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Revision 1.11 by root, Fri Mar 23 17:48:59 2007 UTC vs.
Revision 1.169 by root, Thu Nov 15 20:49:12 2018 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 it and
		46	doesn't 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.
22
23	See MAPPING, below, on how JSON::XS maps perl values to JSON values and	54	See MAPPING, below, on how JSON::XS maps perl values to JSON values and
24	vice versa.	55	vice versa.
25		56
26	=head2 FEATURES	57	=head2 FEATURES
27		58
28	=over 4	59	=over 4
29		60
30	=item * correct handling of unicode issues	61	=item * correct Unicode handling
31		62
32	This module knows how to handle Unicode, and even documents how and when	63	This module knows how to handle Unicode, documents how and when it does
33	it does so.	64	so, and even documents what "correct" means.
34		65
35	=item * round-trip integrity	66	=item * round-trip integrity
36		67
37	When you serialise a perl data structure using only datatypes supported	68	When you serialise a perl data structure using only data types supported
38	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
39	(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.
40		73
41	=item * strict checking of JSON correctness	74	=item * strict checking of JSON correctness
42		75
43	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,
44	and only JSON is accepted as input by default (the latter is a security	77	and only JSON is accepted as input by default (the latter is a security
45	feature).	78	feature).
46		79
47	=item * fast	80	=item * fast
48		81
49	Compared to other JSON modules, this module compares favourably in terms	82	Compared to other JSON modules and other serialisers such as Storable,
50	of speed, too.	83	this module usually compares favourably in terms of speed, too.
51		84
52	=item * simple to use	85	=item * simple to use
53		86
54	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
55	interface.	88	oriented interface.
56		89
57	=item * reasonably versatile output formats	90	=item * reasonably versatile output formats
58		91
59	You can choose between the most compact guarenteed single-line format	92	You can choose between the most compact guaranteed-single-line format
60	possible (nice for simple line-based protocols), a pure-ascii format (for	93	possible (nice for simple line-based protocols), a pure-ASCII format
61	when your transport is not 8-bit clean), or a pretty-printed format (for	94	(for when your transport is not 8-bit clean, still supports the whole
62	when you want to read that stuff). Or you can combine those features in	95	Unicode range), or a pretty-printed format (for when you want to read that
63	whatever way you like.	96	stuff). Or you can combine those features in whatever way you like.
64		97
65	=back	98	=back
66		99
67	=cut	100	=cut
68		101
69	package JSON::XS;	102	package JSON::XS;
70		103
71	BEGIN {	104	use common::sense;
72	$VERSION = '0.3';	105
		106	our $VERSION = 3.04;
73	@ISA = qw(Exporter);	107	our @ISA = qw(Exporter);
74		108
75	@EXPORT = qw(to_json from_json);	109	our @EXPORT = qw(encode_json decode_json);
76	require Exporter;
77		110
78	require XSLoader;	111	use Exporter;
79	XSLoader::load JSON::XS::, $VERSION;	112	use XSLoader;
80	}	113
		114	use Types::Serialiser ();
81		115
82	=head1 FUNCTIONAL INTERFACE	116	=head1 FUNCTIONAL INTERFACE
83		117
84	The following convinience methods are provided by this module. They are	118	The following convenience methods are provided by this module. They are
85	exported by default:	119	exported by default:
86		120
87	=over 4	121	=over 4
88		122
89	=item $json_string = to_json $perl_scalar	123	=item $json_text = encode_json $perl_scalar
90		124
91	Converts the given Perl data structure (a simple scalar or a reference to	125	Converts the given Perl data structure to a UTF-8 encoded, binary string
92	a hash or array) to a UTF-8 encoded, binary string (that is, the string contains	126	(that is, the string contains octets only). Croaks on error.
93	octets only). Croaks on error.
94		127
95	This function call is functionally identical to C<< JSON::XS->new->utf8->encode ($perl_scalar) >>.	128	This function call is functionally identical to:
96		129
		130	$json_text = JSON::XS->new->utf8->encode ($perl_scalar)
		131
		132	Except being faster.
		133
97	=item $perl_scalar = from_json $json_string	134	=item $perl_scalar = decode_json $json_text
98		135
99	The opposite of C<to_json>: expects an UTF-8 (binary) string and tries to	136	The opposite of C<encode_json>: expects a UTF-8 (binary) string and tries
100	parse that as an UTF-8 encoded JSON string, returning the resulting simple	137	to parse that as a UTF-8 encoded JSON text, returning the resulting
101	scalar or reference. Croaks on error.	138	reference. Croaks on error.
102		139
103	This function call is functionally identical to C<< JSON::XS->new->utf8->decode ($json_string) >>.	140	This function call is functionally identical to:
		141
		142	$perl_scalar = JSON::XS->new->utf8->decode ($json_text)
		143
		144	Except being faster.
104		145
105	=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
106		196
107	=head1 OBJECT-ORIENTED INTERFACE	197	=head1 OBJECT-ORIENTED INTERFACE
108		198
109	The object oriented interface lets you configure your own encoding or	199	The object oriented interface lets you configure your own encoding or
110	decoding style, within the limits of supported formats.	200	decoding style, within the limits of supported formats.
…		…
112	=over 4	202	=over 4
113		203
114	=item $json = new JSON::XS	204	=item $json = new JSON::XS
115		205
116	Creates a new JSON::XS object that can be used to de/encode JSON	206	Creates a new JSON::XS object that can be used to de/encode JSON
117	strings. All boolean flags described below are by default I<disabled>.	207	strings. All boolean flags described below are by default I<disabled>
		208	(with the exception of C<allow_nonref>, which defaults to I<enabled> since
		209	version C<4.0>).
118		210
119	The mutators for flags all return the JSON object again and thus calls can	211	The mutators for flags all return the JSON object again and thus calls can
120	be chained:	212	be chained:
121		213
122	my $json = JSON::XS->new->utf8(1)->space_after(1)->encode ({a => [1,2]})	214	my $json = JSON::XS->new->utf8->space_after->encode ({a => [1,2]})
123	=> {"a": [1, 2]}	215	=> {"a": [1, 2]}
124		216
125	=item $json = $json->ascii ([$enable])	217	=item $json = $json->ascii ([$enable])
126		218
		219	=item $enabled = $json->get_ascii
		220
127	If C<$enable> is true (or missing), then the C<encode> method will	221	If C<$enable> is true (or missing), then the C<encode> method will not
128	not generate characters outside the code range C<0..127>. Any unicode	222	generate characters outside the code range C<0..127> (which is ASCII). Any
129	characters outside that range will be escaped using either a single	223	Unicode characters outside that range will be escaped using either a
130	\uXXXX (BMP characters) or a double \uHHHH\uLLLLL escape sequence, as per	224	single \uXXXX (BMP characters) or a double \uHHHH\uLLLLL escape sequence,
131	RFC4627.	225	as per RFC4627. The resulting encoded JSON text can be treated as a native
		226	Unicode string, an ascii-encoded, latin1-encoded or UTF-8 encoded string,
		227	or any other superset of ASCII.
132		228
133	If C<$enable> is false, then the C<encode> method will not escape Unicode	229	If C<$enable> is false, then the C<encode> method will not escape Unicode
134	characters unless necessary.	230	characters unless required by the JSON syntax or other flags. This results
		231	in a faster and more compact format.
135		232
		233	See also the section I<ENCODING/CODESET FLAG NOTES> later in this
		234	document.
		235
		236	The main use for this flag is to produce JSON texts that can be
		237	transmitted over a 7-bit channel, as the encoded JSON texts will not
		238	contain any 8 bit characters.
		239
136	JSON::XS->new->ascii (1)->encode (chr 0x10401)	240	JSON::XS->new->ascii (1)->encode ([chr 0x10401])
137	=> \ud801\udc01	241	=> ["\ud801\udc01"]
		242
		243	=item $json = $json->latin1 ([$enable])
		244
		245	=item $enabled = $json->get_latin1
		246
		247	If C<$enable> is true (or missing), then the C<encode> method will encode
		248	the resulting JSON text as latin1 (or iso-8859-1), escaping any characters
		249	outside the code range C<0..255>. The resulting string can be treated as a
		250	latin1-encoded JSON text or a native Unicode string. The C<decode> method
		251	will not be affected in any way by this flag, as C<decode> by default
		252	expects Unicode, which is a strict superset of latin1.
		253
		254	If C<$enable> is false, then the C<encode> method will not escape Unicode
		255	characters unless required by the JSON syntax or other flags.
		256
		257	See also the section I<ENCODING/CODESET FLAG NOTES> later in this
		258	document.
		259
		260	The main use for this flag is efficiently encoding binary data as JSON
		261	text, as most octets will not be escaped, resulting in a smaller encoded
		262	size. The disadvantage is that the resulting JSON text is encoded
		263	in latin1 (and must correctly be treated as such when storing and
		264	transferring), a rare encoding for JSON. It is therefore most useful when
		265	you want to store data structures known to contain binary data efficiently
		266	in files or databases, not when talking to other JSON encoders/decoders.
		267
		268	JSON::XS->new->latin1->encode (["\x{89}\x{abc}"]
		269	=> ["\x{89}\\u0abc"] # (perl syntax, U+abc escaped, U+89 not)
138		270
139	=item $json = $json->utf8 ([$enable])	271	=item $json = $json->utf8 ([$enable])
140		272
		273	=item $enabled = $json->get_utf8
		274
141	If C<$enable> is true (or missing), then the C<encode> method will encode	275	If C<$enable> is true (or missing), then the C<encode> method will encode
142	the JSON string into UTF-8, as required by many protocols, while the	276	the JSON result into UTF-8, as required by many protocols, while the
143	C<decode> method expects to be handled an UTF-8-encoded string. Please	277	C<decode> method expects to be handed a UTF-8-encoded string. Please
144	note that UTF-8-encoded strings do not contain any characters outside the	278	note that UTF-8-encoded strings do not contain any characters outside the
145	range C<0..255>, they are thus useful for bytewise/binary I/O.	279	range C<0..255>, they are thus useful for bytewise/binary I/O. In future
		280	versions, enabling this option might enable autodetection of the UTF-16
		281	and UTF-32 encoding families, as described in RFC4627.
146		282
147	If C<$enable> is false, then the C<encode> method will return the JSON	283	If C<$enable> is false, then the C<encode> method will return the JSON
148	string as a (non-encoded) unicode string, while C<decode> expects thus a	284	string as a (non-encoded) Unicode string, while C<decode> expects thus a
149	unicode string. Any decoding or encoding (e.g. to UTF-8 or UTF-16) needs	285	Unicode string. Any decoding or encoding (e.g. to UTF-8 or UTF-16) needs
150	to be done yourself, e.g. using the Encode module.	286	to be done yourself, e.g. using the Encode module.
		287
		288	See also the section I<ENCODING/CODESET FLAG NOTES> later in this
		289	document.
		290
		291	Example, output UTF-16BE-encoded JSON:
		292
		293	use Encode;
		294	$jsontext = encode "UTF-16BE", JSON::XS->new->encode ($object);
		295
		296	Example, decode UTF-32LE-encoded JSON:
		297
		298	use Encode;
		299	$object = JSON::XS->new->decode (decode "UTF-32LE", $jsontext);
151		300
152	=item $json = $json->pretty ([$enable])	301	=item $json = $json->pretty ([$enable])
153		302
154	This enables (or disables) all of the C<indent>, C<space_before> and	303	This enables (or disables) all of the C<indent>, C<space_before> and
155	C<space_after> (and in the future possibly more) flags in one call to	304	C<space_after> (and in the future possibly more) flags in one call to
156	generate the most readable (or most compact) form possible.	305	generate the most readable (or most compact) form possible.
		306
		307	Example, pretty-print some simple structure:
157		308
158	my $json = JSON::XS->new->pretty(1)->encode ({a => [1,2]})	309	my $json = JSON::XS->new->pretty(1)->encode ({a => [1,2]})
159	=>	310	=>
160	{	311	{
161	"a" : [	312	"a" : [
…		…
164	]	315	]
165	}	316	}
166		317
167	=item $json = $json->indent ([$enable])	318	=item $json = $json->indent ([$enable])
168		319
		320	=item $enabled = $json->get_indent
		321
169	If C<$enable> is true (or missing), then the C<encode> method will use a multiline	322	If C<$enable> is true (or missing), then the C<encode> method will use a multiline
170	format as output, putting every array member or object/hash key-value pair	323	format as output, putting every array member or object/hash key-value pair
171	into its own line, identing them properly.	324	into its own line, indenting them properly.
172		325
173	If C<$enable> is false, no newlines or indenting will be produced, and the	326	If C<$enable> is false, no newlines or indenting will be produced, and the
174	resulting JSON strings is guarenteed not to contain any C<newlines>.	327	resulting JSON text is guaranteed not to contain any C<newlines>.
175		328
176	This setting has no effect when decoding JSON strings.	329	This setting has no effect when decoding JSON texts.
177		330
178	=item $json = $json->space_before ([$enable])	331	=item $json = $json->space_before ([$enable])
		332
		333	=item $enabled = $json->get_space_before
179		334
180	If C<$enable> is true (or missing), then the C<encode> method will add an extra	335	If C<$enable> is true (or missing), then the C<encode> method will add an extra
181	optional space before the C<:> separating keys from values in JSON objects.	336	optional space before the C<:> separating keys from values in JSON objects.
182		337
183	If C<$enable> is false, then the C<encode> method will not add any extra	338	If C<$enable> is false, then the C<encode> method will not add any extra
184	space at those places.	339	space at those places.
185		340
186	This setting has no effect when decoding JSON strings. You will also most	341	This setting has no effect when decoding JSON texts. You will also
187	likely combine this setting with C<space_after>.	342	most likely combine this setting with C<space_after>.
		343
		344	Example, space_before enabled, space_after and indent disabled:
		345
		346	{"key" :"value"}
188		347
189	=item $json = $json->space_after ([$enable])	348	=item $json = $json->space_after ([$enable])
		349
		350	=item $enabled = $json->get_space_after
190		351
191	If C<$enable> is true (or missing), then the C<encode> method will add an extra	352	If C<$enable> is true (or missing), then the C<encode> method will add an extra
192	optional space after the C<:> separating keys from values in JSON objects	353	optional space after the C<:> separating keys from values in JSON objects
193	and extra whitespace after the C<,> separating key-value pairs and array	354	and extra whitespace after the C<,> separating key-value pairs and array
194	members.	355	members.
195		356
196	If C<$enable> is false, then the C<encode> method will not add any extra	357	If C<$enable> is false, then the C<encode> method will not add any extra
197	space at those places.	358	space at those places.
198		359
199	This setting has no effect when decoding JSON strings.	360	This setting has no effect when decoding JSON texts.
		361
		362	Example, space_before and indent disabled, space_after enabled:
		363
		364	{"key": "value"}
		365
		366	=item $json = $json->relaxed ([$enable])
		367
		368	=item $enabled = $json->get_relaxed
		369
		370	If C<$enable> is true (or missing), then C<decode> will accept some
		371	extensions to normal JSON syntax (see below). C<encode> will not be
		372	affected in any way. I<Be aware that this option makes you accept invalid
		373	JSON texts as if they were valid!>. I suggest only to use this option to
		374	parse application-specific files written by humans (configuration files,
		375	resource files etc.)
		376
		377	If C<$enable> is false (the default), then C<decode> will only accept
		378	valid JSON texts.
		379
		380	Currently accepted extensions are:
		381
		382	=over 4
		383
		384	=item * list items can have an end-comma
		385
		386	JSON I<separates> array elements and key-value pairs with commas. This
		387	can be annoying if you write JSON texts manually and want to be able to
		388	quickly append elements, so this extension accepts comma at the end of
		389	such items not just between them:
		390
		391	[
		392	1,
		393	2, <- this comma not normally allowed
		394	]
		395	{
		396	"k1": "v1",
		397	"k2": "v2", <- this comma not normally allowed
		398	}
		399
		400	=item * shell-style '#'-comments
		401
		402	Whenever JSON allows whitespace, shell-style comments are additionally
		403	allowed. They are terminated by the first carriage-return or line-feed
		404	character, after which more white-space and comments are allowed.
		405
		406	[
		407	1, # this comment not allowed in JSON
		408	# neither this one...
		409	]
		410
		411	=item * literal ASCII TAB characters in strings
		412
		413	Literal ASCII TAB characters are now allowed in strings (and treated as
		414	C<\t>).
		415
		416	[
		417	"Hello\tWorld",
		418	"Hello<TAB>World", # literal <TAB> would not normally be allowed
		419	]
		420
		421	=back
200		422
201	=item $json = $json->canonical ([$enable])	423	=item $json = $json->canonical ([$enable])
		424
		425	=item $enabled = $json->get_canonical
202		426
203	If C<$enable> is true (or missing), then the C<encode> method will output JSON objects	427	If C<$enable> is true (or missing), then the C<encode> method will output JSON objects
204	by sorting their keys. This is adding a comparatively high overhead.	428	by sorting their keys. This is adding a comparatively high overhead.
205		429
206	If C<$enable> is false, then the C<encode> method will output key-value	430	If C<$enable> is false, then the C<encode> method will output key-value
207	pairs in the order Perl stores them (which will likely change between runs	431	pairs in the order Perl stores them (which will likely change between runs
208	of the same script).	432	of the same script, and can change even within the same run from 5.18
		433	onwards).
209		434
210	This option is useful if you want the same data structure to be encoded as	435	This option is useful if you want the same data structure to be encoded as
211	the same JSON string (given the same overall settings). If it is disabled,	436	the same JSON text (given the same overall settings). If it is disabled,
212	the same hash migh be encoded differently even if contains the same data,	437	the same hash might be encoded differently even if contains the same data,
213	as key-value pairs have no inherent ordering in Perl.	438	as key-value pairs have no inherent ordering in Perl.
214		439
215	This setting has no effect when decoding JSON strings.	440	This setting has no effect when decoding JSON texts.
		441
		442	This setting has currently no effect on tied hashes.
216		443
217	=item $json = $json->allow_nonref ([$enable])	444	=item $json = $json->allow_nonref ([$enable])
		445
		446	=item $enabled = $json->get_allow_nonref
		447
		448	Unlike other boolean options, this opotion is enabled by default beginning
		449	with version C<4.0>. See L<SECURITY CONSIDERATIONS> for the gory details.
218		450
219	If C<$enable> is true (or missing), then the C<encode> method can convert a	451	If C<$enable> is true (or missing), then the C<encode> method can convert a
220	non-reference into its corresponding string, number or null JSON value,	452	non-reference into its corresponding string, number or null JSON value,
221	which is an extension to RFC4627. Likewise, C<decode> will accept those JSON	453	which is an extension to RFC4627. Likewise, C<decode> will accept those JSON
222	values instead of croaking.	454	values instead of croaking.
223		455
224	If C<$enable> is false, then the C<encode> method will croak if it isn't	456	If C<$enable> is false, then the C<encode> method will croak if it isn't
225	passed an arrayref or hashref, as JSON strings must either be an object	457	passed an arrayref or hashref, as JSON texts must either be an object
226	or array. Likewise, C<decode> will croak if given something that is not a	458	or array. Likewise, C<decode> will croak if given something that is not a
227	JSON object or array.	459	JSON object or array.
228		460
		461	Example, encode a Perl scalar as JSON value without enabled C<allow_nonref>,
		462	resulting in an error:
		463
		464	JSON::XS->new->allow_nonref (0)->encode ("Hello, World!")
		465	=> hash- or arrayref expected...
		466
		467	=item $json = $json->allow_unknown ([$enable])
		468
		469	=item $enabled = $json->get_allow_unknown
		470
		471	If C<$enable> is true (or missing), then C<encode> will I<not> throw an
		472	exception when it encounters values it cannot represent in JSON (for
		473	example, filehandles) but instead will encode a JSON C<null> value. Note
		474	that blessed objects are not included here and are handled separately by
		475	c<allow_nonref>.
		476
		477	If C<$enable> is false (the default), then C<encode> will throw an
		478	exception when it encounters anything it cannot encode as JSON.
		479
		480	This option does not affect C<decode> in any way, and it is recommended to
		481	leave it off unless you know your communications partner.
		482
		483	=item $json = $json->allow_blessed ([$enable])
		484
		485	=item $enabled = $json->get_allow_blessed
		486
		487	See L<OBJECT SERIALISATION> for details.
		488
		489	If C<$enable> is true (or missing), then the C<encode> method will not
		490	barf when it encounters a blessed reference that it cannot convert
		491	otherwise. Instead, a JSON C<null> value is encoded instead of the object.
		492
		493	If C<$enable> is false (the default), then C<encode> will throw an
		494	exception when it encounters a blessed object that it cannot convert
		495	otherwise.
		496
		497	This setting has no effect on C<decode>.
		498
		499	=item $json = $json->convert_blessed ([$enable])
		500
		501	=item $enabled = $json->get_convert_blessed
		502
		503	See L<OBJECT SERIALISATION> for details.
		504
		505	If C<$enable> is true (or missing), then C<encode>, upon encountering a
		506	blessed object, will check for the availability of the C<TO_JSON> method
		507	on the object's class. If found, it will be called in scalar context and
		508	the resulting scalar will be encoded instead of the object.
		509
		510	The C<TO_JSON> method may safely call die if it wants. If C<TO_JSON>
		511	returns other blessed objects, those will be handled in the same
		512	way. C<TO_JSON> must take care of not causing an endless recursion cycle
		513	(== crash) in this case. The name of C<TO_JSON> was chosen because other
		514	methods called by the Perl core (== not by the user of the object) are
		515	usually in upper case letters and to avoid collisions with any C<to_json>
		516	function or method.
		517
		518	If C<$enable> is false (the default), then C<encode> will not consider
		519	this type of conversion.
		520
		521	This setting has no effect on C<decode>.
		522
		523	=item $json = $json->allow_tags ([$enable])
		524
		525	=item $enabled = $json->get_allow_tags
		526
		527	See L<OBJECT SERIALISATION> for details.
		528
		529	If C<$enable> is true (or missing), then C<encode>, upon encountering a
		530	blessed object, will check for the availability of the C<FREEZE> method on
		531	the object's class. If found, it will be used to serialise the object into
		532	a nonstandard tagged JSON value (that JSON decoders cannot decode).
		533
		534	It also causes C<decode> to parse such tagged JSON values and deserialise
		535	them via a call to the C<THAW> method.
		536
		537	If C<$enable> is false (the default), then C<encode> will not consider
		538	this type of conversion, and tagged JSON values will cause a parse error
		539	in C<decode>, as if tags were not part of the grammar.
		540
		541	=item $json = $json->filter_json_object ([$coderef->($hashref)])
		542
		543	When C<$coderef> is specified, it will be called from C<decode> each
		544	time it decodes a JSON object. The only argument is a reference to
		545	the newly-created hash. If the code reference returns a single scalar
		546	(which need not be a reference), this value (or rather a copy of it) is
		547	inserted into the deserialised data structure. If it returns an empty
		548	list (NOTE: I<not> C<undef>, which is a valid scalar), the original
		549	deserialised hash will be inserted. This setting can slow down decoding
		550	considerably.
		551
		552	When C<$coderef> is omitted or undefined, any existing callback will
		553	be removed and C<decode> will not change the deserialised hash in any
		554	way.
		555
		556	Example, convert all JSON objects into the integer 5:
		557
		558	my $js = JSON::XS->new->filter_json_object (sub { 5 });
		559	# returns [5]
		560	$js->decode ('[{}]')
		561	# throw an exception because allow_nonref is not enabled
		562	# so a lone 5 is not allowed.
		563	$js->decode ('{"a":1, "b":2}');
		564
		565	=item $json = $json->filter_json_single_key_object ($key [=> $coderef->($value)])
		566
		567	Works remotely similar to C<filter_json_object>, but is only called for
		568	JSON objects having a single key named C<$key>.
		569
		570	This C<$coderef> is called before the one specified via
		571	C<filter_json_object>, if any. It gets passed the single value in the JSON
		572	object. If it returns a single value, it will be inserted into the data
		573	structure. If it returns nothing (not even C<undef> but the empty list),
		574	the callback from C<filter_json_object> will be called next, as if no
		575	single-key callback were specified.
		576
		577	If C<$coderef> is omitted or undefined, the corresponding callback will be
		578	disabled. There can only ever be one callback for a given key.
		579
		580	As this callback gets called less often then the C<filter_json_object>
		581	one, decoding speed will not usually suffer as much. Therefore, single-key
		582	objects make excellent targets to serialise Perl objects into, especially
		583	as single-key JSON objects are as close to the type-tagged value concept
		584	as JSON gets (it's basically an ID/VALUE tuple). Of course, JSON does not
		585	support this in any way, so you need to make sure your data never looks
		586	like a serialised Perl hash.
		587
		588	Typical names for the single object key are C<__class_whatever__>, or
		589	C<$__dollars_are_rarely_used__$> or C<}ugly_brace_placement>, or even
		590	things like C<__class_md5sum(classname)__>, to reduce the risk of clashing
		591	with real hashes.
		592
		593	Example, decode JSON objects of the form C<< { "__widget__" => <id> } >>
		594	into the corresponding C<< $WIDGET{<id>} >> object:
		595
		596	# return whatever is in $WIDGET{5}:
		597	JSON::XS
		598	->new
		599	->filter_json_single_key_object (__widget__ => sub {
		600	$WIDGET{ $_[0] }
		601	})
		602	->decode ('{"__widget__": 5')
		603
		604	# this can be used with a TO_JSON method in some "widget" class
		605	# for serialisation to json:
		606	sub WidgetBase::TO_JSON {
		607	my ($self) = @_;
		608
		609	unless ($self->{id}) {
		610	$self->{id} = ..get..some..id..;
		611	$WIDGET{$self->{id}} = $self;
		612	}
		613
		614	{ __widget__ => $self->{id} }
		615	}
		616
229	=item $json = $json->shrink ([$enable])	617	=item $json = $json->shrink ([$enable])
230		618
		619	=item $enabled = $json->get_shrink
		620
231	Perl usually over-allocates memory a bit when allocating space for	621	Perl usually over-allocates memory a bit when allocating space for
232	strings. This flag optionally resizes strings generated by either	622	strings. This flag optionally resizes strings generated by either
233	C<encode> or C<decode> to their minimum size possible. This can save	623	C<encode> or C<decode> to their minimum size possible. This can save
234	memory when your JSON strings are either very very long or you have many	624	memory when your JSON texts are either very very long or you have many
235	short strings. It will also try to downgrade any strings to octet-form	625	short strings. It will also try to downgrade any strings to octet-form
236	if possible: perl stores strings internally either in an encoding called	626	if possible: perl stores strings internally either in an encoding called
237	UTF-X or in octet-form. The latter cannot store everything but uses less	627	UTF-X or in octet-form. The latter cannot store everything but uses less
238	space in general.	628	space in general (and some buggy Perl or C code might even rely on that
		629	internal representation being used).
239		630
		631	The actual definition of what shrink does might change in future versions,
		632	but it will always try to save space at the expense of time.
		633
240	If C<$enable> is true (or missing), the string returned by C<encode> will be shrunk-to-fit,	634	If C<$enable> is true (or missing), the string returned by C<encode> will
241	while all strings generated by C<decode> will also be shrunk-to-fit.	635	be shrunk-to-fit, while all strings generated by C<decode> will also be
		636	shrunk-to-fit.
242		637
243	If C<$enable> is false, then the normal perl allocation algorithms are used.	638	If C<$enable> is false, then the normal perl allocation algorithms are used.
244	If you work with your data, then this is likely to be faster.	639	If you work with your data, then this is likely to be faster.
245		640
246	In the future, this setting might control other things, such as converting	641	In the future, this setting might control other things, such as converting
247	strings that look like integers or floats into integers or floats	642	strings that look like integers or floats into integers or floats
248	internally (there is no difference on the Perl level), saving space.	643	internally (there is no difference on the Perl level), saving space.
249		644
		645	=item $json = $json->max_depth ([$maximum_nesting_depth])
		646
		647	=item $max_depth = $json->get_max_depth
		648
		649	Sets the maximum nesting level (default C<512>) accepted while encoding
		650	or decoding. If a higher nesting level is detected in JSON text or a Perl
		651	data structure, then the encoder and decoder will stop and croak at that
		652	point.
		653
		654	Nesting level is defined by number of hash- or arrayrefs that the encoder
		655	needs to traverse to reach a given point or the number of C<{> or C<[>
		656	characters without their matching closing parenthesis crossed to reach a
		657	given character in a string.
		658
		659	Setting the maximum depth to one disallows any nesting, so that ensures
		660	that the object is only a single hash/object or array.
		661
		662	If no argument is given, the highest possible setting will be used, which
		663	is rarely useful.
		664
		665	Note that nesting is implemented by recursion in C. The default value has
		666	been chosen to be as large as typical operating systems allow without
		667	crashing.
		668
		669	See SECURITY CONSIDERATIONS, below, for more info on why this is useful.
		670
		671	=item $json = $json->max_size ([$maximum_string_size])
		672
		673	=item $max_size = $json->get_max_size
		674
		675	Set the maximum length a JSON text may have (in bytes) where decoding is
		676	being attempted. The default is C<0>, meaning no limit. When C<decode>
		677	is called on a string that is longer then this many bytes, it will not
		678	attempt to decode the string but throw an exception. This setting has no
		679	effect on C<encode> (yet).
		680
		681	If no argument is given, the limit check will be deactivated (same as when
		682	C<0> is specified).
		683
		684	See SECURITY CONSIDERATIONS, below, for more info on why this is useful.
		685
250	=item $json_string = $json->encode ($perl_scalar)	686	=item $json_text = $json->encode ($perl_scalar)
251		687
252	Converts the given Perl data structure (a simple scalar or a reference	688	Converts the given Perl value or data structure to its JSON
253	to a hash or array) to its JSON representation. Simple scalars will be	689	representation. Croaks on error.
254	converted into JSON string or number sequences, while references to arrays
255	become JSON arrays and references to hashes become JSON objects. Undefined
256	Perl values (e.g. C<undef>) become JSON C<null> values. Neither C<true>
257	nor C<false> values will be generated.
258		690
259	=item $perl_scalar = $json->decode ($json_string)	691	=item $perl_scalar = $json->decode ($json_text)
260		692
261	The opposite of C<encode>: expects a JSON string and tries to parse it,	693	The opposite of C<encode>: expects a JSON text and tries to parse it,
262	returning the resulting simple scalar or reference. Croaks on error.	694	returning the resulting simple scalar or reference. Croaks on error.
263		695
264	JSON numbers and strings become simple Perl scalars. JSON arrays become	696	=item ($perl_scalar, $characters) = $json->decode_prefix ($json_text)
265	Perl arrayrefs and JSON objects become Perl hashrefs. C<true> becomes	697
266	C<1>, C<false> becomes C<0> and C<null> becomes C<undef>.	698	This works like the C<decode> method, but instead of raising an exception
		699	when there is trailing garbage after the first JSON object, it will
		700	silently stop parsing there and return the number of characters consumed
		701	so far.
		702
		703	This is useful if your JSON texts are not delimited by an outer protocol
		704	and you need to know where the JSON text ends.
		705
		706	JSON::XS->new->decode_prefix ("[1] the tail")
		707	=> ([1], 3)
267		708
268	=back	709	=back
		710
		711
		712	=head1 INCREMENTAL PARSING
		713
		714	In some cases, there is the need for incremental parsing of JSON
		715	texts. While this module always has to keep both JSON text and resulting
		716	Perl data structure in memory at one time, it does allow you to parse a
		717	JSON stream incrementally. It does so by accumulating text until it has
		718	a full JSON object, which it then can decode. This process is similar to
		719	using C<decode_prefix> to see if a full JSON object is available, but
		720	is much more efficient (and can be implemented with a minimum of method
		721	calls).
		722
		723	JSON::XS will only attempt to parse the JSON text once it is sure it
		724	has enough text to get a decisive result, using a very simple but
		725	truly incremental parser. This means that it sometimes won't stop as
		726	early as the full parser, for example, it doesn't detect mismatched
		727	parentheses. The only thing it guarantees is that it starts decoding as
		728	soon as a syntactically valid JSON text has been seen. This means you need
		729	to set resource limits (e.g. C<max_size>) to ensure the parser will stop
		730	parsing in the presence if syntax errors.
		731
		732	The following methods implement this incremental parser.
		733
		734	=over 4
		735
		736	=item [void, scalar or list context] = $json->incr_parse ([$string])
		737
		738	This is the central parsing function. It can both append new text and
		739	extract objects from the stream accumulated so far (both of these
		740	functions are optional).
		741
		742	If C<$string> is given, then this string is appended to the already
		743	existing JSON fragment stored in the C<$json> object.
		744
		745	After that, if the function is called in void context, it will simply
		746	return without doing anything further. This can be used to add more text
		747	in as many chunks as you want.
		748
		749	If the method is called in scalar context, then it will try to extract
		750	exactly I<one> JSON object. If that is successful, it will return this
		751	object, otherwise it will return C<undef>. If there is a parse error,
		752	this method will croak just as C<decode> would do (one can then use
		753	C<incr_skip> to skip the erroneous part). This is the most common way of
		754	using the method.
		755
		756	And finally, in list context, it will try to extract as many objects
		757	from the stream as it can find and return them, or the empty list
		758	otherwise. For this to work, there must be no separators (other than
		759	whitespace) between the JSON objects or arrays, instead they must be
		760	concatenated back-to-back. If an error occurs, an exception will be
		761	raised as in the scalar context case. Note that in this case, any
		762	previously-parsed JSON texts will be lost.
		763
		764	Example: Parse some JSON arrays/objects in a given string and return
		765	them.
		766
		767	my @objs = JSON::XS->new->incr_parse ("[5][7][1,2]");
		768
		769	=item $lvalue_string = $json->incr_text
		770
		771	This method returns the currently stored JSON fragment as an lvalue, that
		772	is, you can manipulate it. This I<only> works when a preceding call to
		773	C<incr_parse> in I<scalar context> successfully returned an object. Under
		774	all other circumstances you must not call this function (I mean it.
		775	although in simple tests it might actually work, it I<will> fail under
		776	real world conditions). As a special exception, you can also call this
		777	method before having parsed anything.
		778
		779	That means you can only use this function to look at or manipulate text
		780	before or after complete JSON objects, not while the parser is in the
		781	middle of parsing a JSON object.
		782
		783	This function is useful in two cases: a) finding the trailing text after a
		784	JSON object or b) parsing multiple JSON objects separated by non-JSON text
		785	(such as commas).
		786
		787	=item $json->incr_skip
		788
		789	This will reset the state of the incremental parser and will remove
		790	the parsed text from the input buffer so far. This is useful after
		791	C<incr_parse> died, in which case the input buffer and incremental parser
		792	state is left unchanged, to skip the text parsed so far and to reset the
		793	parse state.
		794
		795	The difference to C<incr_reset> is that only text until the parse error
		796	occurred is removed.
		797
		798	=item $json->incr_reset
		799
		800	This completely resets the incremental parser, that is, after this call,
		801	it will be as if the parser had never parsed anything.
		802
		803	This is useful if you want to repeatedly parse JSON objects and want to
		804	ignore any trailing data, which means you have to reset the parser after
		805	each successful decode.
		806
		807	=back
		808
		809	=head2 LIMITATIONS
		810
		811	All options that affect decoding are supported, except
		812	C<allow_nonref>. The reason for this is that it cannot be made to work
		813	sensibly: JSON objects and arrays are self-delimited, i.e. you can
		814	concatenate them back to back and still decode them perfectly. This does
		815	not hold true for JSON numbers, however.
		816
		817	For example, is the string C<1> a single JSON number, or is it simply the
		818	start of C<12>? Or is C<12> a single JSON number, or the concatenation
		819	of C<1> and C<2>? In neither case you can tell, and this is why JSON::XS
		820	takes the conservative route and disallows this case.
		821
		822	=head2 EXAMPLES
		823
		824	Some examples will make all this clearer. First, a simple example that
		825	works similarly to C<decode_prefix>: We want to decode the JSON object at
		826	the start of a string and identify the portion after the JSON object:
		827
		828	my $text = "[1,2,3] hello";
		829
		830	my $json = new JSON::XS;
		831
		832	my $obj = $json->incr_parse ($text)
		833	or die "expected JSON object or array at beginning of string";
		834
		835	my $tail = $json->incr_text;
		836	# $tail now contains " hello"
		837
		838	Easy, isn't it?
		839
		840	Now for a more complicated example: Imagine a hypothetical protocol where
		841	you read some requests from a TCP stream, and each request is a JSON
		842	array, without any separation between them (in fact, it is often useful to
		843	use newlines as "separators", as these get interpreted as whitespace at
		844	the start of the JSON text, which makes it possible to test said protocol
		845	with C<telnet>...).
		846
		847	Here is how you'd do it (it is trivial to write this in an event-based
		848	manner):
		849
		850	my $json = new JSON::XS;
		851
		852	# read some data from the socket
		853	while (sysread $socket, my $buf, 4096) {
		854
		855	# split and decode as many requests as possible
		856	for my $request ($json->incr_parse ($buf)) {
		857	# act on the $request
		858	}
		859	}
		860
		861	Another complicated example: Assume you have a string with JSON objects
		862	or arrays, all separated by (optional) comma characters (e.g. C<[1],[2],
		863	[3]>). To parse them, we have to skip the commas between the JSON texts,
		864	and here is where the lvalue-ness of C<incr_text> comes in useful:
		865
		866	my $text = "[1],[2], [3]";
		867	my $json = new JSON::XS;
		868
		869	# void context, so no parsing done
		870	$json->incr_parse ($text);
		871
		872	# now extract as many objects as possible. note the
		873	# use of scalar context so incr_text can be called.
		874	while (my $obj = $json->incr_parse) {
		875	# do something with $obj
		876
		877	# now skip the optional comma
		878	$json->incr_text =~ s/^ \s* , //x;
		879	}
		880
		881	Now lets go for a very complex example: Assume that you have a gigantic
		882	JSON array-of-objects, many gigabytes in size, and you want to parse it,
		883	but you cannot load it into memory fully (this has actually happened in
		884	the real world :).
		885
		886	Well, you lost, you have to implement your own JSON parser. But JSON::XS
		887	can still help you: You implement a (very simple) array parser and let
		888	JSON decode the array elements, which are all full JSON objects on their
		889	own (this wouldn't work if the array elements could be JSON numbers, for
		890	example):
		891
		892	my $json = new JSON::XS;
		893
		894	# open the monster
		895	open my $fh, "<bigfile.json"
		896	or die "bigfile: $!";
		897
		898	# first parse the initial "["
		899	for (;;) {
		900	sysread $fh, my $buf, 65536
		901	or die "read error: $!";
		902	$json->incr_parse ($buf); # void context, so no parsing
		903
		904	# Exit the loop once we found and removed(!) the initial "[".
		905	# In essence, we are (ab-)using the $json object as a simple scalar
		906	# we append data to.
		907	last if $json->incr_text =~ s/^ \s* \[ //x;
		908	}
		909
		910	# now we have the skipped the initial "[", so continue
		911	# parsing all the elements.
		912	for (;;) {
		913	# in this loop we read data until we got a single JSON object
		914	for (;;) {
		915	if (my $obj = $json->incr_parse) {
		916	# do something with $obj
		917	last;
		918	}
		919
		920	# add more data
		921	sysread $fh, my $buf, 65536
		922	or die "read error: $!";
		923	$json->incr_parse ($buf); # void context, so no parsing
		924	}
		925
		926	# in this loop we read data until we either found and parsed the
		927	# separating "," between elements, or the final "]"
		928	for (;;) {
		929	# first skip whitespace
		930	$json->incr_text =~ s/^\s*//;
		931
		932	# if we find "]", we are done
		933	if ($json->incr_text =~ s/^\]//) {
		934	print "finished.\n";
		935	exit;
		936	}
		937
		938	# if we find ",", we can continue with the next element
		939	if ($json->incr_text =~ s/^,//) {
		940	last;
		941	}
		942
		943	# if we find anything else, we have a parse error!
		944	if (length $json->incr_text) {
		945	die "parse error near ", $json->incr_text;
		946	}
		947
		948	# else add more data
		949	sysread $fh, my $buf, 65536
		950	or die "read error: $!";
		951	$json->incr_parse ($buf); # void context, so no parsing
		952	}
		953
		954	This is a complex example, but most of the complexity comes from the fact
		955	that we are trying to be correct (bear with me if I am wrong, I never ran
		956	the above example :).
		957
		958
269		959
270	=head1 MAPPING	960	=head1 MAPPING
271		961
272	This section describes how JSON::XS maps Perl values to JSON values and	962	This section describes how JSON::XS maps Perl values to JSON values and
273	vice versa. These mappings are designed to "do the right thing" in most	963	vice versa. These mappings are designed to "do the right thing" in most
274	circumstances automatically, preserving round-tripping characteristics	964	circumstances automatically, preserving round-tripping characteristics
275	(what you put in comes out as something equivalent).	965	(what you put in comes out as something equivalent).
276		966
277	For the more enlightened: note that in the following descriptions,	967	For the more enlightened: note that in the following descriptions,
278	lowercase I<perl> refers to the Perl interpreter, while uppcercase I<Perl>	968	lowercase I<perl> refers to the Perl interpreter, while uppercase I<Perl>
279	refers to the abstract Perl language itself.	969	refers to the abstract Perl language itself.
280		970
		971
281	=head2 JSON -> PERL	972	=head2 JSON -> PERL
282		973
283	=over 4	974	=over 4
284		975
285	=item object	976	=item object
286		977
287	A JSON object becomes a reference to a hash in Perl. No ordering of object	978	A JSON object becomes a reference to a hash in Perl. No ordering of object
288	keys is preserved.	979	keys is preserved (JSON does not preserve object key ordering itself).
289		980
290	=item array	981	=item array
291		982
292	A JSON array becomes a reference to an array in Perl.	983	A JSON array becomes a reference to an array in Perl.
293		984
…		…
297	are represented by the same codepoints in the Perl string, so no manual	988	are represented by the same codepoints in the Perl string, so no manual
298	decoding is necessary.	989	decoding is necessary.
299		990
300	=item number	991	=item number
301		992
302	A JSON number becomes either an integer or numeric (floating point)	993	A JSON number becomes either an integer, numeric (floating point) or
303	scalar in perl, depending on its range and any fractional parts. On the	994	string scalar in perl, depending on its range and any fractional parts. On
304	Perl level, there is no difference between those as Perl handles all the	995	the Perl level, there is no difference between those as Perl handles all
305	conversion details, but an integer may take slightly less memory and might	996	the conversion details, but an integer may take slightly less memory and
306	represent more values exactly than (floating point) numbers.	997	might represent more values exactly than floating point numbers.
		998
		999	If the number consists of digits only, JSON::XS will try to represent
		1000	it as an integer value. If that fails, it will try to represent it as
		1001	a numeric (floating point) value if that is possible without loss of
		1002	precision. Otherwise it will preserve the number as a string value (in
		1003	which case you lose roundtripping ability, as the JSON number will be
		1004	re-encoded to a JSON string).
		1005
		1006	Numbers containing a fractional or exponential part will always be
		1007	represented as numeric (floating point) values, possibly at a loss of
		1008	precision (in which case you might lose perfect roundtripping ability, but
		1009	the JSON number will still be re-encoded as a JSON number).
		1010
		1011	Note that precision is not accuracy - binary floating point values cannot
		1012	represent most decimal fractions exactly, and when converting from and to
		1013	floating point, JSON::XS only guarantees precision up to but not including
		1014	the least significant bit.
307		1015
308	=item true, false	1016	=item true, false
309		1017
310	These JSON atoms become C<0>, C<1>, respectively. Information is lost in	1018	These JSON atoms become C<Types::Serialiser::true> and
311	this process. Future versions might represent those values differently,	1019	C<Types::Serialiser::false>, respectively. They are overloaded to act
312	but they will be guarenteed to act like these integers would normally in	1020	almost exactly like the numbers C<1> and C<0>. You can check whether
313	Perl.	1021	a scalar is a JSON boolean by using the C<Types::Serialiser::is_bool>
		1022	function (after C<use Types::Serialier>, of course).
314		1023
315	=item null	1024	=item null
316		1025
317	A JSON null atom becomes C<undef> in Perl.	1026	A JSON null atom becomes C<undef> in Perl.
318		1027
		1028	=item shell-style comments (C<< # I<text> >>)
		1029
		1030	As a nonstandard extension to the JSON syntax that is enabled by the
		1031	C<relaxed> setting, shell-style comments are allowed. They can start
		1032	anywhere outside strings and go till the end of the line.
		1033
		1034	=item tagged values (C<< (I<tag>)I<value> >>).
		1035
		1036	Another nonstandard extension to the JSON syntax, enabled with the
		1037	C<allow_tags> setting, are tagged values. In this implementation, the
		1038	I<tag> must be a perl package/class name encoded as a JSON string, and the
		1039	I<value> must be a JSON array encoding optional constructor arguments.
		1040
		1041	See L<OBJECT SERIALISATION>, below, for details.
		1042
319	=back	1043	=back
		1044
320		1045
321	=head2 PERL -> JSON	1046	=head2 PERL -> JSON
322		1047
323	The mapping from Perl to JSON is slightly more difficult, as Perl is a	1048	The mapping from Perl to JSON is slightly more difficult, as Perl is a
324	truly typeless language, so we can only guess which JSON type is meant by	1049	truly typeless language, so we can only guess which JSON type is meant by
…		…
326		1051
327	=over 4	1052	=over 4
328		1053
329	=item hash references	1054	=item hash references
330		1055
331	Perl hash references become JSON objects. As there is no inherent ordering	1056	Perl hash references become JSON objects. As there is no inherent
332	in hash keys, they will usually be encoded in a pseudo-random order that	1057	ordering in hash keys (or JSON objects), they will usually be encoded
333	can change between runs of the same program but stays generally the same	1058	in a pseudo-random order. JSON::XS can optionally sort the hash keys
334	within the single run of a program. JSON::XS can optionally sort the hash
335	keys (determined by the I<canonical> flag), so the same datastructure	1059	(determined by the I<canonical> flag), so the same datastructure will
336	will serialise to the same JSON text (given same settings and version of	1060	serialise to the same JSON text (given same settings and version of
337	JSON::XS), but this incurs a runtime overhead.	1061	JSON::XS), but this incurs a runtime overhead and is only rarely useful,
		1062	e.g. when you want to compare some JSON text against another for equality.
338		1063
339	=item array references	1064	=item array references
340		1065
341	Perl array references become JSON arrays.	1066	Perl array references become JSON arrays.
342		1067
		1068	=item other references
		1069
		1070	Other unblessed references are generally not allowed and will cause an
		1071	exception to be thrown, except for references to the integers C<0> and
		1072	C<1>, which get turned into C<false> and C<true> atoms in JSON.
		1073
		1074	Since C<JSON::XS> uses the boolean model from L<Types::Serialiser>, you
		1075	can also C<use Types::Serialiser> and then use C<Types::Serialiser::false>
		1076	and C<Types::Serialiser::true> to improve readability.
		1077
		1078	use Types::Serialiser;
		1079	encode_json [\0, Types::Serialiser::true] # yields [false,true]
		1080
		1081	=item Types::Serialiser::true, Types::Serialiser::false
		1082
		1083	These special values from the L<Types::Serialiser> module become JSON true
		1084	and JSON false values, respectively. You can also use C<\1> and C<\0>
		1085	directly if you want.
		1086
343	=item blessed objects	1087	=item blessed objects
344		1088
345	Blessed objects are not allowed. JSON::XS currently tries to encode their	1089	Blessed objects are not directly representable in JSON, but C<JSON::XS>
346	underlying representation (hash- or arrayref), but this behaviour might	1090	allows various ways of handling objects. See L<OBJECT SERIALISATION>,
347	change in future versions.	1091	below, for details.
348		1092
349	=item simple scalars	1093	=item simple scalars
350		1094
351	Simple Perl scalars (any scalar that is not a reference) are the most	1095	Simple Perl scalars (any scalar that is not a reference) are the most
352	difficult objects to encode: JSON::XS will encode undefined scalars as	1096	difficult objects to encode: JSON::XS will encode undefined scalars as
353	JSON null value, scalars that have last been used in a string context	1097	JSON C<null> values, scalars that have last been used in a string context
354	before encoding as JSON strings and anything else as number value:	1098	before encoding as JSON strings, and anything else as number value:
355		1099
356	# dump as number	1100	# dump as number
357	to_json [2] # yields [2]	1101	encode_json [2] # yields [2]
358	to_json [-3.0e17] # yields [-3e+17]	1102	encode_json [-3.0e17] # yields [-3e+17]
359	my $value = 5; to_json [$value] # yields [5]	1103	my $value = 5; encode_json [$value] # yields [5]
360		1104
361	# used as string, so dump as string	1105	# used as string, so dump as string
362	print $value;	1106	print $value;
363	to_json [$value] # yields ["5"]	1107	encode_json [$value] # yields ["5"]
364		1108
365	# undef becomes null	1109	# undef becomes null
366	to_json [undef] # yields [null]	1110	encode_json [undef] # yields [null]
367		1111
368	You can force the type to be a string by stringifying it:	1112	You can force the type to be a JSON string by stringifying it:
369		1113
370	my $x = 3.1; # some variable containing a number	1114	my $x = 3.1; # some variable containing a number
371	"$x"; # stringified	1115	"$x"; # stringified
372	$x .= ""; # another, more awkward way to stringify	1116	$x .= ""; # another, more awkward way to stringify
373	print $x; # perl does it for you, too, quite often	1117	print $x; # perl does it for you, too, quite often
374		1118
375	You can force the type to be a number by numifying it:	1119	You can force the type to be a JSON number by numifying it:
376		1120
377	my $x = "3"; # some variable containing a string	1121	my $x = "3"; # some variable containing a string
378	$x += 0; # numify it, ensuring it will be dumped as a number	1122	$x += 0; # numify it, ensuring it will be dumped as a number
379	$x *= 1; # same thing, the choise is yours.	1123	$x *= 1; # same thing, the choice is yours.
380		1124
381	You can not currently output JSON booleans or force the type in other,	1125	You can not currently force the type in other, less obscure, ways. Tell me
382	less obscure, ways. Tell me if you need this capability.	1126	if you need this capability (but don't forget to explain why it's needed
		1127	:).
383		1128
384	=item circular data structures	1129	Note that numerical precision has the same meaning as under Perl (so
385		1130	binary to decimal conversion follows the same rules as in Perl, which
386	Those will be encoded until memory or stackspace runs out.	1131	can differ to other languages). Also, your perl interpreter might expose
		1132	extensions to the floating point numbers of your platform, such as
		1133	infinities or NaN's - these cannot be represented in JSON, and it is an
		1134	error to pass those in.
387		1135
388	=back	1136	=back
389		1137
390	=head1 COMPARISON	1138	=head2 OBJECT SERIALISATION
391		1139
392	As already mentioned, this module was created because none of the existing	1140	As JSON cannot directly represent Perl objects, you have to choose between
393	JSON modules could be made to work correctly. First I will describe the	1141	a pure JSON representation (without the ability to deserialise the object
394	problems (or pleasures) I encountered with various existing JSON modules,	1142	automatically again), and a nonstandard extension to the JSON syntax,
395	followed by some benchmark values. JSON::XS was designed not to suffer	1143	tagged values.
396	from any of these problems or limitations.	1144
		1145	=head3 SERIALISATION
		1146
		1147	What happens when C<JSON::XS> encounters a Perl object depends on the
		1148	C<allow_blessed>, C<convert_blessed> and C<allow_tags> settings, which are
		1149	used in this order:
397		1150
398	=over 4	1151	=over 4
399		1152
400	=item JSON 1.07	1153	=item 1. C<allow_tags> is enabled and the object has a C<FREEZE> method.
401		1154
402	Slow (but very portable, as it is written in pure Perl).	1155	In this case, C<JSON::XS> uses the L<Types::Serialiser> object
		1156	serialisation protocol to create a tagged JSON value, using a nonstandard
		1157	extension to the JSON syntax.
403		1158
404	Undocumented/buggy Unicode handling (how JSON handles unicode values is	1159	This works by invoking the C<FREEZE> method on the object, with the first
405	undocumented. One can get far by feeding it unicode strings and doing	1160	argument being the object to serialise, and the second argument being the
406	en-/decoding oneself, but unicode escapes are not working properly).	1161	constant string C<JSON> to distinguish it from other serialisers.
407		1162
408	No roundtripping (strings get clobbered if they look like numbers, e.g.	1163	The C<FREEZE> method can return any number of values (i.e. zero or
409	the string C<2.0> will encode to C<2.0> instead of C<"2.0">, and that will	1164	more). These values and the paclkage/classname of the object will then be
410	decode into the number 2.	1165	encoded as a tagged JSON value in the following format:
411		1166
412	=item JSON::PC 0.01	1167	("classname")[FREEZE return values...]
413		1168
414	Very fast.	1169	e.g.:
415		1170
416	Undocumented/buggy Unicode handling.	1171	("URI")["http://www.google.com/"]
		1172	("MyDate")[2013,10,29]
		1173	("ImageData::JPEG")["Z3...VlCg=="]
417		1174
418	No roundtripping.	1175	For example, the hypothetical C<My::Object> C<FREEZE> method might use the
		1176	objects C<type> and C<id> members to encode the object:
419		1177
420	Has problems handling many Perl values (e.g. regex results and other magic	1178	sub My::Object::FREEZE {
421	values will make it croak).	1179	my ($self, $serialiser) = @_;
422		1180
423	Does not even generate valid JSON (C<{1,2}> gets converted to C<{1:2}>	1181	($self->{type}, $self->{id})
424	which is not a valid JSON string.	1182	}
425		1183
426	Unmaintained (maintainer unresponsive for many months, bugs are not	1184	=item 2. C<convert_blessed> is enabled and the object has a C<TO_JSON> method.
427	getting fixed).
428		1185
429	=item JSON::Syck 0.21	1186	In this case, the C<TO_JSON> method of the object is invoked in scalar
		1187	context. It must return a single scalar that can be directly encoded into
		1188	JSON. This scalar replaces the object in the JSON text.
430		1189
431	Very buggy (often crashes).	1190	For example, the following C<TO_JSON> method will convert all L<URI>
		1191	objects to JSON strings when serialised. The fatc that these values
		1192	originally were L<URI> objects is lost.
432		1193
433	Very inflexible (no human-readable format supported, format pretty much	1194	sub URI::TO_JSON {
434	undocumented. I need at least a format for easy reading by humans and a	1195	my ($uri) = @_;
435	single-line compact format for use in a protocol, and preferably a way to	1196	$uri->as_string
436	generate ASCII-only JSON strings).	1197	}
437		1198
438	Completely broken (and confusingly documented) Unicode handling (unicode	1199	=item 3. C<allow_blessed> is enabled.
439	escapes are not working properly, you need to set ImplicitUnicode to
440	I<different> values on en- and decoding to get symmetric behaviour).
441		1200
442	No roundtripping (simple cases work, but this depends on wether the scalar	1201	The object will be serialised as a JSON null value.
443	value was used in a numeric context or not).
444		1202
445	Dumping hashes may skip hash values depending on iterator state.	1203	=item 4. none of the above
446		1204
447	Unmaintained (maintainer unresponsive for many months, bugs are not	1205	If none of the settings are enabled or the respective methods are missing,
448	getting fixed).	1206	C<JSON::XS> throws an exception.
449
450	Does not check input for validity (i.e. will accept non-JSON input and
451	return "something" instead of raising an exception. This is a security
452	issue: imagine two banks transfering money between each other using
453	JSON. One bank might parse a given non-JSON request and deduct money,
454	while the other might reject the transaction with a syntax error. While a
455	good protocol will at least recover, that is extra unnecessary work and
456	the transaction will still not succeed).
457
458	=item JSON::DWIW 0.04
459
460	Very fast. Very natural. Very nice.
461
462	Undocumented unicode handling (but the best of the pack. Unicode escapes
463	still don't get parsed properly).
464
465	Very inflexible.
466
467	No roundtripping.
468
469	Does not generate valid JSON (key strings are often unquoted, empty keys
470	result in nothing being output)
471
472	Does not check input for validity.
473		1207
474	=back	1208	=back
		1209
		1210	=head3 DESERIALISATION
		1211
		1212	For deserialisation there are only two cases to consider: either
		1213	nonstandard tagging was used, in which case C<allow_tags> decides,
		1214	or objects cannot be automatically be deserialised, in which
		1215	case you can use postprocessing or the C<filter_json_object> or
		1216	C<filter_json_single_key_object> callbacks to get some real objects our of
		1217	your JSON.
		1218
		1219	This section only considers the tagged value case: I a tagged JSON object
		1220	is encountered during decoding and C<allow_tags> is disabled, a parse
		1221	error will result (as if tagged values were not part of the grammar).
		1222
		1223	If C<allow_tags> is enabled, C<JSON::XS> will look up the C<THAW> method
		1224	of the package/classname used during serialisation (it will not attempt
		1225	to load the package as a Perl module). If there is no such method, the
		1226	decoding will fail with an error.
		1227
		1228	Otherwise, the C<THAW> method is invoked with the classname as first
		1229	argument, the constant string C<JSON> as second argument, and all the
		1230	values from the JSON array (the values originally returned by the
		1231	C<FREEZE> method) as remaining arguments.
		1232
		1233	The method must then return the object. While technically you can return
		1234	any Perl scalar, you might have to enable the C<enable_nonref> setting to
		1235	make that work in all cases, so better return an actual blessed reference.
		1236
		1237	As an example, let's implement a C<THAW> function that regenerates the
		1238	C<My::Object> from the C<FREEZE> example earlier:
		1239
		1240	sub My::Object::THAW {
		1241	my ($class, $serialiser, $type, $id) = @_;
		1242
		1243	$class->new (type => $type, id => $id)
		1244	}
		1245
		1246
		1247	=head1 ENCODING/CODESET FLAG NOTES
		1248
		1249	The interested reader might have seen a number of flags that signify
		1250	encodings or codesets - C<utf8>, C<latin1> and C<ascii>. There seems to be
		1251	some confusion on what these do, so here is a short comparison:
		1252
		1253	C<utf8> controls whether the JSON text created by C<encode> (and expected
		1254	by C<decode>) is UTF-8 encoded or not, while C<latin1> and C<ascii> only
		1255	control whether C<encode> escapes character values outside their respective
		1256	codeset range. Neither of these flags conflict with each other, although
		1257	some combinations make less sense than others.
		1258
		1259	Care has been taken to make all flags symmetrical with respect to
		1260	C<encode> and C<decode>, that is, texts encoded with any combination of
		1261	these flag values will be correctly decoded when the same flags are used
		1262	- in general, if you use different flag settings while encoding vs. when
		1263	decoding you likely have a bug somewhere.
		1264
		1265	Below comes a verbose discussion of these flags. Note that a "codeset" is
		1266	simply an abstract set of character-codepoint pairs, while an encoding
		1267	takes those codepoint numbers and I<encodes> them, in our case into
		1268	octets. Unicode is (among other things) a codeset, UTF-8 is an encoding,
		1269	and ISO-8859-1 (= latin 1) and ASCII are both codesets I<and> encodings at
		1270	the same time, which can be confusing.
		1271
		1272	=over 4
		1273
		1274	=item C<utf8> flag disabled
		1275
		1276	When C<utf8> is disabled (the default), then C<encode>/C<decode> generate
		1277	and expect Unicode strings, that is, characters with high ordinal Unicode
		1278	values (> 255) will be encoded as such characters, and likewise such
		1279	characters are decoded as-is, no changes to them will be done, except
		1280	"(re-)interpreting" them as Unicode codepoints or Unicode characters,
		1281	respectively (to Perl, these are the same thing in strings unless you do
		1282	funny/weird/dumb stuff).
		1283
		1284	This is useful when you want to do the encoding yourself (e.g. when you
		1285	want to have UTF-16 encoded JSON texts) or when some other layer does
		1286	the encoding for you (for example, when printing to a terminal using a
		1287	filehandle that transparently encodes to UTF-8 you certainly do NOT want
		1288	to UTF-8 encode your data first and have Perl encode it another time).
		1289
		1290	=item C<utf8> flag enabled
		1291
		1292	If the C<utf8>-flag is enabled, C<encode>/C<decode> will encode all
		1293	characters using the corresponding UTF-8 multi-byte sequence, and will
		1294	expect your input strings to be encoded as UTF-8, that is, no "character"
		1295	of the input string must have any value > 255, as UTF-8 does not allow
		1296	that.
		1297
		1298	The C<utf8> flag therefore switches between two modes: disabled means you
		1299	will get a Unicode string in Perl, enabled means you get a UTF-8 encoded
		1300	octet/binary string in Perl.
		1301
		1302	=item C<latin1> or C<ascii> flags enabled
		1303
		1304	With C<latin1> (or C<ascii>) enabled, C<encode> will escape characters
		1305	with ordinal values > 255 (> 127 with C<ascii>) and encode the remaining
		1306	characters as specified by the C<utf8> flag.
		1307
		1308	If C<utf8> is disabled, then the result is also correctly encoded in those
		1309	character sets (as both are proper subsets of Unicode, meaning that a
		1310	Unicode string with all character values < 256 is the same thing as a
		1311	ISO-8859-1 string, and a Unicode string with all character values < 128 is
		1312	the same thing as an ASCII string in Perl).
		1313
		1314	If C<utf8> is enabled, you still get a correct UTF-8-encoded string,
		1315	regardless of these flags, just some more characters will be escaped using
		1316	C<\uXXXX> then before.
		1317
		1318	Note that ISO-8859-1-I<encoded> strings are not compatible with UTF-8
		1319	encoding, while ASCII-encoded strings are. That is because the ISO-8859-1
		1320	encoding is NOT a subset of UTF-8 (despite the ISO-8859-1 I<codeset> being
		1321	a subset of Unicode), while ASCII is.
		1322
		1323	Surprisingly, C<decode> will ignore these flags and so treat all input
		1324	values as governed by the C<utf8> flag. If it is disabled, this allows you
		1325	to decode ISO-8859-1- and ASCII-encoded strings, as both strict subsets of
		1326	Unicode. If it is enabled, you can correctly decode UTF-8 encoded strings.
		1327
		1328	So neither C<latin1> nor C<ascii> are incompatible with the C<utf8> flag -
		1329	they only govern when the JSON output engine escapes a character or not.
		1330
		1331	The main use for C<latin1> is to relatively efficiently store binary data
		1332	as JSON, at the expense of breaking compatibility with most JSON decoders.
		1333
		1334	The main use for C<ascii> is to force the output to not contain characters
		1335	with values > 127, which means you can interpret the resulting string
		1336	as UTF-8, ISO-8859-1, ASCII, KOI8-R or most about any character set and
		1337	8-bit-encoding, and still get the same data structure back. This is useful
		1338	when your channel for JSON transfer is not 8-bit clean or the encoding
		1339	might be mangled in between (e.g. in mail), and works because ASCII is a
		1340	proper subset of most 8-bit and multibyte encodings in use in the world.
		1341
		1342	=back
		1343
		1344
		1345	=head2 JSON and ECMAscript
		1346
		1347	JSON syntax is based on how literals are represented in javascript (the
		1348	not-standardised predecessor of ECMAscript) which is presumably why it is
		1349	called "JavaScript Object Notation".
		1350
		1351	However, JSON is not a subset (and also not a superset of course) of
		1352	ECMAscript (the standard) or javascript (whatever browsers actually
		1353	implement).
		1354
		1355	If you want to use javascript's C<eval> function to "parse" JSON, you
		1356	might run into parse errors for valid JSON texts, or the resulting data
		1357	structure might not be queryable:
		1358
		1359	One of the problems is that U+2028 and U+2029 are valid characters inside
		1360	JSON strings, but are not allowed in ECMAscript string literals, so the
		1361	following Perl fragment will not output something that can be guaranteed
		1362	to be parsable by javascript's C<eval>:
		1363
		1364	use JSON::XS;
		1365
		1366	print encode_json [chr 0x2028];
		1367
		1368	The right fix for this is to use a proper JSON parser in your javascript
		1369	programs, and not rely on C<eval> (see for example Douglas Crockford's
		1370	F<json2.js> parser).
		1371
		1372	If this is not an option, you can, as a stop-gap measure, simply encode to
		1373	ASCII-only JSON:
		1374
		1375	use JSON::XS;
		1376
		1377	print JSON::XS->new->ascii->encode ([chr 0x2028]);
		1378
		1379	Note that this will enlarge the resulting JSON text quite a bit if you
		1380	have many non-ASCII characters. You might be tempted to run some regexes
		1381	to only escape U+2028 and U+2029, e.g.:
		1382
		1383	# DO NOT USE THIS!
		1384	my $json = JSON::XS->new->utf8->encode ([chr 0x2028]);
		1385	$json =~ s/\xe2\x80\xa8/\\u2028/g; # escape U+2028
		1386	$json =~ s/\xe2\x80\xa9/\\u2029/g; # escape U+2029
		1387	print $json;
		1388
		1389	Note that I<this is a bad idea>: the above only works for U+2028 and
		1390	U+2029 and thus only for fully ECMAscript-compliant parsers. Many existing
		1391	javascript implementations, however, have issues with other characters as
		1392	well - using C<eval> naively simply I<will> cause problems.
		1393
		1394	Another problem is that some javascript implementations reserve
		1395	some property names for their own purposes (which probably makes
		1396	them non-ECMAscript-compliant). For example, Iceweasel reserves the
		1397	C<__proto__> property name for its own purposes.
		1398
		1399	If that is a problem, you could parse try to filter the resulting JSON
		1400	output for these property strings, e.g.:
		1401
		1402	$json =~ s/"__proto__"\s*:/"__proto__renamed":/g;
		1403
		1404	This works because C<__proto__> is not valid outside of strings, so every
		1405	occurrence of C<"__proto__"\s*:> must be a string used as property name.
		1406
		1407	If you know of other incompatibilities, please let me know.
		1408
		1409
		1410	=head2 JSON and YAML
		1411
		1412	You often hear that JSON is a subset of YAML. This is, however, a mass
		1413	hysteria(*) and very far from the truth (as of the time of this writing),
		1414	so let me state it clearly: I<in general, there is no way to configure
		1415	JSON::XS to output a data structure as valid YAML> that works in all
		1416	cases.
		1417
		1418	If you really must use JSON::XS to generate YAML, you should use this
		1419	algorithm (subject to change in future versions):
		1420
		1421	my $to_yaml = JSON::XS->new->utf8->space_after (1);
		1422	my $yaml = $to_yaml->encode ($ref) . "\n";
		1423
		1424	This will I<usually> generate JSON texts that also parse as valid
		1425	YAML. Please note that YAML has hardcoded limits on (simple) object key
		1426	lengths that JSON doesn't have and also has different and incompatible
		1427	unicode character escape syntax, so you should make sure that your hash
		1428	keys are noticeably shorter than the 1024 "stream characters" YAML allows
		1429	and that you do not have characters with codepoint values outside the
		1430	Unicode BMP (basic multilingual page). YAML also does not allow C<\/>
		1431	sequences in strings (which JSON::XS does not I<currently> generate, but
		1432	other JSON generators might).
		1433
		1434	There might be other incompatibilities that I am not aware of (or the YAML
		1435	specification has been changed yet again - it does so quite often). In
		1436	general you should not try to generate YAML with a JSON generator or vice
		1437	versa, or try to parse JSON with a YAML parser or vice versa: chances are
		1438	high that you will run into severe interoperability problems when you
		1439	least expect it.
		1440
		1441	=over 4
		1442
		1443	=item (*)
		1444
		1445	I have been pressured multiple times by Brian Ingerson (one of the
		1446	authors of the YAML specification) to remove this paragraph, despite him
		1447	acknowledging that the actual incompatibilities exist. As I was personally
		1448	bitten by this "JSON is YAML" lie, I refused and said I will continue to
		1449	educate people about these issues, so others do not run into the same
		1450	problem again and again. After this, Brian called me a (quote)I<complete
		1451	and worthless idiot>(unquote).
		1452
		1453	In my opinion, instead of pressuring and insulting people who actually
		1454	clarify issues with YAML and the wrong statements of some of its
		1455	proponents, I would kindly suggest reading the JSON spec (which is not
		1456	that difficult or long) and finally make YAML compatible to it, and
		1457	educating users about the changes, instead of spreading lies about the
		1458	real compatibility for many I<years> and trying to silence people who
		1459	point out that it isn't true.
		1460
		1461	Addendum/2009: the YAML 1.2 spec is still incompatible with JSON, even
		1462	though the incompatibilities have been documented (and are known to Brian)
		1463	for many years and the spec makes explicit claims that YAML is a superset
		1464	of JSON. It would be so easy to fix, but apparently, bullying people and
		1465	corrupting userdata is so much easier.
		1466
		1467	=back
		1468
475		1469
476	=head2 SPEED	1470	=head2 SPEED
477		1471
478	It seems that JSON::XS is surprisingly fast, as shown in the following	1472	It seems that JSON::XS is surprisingly fast, as shown in the following
479	tables. They have been generated with the help of the C<eg/bench> program	1473	tables. They have been generated with the help of the C<eg/bench> program
480	in the JSON::XS distribution, to make it easy to compare on your own	1474	in the JSON::XS distribution, to make it easy to compare on your own
481	system.	1475	system.
482		1476
483	First is a comparison between various modules using a very simple JSON	1477	First comes a comparison between various modules using
		1478	a very short single-line JSON string (also available at
		1479	L<http://dist.schmorp.de/misc/json/short.json>).
		1480
		1481	{"method": "handleMessage", "params": ["user1",
		1482	"we were just talking"], "id": null, "array":[1,11,234,-5,1e5,1e7,
		1483	1, 0]}
		1484
484	string, showing the number of encodes/decodes per second (JSON::XS is	1485	It shows the number of encodes/decodes per second (JSON::XS uses
485	the functional interface, while JSON::XS/2 is the OO interface with	1486	the functional interface, while JSON::XS/2 uses the OO interface
486	pretty-printing and hashkey sorting enabled).	1487	with pretty-printing and hashkey sorting enabled, JSON::XS/3 enables
		1488	shrink. JSON::DWIW/DS uses the deserialise function, while JSON::DWIW::FJ
		1489	uses the from_json method). Higher is better:
487		1490
488	module | encode | decode |	1491	module | encode | decode |
489	-----------|------------|------------|	1492	--------------|------------|------------|
490	JSON | 14006 | 6820 |	1493	JSON::DWIW/DS | 86302.551 | 102300.098 |
491	JSON::DWIW | 200937 | 120386 |	1494	JSON::DWIW/FJ | 86302.551 | 75983.768 |
492	JSON::PC | 85065 | 129366 |	1495	JSON::PP | 15827.562 | 6638.658 |
493	JSON::Syck | 59898 | 44232 |	1496	JSON::Syck | 63358.066 | 47662.545 |
494	JSON::XS | 1171478 | 342435 |	1497	JSON::XS | 511500.488 | 511500.488 |
495	JSON::XS/2 | 730760 | 328714 |	1498	JSON::XS/2 | 291271.111 | 388361.481 |
		1499	JSON::XS/3 | 361577.931 | 361577.931 |
		1500	Storable | 66788.280 | 265462.278 |
496	-----------+------------+------------+	1501	--------------+------------+------------+
497		1502
498	That is, JSON::XS is 6 times faster than than JSON::DWIW and about 80	1503	That is, JSON::XS is almost six times faster than JSON::DWIW on encoding,
499	times faster than JSON, even with pretty-printing and key sorting.	1504	about five times faster on decoding, and over thirty to seventy times
		1505	faster than JSON's pure perl implementation. It also compares favourably
		1506	to Storable for small amounts of data.
500		1507
501	Using a longer test string (roughly 8KB, generated from Yahoo! Locals	1508	Using a longer test string (roughly 18KB, generated from Yahoo! Locals
502	search API (http://nanoref.com/yahooapis/mgPdGg):	1509	search API (L<http://dist.schmorp.de/misc/json/long.json>).
503		1510
504	module | encode | decode |	1511	module | encode | decode |
505	-----------|------------|------------|	1512	--------------|------------|------------|
506	JSON | 673 | 38 |	1513	JSON::DWIW/DS | 1647.927 | 2673.916 |
507	JSON::DWIW | 5271 | 770 |	1514	JSON::DWIW/FJ | 1630.249 | 2596.128 |
508	JSON::PC | 9901 | 2491 |	1515	JSON::PP | 400.640 | 62.311 |
509	JSON::Syck | 2360 | 786 |	1516	JSON::Syck | 1481.040 | 1524.869 |
510	JSON::XS | 37398 | 3202 |	1517	JSON::XS | 20661.596 | 9541.183 |
511	JSON::XS/2 | 13765 | 3153 |	1518	JSON::XS/2 | 10683.403 | 9416.938 |
		1519	JSON::XS/3 | 20661.596 | 9400.054 |
		1520	Storable | 19765.806 | 10000.725 |
512	-----------+------------+------------+	1521	--------------+------------+------------+
513		1522
514	Again, JSON::XS leads by far in the encoding case, while still beating	1523	Again, JSON::XS leads by far (except for Storable which non-surprisingly
515	every other module in the decoding case.	1524	decodes a bit faster).
516		1525
517	Last example is an almost 8MB large hash with many large binary values	1526	On large strings containing lots of high Unicode characters, some modules
518	(PNG files), resulting in a lot of escaping:	1527	(such as JSON::PC) seem to decode faster than JSON::XS, but the result
		1528	will be broken due to missing (or wrong) Unicode handling. Others refuse
		1529	to decode or encode properly, so it was impossible to prepare a fair
		1530	comparison table for that case.
519		1531
520	=head1 RESOURCE LIMITS
521		1532
522	JSON::XS does not impose any limits on the size of JSON texts or Perl	1533	=head1 SECURITY CONSIDERATIONS
523	values they represent - if your machine cna handle it, JSON::XS will	1534
524	encode or decode it. Future versions might optionally impose structure	1535	When you are using JSON in a protocol, talking to untrusted potentially
525	depth and memory use resource limits.	1536	hostile creatures requires relatively few measures.
		1537
		1538	First of all, your JSON decoder should be secure, that is, should not have
		1539	any buffer overflows. Obviously, this module should ensure that and I am
		1540	trying hard on making that true, but you never know.
		1541
		1542	Second, you need to avoid resource-starving attacks. That means you should
		1543	limit the size of JSON texts you accept, or make sure then when your
		1544	resources run out, that's just fine (e.g. by using a separate process that
		1545	can crash safely). The size of a JSON text in octets or characters is
		1546	usually a good indication of the size of the resources required to decode
		1547	it into a Perl structure. While JSON::XS can check the size of the JSON
		1548	text, it might be too late when you already have it in memory, so you
		1549	might want to check the size before you accept the string.
		1550
		1551	Third, JSON::XS recurses using the C stack when decoding objects and
		1552	arrays. The C stack is a limited resource: for instance, on my amd64
		1553	machine with 8MB of stack size I can decode around 180k nested arrays but
		1554	only 14k nested JSON objects (due to perl itself recursing deeply on croak
		1555	to free the temporary). If that is exceeded, the program crashes. To be
		1556	conservative, the default nesting limit is set to 512. If your process
		1557	has a smaller stack, you should adjust this setting accordingly with the
		1558	C<max_depth> method.
		1559
		1560	Something else could bomb you, too, that I forgot to think of. In that
		1561	case, you get to keep the pieces. I am always open for hints, though...
		1562
		1563	Also keep in mind that JSON::XS might leak contents of your Perl data
		1564	structures in its error messages, so when you serialise sensitive
		1565	information you might want to make sure that exceptions thrown by JSON::XS
		1566	will not end up in front of untrusted eyes.
		1567
		1568	If you are using JSON::XS to return packets to consumption
		1569	by JavaScript scripts in a browser you should have a look at
		1570	L<http://blog.archive.jpsykes.com/47/practical-csrf-and-json-security/> to
		1571	see whether you are vulnerable to some common attack vectors (which really
		1572	are browser design bugs, but it is still you who will have to deal with
		1573	it, as major browser developers care only for features, not about getting
		1574	security right).
		1575
		1576
		1577	=head2 "OLD" VS. "NEW" JSON (RFC 4627 VS. RFC 7159)
		1578
		1579	JSON originally required JSON texts to represent an array or object -
		1580	scalar values were explicitly not allowed. This has changed, and versions
		1581	of JSON::XS beginning with C<4.0> reflect this by allowing scalar values
		1582	by default.
		1583
		1584	One reason why one might not want this is that this removes a fundamental
		1585	property of JSON texts, namely that they are self-delimited and
		1586	self-contained, or in other words, you could take any number of "old"
		1587	JSON texts and paste them together, and the result would be unambiguously
		1588	parseable:
		1589
		1590	[1,3]{"k":5}[][null] # four JSON texts, without doubt
		1591
		1592	By allowing scalars, this property is lost: in the following example, is
		1593	this one JSON text (the number 12) or two JSON texts (the numbers 1 and
		1594	2):
		1595
		1596	12 # could be 12, or 1 and 2
		1597
		1598	Another lost property of "old" JSON is that no lookahead is required to
		1599	know the end of a JSON text, i.e. the JSON text definitely ended at the
		1600	last C<]> or C<}> character, there was no need to read extra characters.
		1601
		1602	For example, a viable network protocol with "old" JSON was to simply
		1603	exchange JSON texts without delimiter. For "new" JSON, you have to use a
		1604	suitable delimiter (such as a newline) after every JSON text or ensure you
		1605	never encode/decode scalar values.
		1606
		1607	Most protocols do work by only transferring arrays or objects, and the
		1608	easiest way to avoid problems with the "new" JSON definition is to
		1609	explicitly disallow scalar values in your encoder and decoder:
		1610
		1611	$json_coder = JSON::XS->new->allow_nonref (0)
		1612
		1613	This is a somewhat unhappy situation, and the blame can fully be put on
		1614	JSON's inmventor, Douglas Crockford, who unilaterally changed the format
		1615	in 2006 without consulting the IETF, forcing the IETF to either fork the
		1616	format or go with it (as I was told, the IETF wasn't amused).
		1617
		1618
		1619	=head1 INTEROPERABILITY WITH OTHER MODULES
		1620
		1621	C<JSON::XS> uses the L<Types::Serialiser> module to provide boolean
		1622	constants. That means that the JSON true and false values will be
		1623	comaptible to true and false values of other modules that do the same,
		1624	such as L<JSON::PP> and L<CBOR::XS>.
		1625
		1626
		1627	=head1 INTEROPERABILITY WITH OTHER JSON DECODERS
		1628
		1629	As long as you only serialise data that can be directly expressed in JSON,
		1630	C<JSON::XS> is incapable of generating invalid JSON output (modulo bugs,
		1631	but C<JSON::XS> has found more bugs in the official JSON testsuite (1)
		1632	than the official JSON testsuite has found in C<JSON::XS> (0)).
		1633
		1634	When you have trouble decoding JSON generated by this module using other
		1635	decoders, then it is very likely that you have an encoding mismatch or the
		1636	other decoder is broken.
		1637
		1638	When decoding, C<JSON::XS> is strict by default and will likely catch all
		1639	errors. There are currently two settings that change this: C<relaxed>
		1640	makes C<JSON::XS> accept (but not generate) some non-standard extensions,
		1641	and C<allow_tags> will allow you to encode and decode Perl objects, at the
		1642	cost of not outputting valid JSON anymore.
		1643
		1644	=head2 TAGGED VALUE SYNTAX AND STANDARD JSON EN/DECODERS
		1645
		1646	When you use C<allow_tags> to use the extended (and also nonstandard and
		1647	invalid) JSON syntax for serialised objects, and you still want to decode
		1648	the generated When you want to serialise objects, you can run a regex
		1649	to replace the tagged syntax by standard JSON arrays (it only works for
		1650	"normal" package names without comma, newlines or single colons). First,
		1651	the readable Perl version:
		1652
		1653	# if your FREEZE methods return no values, you need this replace first:
		1654	$json =~ s/\( \s* (" (?: [^\\":,]+|\\.|::)* ") \s* \) \s* \[\s*\]/[$1]/gx;
		1655
		1656	# this works for non-empty constructor arg lists:
		1657	$json =~ s/\( \s* (" (?: [^\\":,]+|\\.|::)* ") \s* \) \s* \[/[$1,/gx;
		1658
		1659	And here is a less readable version that is easy to adapt to other
		1660	languages:
		1661
		1662	$json =~ s/\(\s*("([^\\":,]+|\\.|::)*")\s*\)\s*\[/[$1,/g;
		1663
		1664	Here is an ECMAScript version (same regex):
		1665
		1666	json = json.replace (/\(\s*("([^\\":,]+|\\.|::)*")\s*\)\s*\[/g, "[$1,");
		1667
		1668	Since this syntax converts to standard JSON arrays, it might be hard to
		1669	distinguish serialised objects from normal arrays. You can prepend a
		1670	"magic number" as first array element to reduce chances of a collision:
		1671
		1672	$json =~ s/\(\s*("([^\\":,]+|\\.|::)*")\s*\)\s*\[/["XU1peReLzT4ggEllLanBYq4G9VzliwKF",$1,/g;
		1673
		1674	And after decoding the JSON text, you could walk the data
		1675	structure looking for arrays with a first element of
		1676	C<XU1peReLzT4ggEllLanBYq4G9VzliwKF>.
		1677
		1678	The same approach can be used to create the tagged format with another
		1679	encoder. First, you create an array with the magic string as first member,
		1680	the classname as second, and constructor arguments last, encode it as part
		1681	of your JSON structure, and then:
		1682
		1683	$json =~ s/\[\s*"XU1peReLzT4ggEllLanBYq4G9VzliwKF"\s*,\s*("([^\\":,]+|\\.|::)*")\s*,/($1)[/g;
		1684
		1685	Again, this has some limitations - the magic string must not be encoded
		1686	with character escapes, and the constructor arguments must be non-empty.
		1687
		1688
		1689	=head1 RFC7159
		1690
		1691	Since this module was written, Google has written a new JSON RFC, RFC 7159
		1692	(and RFC7158). Unfortunately, this RFC breaks compatibility with both the
		1693	original JSON specification on www.json.org and RFC4627.
		1694
		1695	As far as I can see, you can get partial compatibility when parsing by
		1696	using C<< ->allow_nonref >>. However, consider the security implications
		1697	of doing so.
		1698
		1699	I haven't decided yet when to break compatibility with RFC4627 by default
		1700	(and potentially leave applications insecure) and change the default to
		1701	follow RFC7159, but application authors are well advised to call C<<
		1702	->allow_nonref(0) >> even if this is the current default, if they cannot
		1703	handle non-reference values, in preparation for the day when the default
		1704	will change.
		1705
		1706
		1707	=head1 (I-)THREADS
		1708
		1709	This module is I<not> guaranteed to be ithread (or MULTIPLICITY-) safe
		1710	and there are no plans to change this. Note that perl's builtin so-called
		1711	threads/ithreads are officially deprecated and should not be used.
		1712
		1713
		1714	=head1 THE PERILS OF SETLOCALE
		1715
		1716	Sometimes people avoid the Perl locale support and directly call the
		1717	system's setlocale function with C<LC_ALL>.
		1718
		1719	This breaks both perl and modules such as JSON::XS, as stringification of
		1720	numbers no longer works correctly (e.g. C<$x = 0.1; print "$x"+1> might
		1721	print C<1>, and JSON::XS might output illegal JSON as JSON::XS relies on
		1722	perl to stringify numbers).
		1723
		1724	The solution is simple: don't call C<setlocale>, or use it for only those
		1725	categories you need, such as C<LC_MESSAGES> or C<LC_CTYPE>.
		1726
		1727	If you need C<LC_NUMERIC>, you should enable it only around the code that
		1728	actually needs it (avoiding stringification of numbers), and restore it
		1729	afterwards.
		1730
526		1731
527	=head1 BUGS	1732	=head1 BUGS
528		1733
529	While the goal of this module is to be correct, that unfortunately does	1734	While the goal of this module is to be correct, that unfortunately does
530	not mean its bug-free, only that I think its design is bug-free. It is	1735	not mean it's bug-free, only that I think its design is bug-free. If you
531	still very young and not well-tested. If you keep reporting bugs they will	1736	keep reporting bugs they will be fixed swiftly, though.
532	be fixed swiftly, though.	1737
		1738	Please refrain from using rt.cpan.org or any other bug reporting
		1739	service. I put the contact address into my modules for a reason.
533		1740
534	=cut	1741	=cut
535		1742
536	1;	1743	BEGIN {
		1744	*true = \$Types::Serialiser::true;
		1745	*true = \&Types::Serialiser::true;
		1746	*false = \$Types::Serialiser::false;
		1747	*false = \&Types::Serialiser::false;
		1748	*is_bool = \&Types::Serialiser::is_bool;
		1749
		1750	*JSON::XS::Boolean:: = *Types::Serialiser::Boolean::;
		1751	}
		1752
		1753	XSLoader::load "JSON::XS", $VERSION;
		1754
		1755	=head1 SEE ALSO
		1756
		1757	The F<json_xs> command line utility for quick experiments.
537		1758
538	=head1 AUTHOR	1759	=head1 AUTHOR
539		1760
540	Marc Lehmann <schmorp@schmorp.de>	1761	Marc Lehmann <schmorp@schmorp.de>
541	http://home.schmorp.de/	1762	http://home.schmorp.de/
542		1763
543	=cut	1764	=cut
544		1765
		1766	1
		1767

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