case.
		1373
		1374
		1375	=head1 SECURITY CONSIDERATIONS
		1376
		1377	When you are using JSON in a protocol, talking to untrusted potentially
		1378	hostile creatures requires relatively few measures.
		1379
		1380	First of all, your JSON decoder should be secure, that is, should not have
		1381	any buffer overflows. Obviously, this module should ensure that and I am
		1382	trying hard on making that true, but you never know.
		1383
		1384	Second, you need to avoid resource-starving attacks. That means you should
		1385	limit the size of JSON texts you accept, or make sure then when your
		1386	resources run out, that's just fine (e.g. by using a separate process that
		1387	can crash safely). The size of a JSON text in octets or characters is
		1388	usually a good indication of the size of the resources required to decode
		1389	it into a Perl structure. While JSON::XS can check the size of the JSON
		1390	text, it might be too late when you already have it in memory, so you
		1391	might want to check the size before you accept the string.
		1392
		1393	Third, JSON::XS recurses using the C stack when decoding objects and
		1394	arrays. The C stack is a limited resource: for instance, on my amd64
		1395	machine with 8MB of stack size I can decode around 180k nested arrays but
		1396	only 14k nested JSON objects (due to perl itself recursing deeply on croak
		1397	to free the temporary). If that is exceeded, the program crashes. To be
		1398	conservative, the default nesting limit is set to 512. If your process
		1399	has a smaller stack, you should adjust this setting accordingly with the
		1400	C<max_depth> method.
		1401
		1402	Something else could bomb you, too, that I forgot to think of. In that
		1403	case, you get to keep the pieces. I am always open for hints, though...
		1404
		1405	Also keep in mind that JSON::XS might leak contents of your Perl data
		1406	structures in its error messages, so when you serialise sensitive
		1407	information you might want to make sure that exceptions thrown by JSON::XS
		1408	will not end up in front of untrusted eyes.
		1409
		1410	If you are using JSON::XS to return packets to consumption
		1411	by JavaScript scripts in a browser you should have a look at
		1412	L<http://blog.archive.jpsykes.com/47/practical-csrf-and-json-security/> to
		1413	see whether you are vulnerable to some common attack vectors (which really
		1414	are browser design bugs, but it is still you who will have to deal with
		1415	it, as major browser developers care only for features, not about getting
		1416	security right).
		1417
		1418
		1419	=head1 INTEROPERABILITY WITH OTHER MODULES
		1420
		1421	C<JSON::XS> uses the L<Types::Serialiser> module to provide boolean
		1422	constants. That means that the JSON true and false values will be
		1423	comaptible to true and false values of iother modules that do the same,
		1424	such as L<JSON::PP> and L<CBOR::XS>.
		1425
		1426
		1427	=head1 THREADS
		1428
		1429	This module is I<not> guaranteed to be thread safe and there are no
		1430	plans to change this until Perl gets thread support (as opposed to the
		1431	horribly slow so-called "threads" which are simply slow and bloated
		1432	process simulations - use fork, it's I<much> faster, cheaper, better).
		1433
		1434	(It might actually work, but you have been warned).
		1435
		1436
		1437	=head1 THE PERILS OF SETLOCALE
		1438
		1439	Sometimes people avoid the Perl locale support and directly call the
		1440	system's setlocale function with C<LC_ALL>.
		1441
		1442	This breaks both perl and modules such as JSON::XS, as stringification of
		1443	numbers no longer works correctly (e.g. C<$x = 0.1; print "$x"+1> might
		1444	print C<1>, and JSON::XS might output illegal JSON as JSON::XS relies on
		1445	perl to stringify numbers).
		1446
		1447	The solution is simple: don't call C<setlocale>, or use it for only those
		1448	categories you need, such as C<LC_MESSAGES> or C<LC_CTYPE>.
		1449
		1450	If you need C<LC_NUMERIC>, you should enable it only around the code that
		1451	actually needs it (avoiding stringification of numbers), and restore it
		1452	afterwards.
		1453
		1454
		1455	=head1 BUGS
		1456
		1457	While the goal of this module is to be correct, that unfortunately does
		1458	not mean it's bug-free, only that I think its design is bug-free. If you
		1459	keep reporting bugs they will be fixed swiftly, though.
		1460
		1461	Please refrain from using rt.cpan.org or any other bug reporting
		1462	service. I put the contact address into my modules for a reason.
		1463
218	=cut	1464	=cut
219		1465
220	1;	1466	BEGIN {
		1467	*true = \$Types::Serialiser::true;
		1468	*true = \&Types::Serialiser::true;
		1469	*false = \$Types::Serialiser::false;
		1470	*false = \&Types::Serialiser::false;
		1471	*is_bool = \&Types::Serialiser::is_bool;
		1472
		1473	*JSON::XS::Boolean:: = *Types::Serialiser::Boolean::;
		1474	}
		1475
		1476	XSLoader::load "JSON::XS", $VERSION;
		1477
		1478	=head1 SEE ALSO
		1479
		1480	The F<json_xs> command line utility for quick experiments.
221		1481
222	=head1 AUTHOR	1482	=head1 AUTHOR
223		1483
224	Marc Lehmann <schmorp@schmorp.de>	1484	Marc Lehmann <schmorp@schmorp.de>
225	http://home.schmorp.de/	1485	http://home.schmorp.de/
226		1486
227	=cut	1487	=cut
228		1488
		1489	1
		1490

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