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Revision 1.2 by root, Thu Mar 22 17:28:50 2007 UTC vs.
Revision 1.141 by root, Fri Oct 25 19:53:08 2013 UTC

1	=head1 NAME	1	=head1 NAME
2		2
3	JSON::XS - JSON serialising/deserialising, done correctly and fast	3	JSON::XS - JSON serialising/deserialising, done correctly and fast
4		4
		5	=encoding utf-8
		6
		7	JSON::XS - 正しくて高速な JSON シリアライザ/デシリアライザ
		8	(http://fleur.hio.jp/perldoc/mix/lib/JSON/XS.html)
		9
5	=head1 SYNOPSIS	10	=head1 SYNOPSIS
6		11
7	use JSON::XS;	12	use JSON::XS;
		13
		14	# exported functions, they croak on error
		15	# and expect/generate UTF-8
		16
		17	$utf8_encoded_json_text = encode_json $perl_hash_or_arrayref;
		18	$perl_hash_or_arrayref = decode_json $utf8_encoded_json_text;
		19
		20	# OO-interface
		21
		22	$coder = JSON::XS->new->ascii->pretty->allow_nonref;
		23	$pretty_printed_unencoded = $coder->encode ($perl_scalar);
		24	$perl_scalar = $coder->decode ($unicode_json_text);
		25
		26	# Note that JSON version 2.0 and above will automatically use JSON::XS
		27	# if available, at virtually no speed overhead either, so you should
		28	# be able to just:
		29
		30	use JSON;
		31
		32	# and do the same things, except that you have a pure-perl fallback now.
8		33
9	=head1 DESCRIPTION	34	=head1 DESCRIPTION
10		35
11	This module converts Perl data structures to JSON and vice versa. Its	36	This module converts Perl data structures to JSON and vice versa. Its
12	primary goal is to be I<correct> and its secondary goal is to be	37	primary goal is to be I<correct> and its secondary goal is to be
13	I<fast>. To reach the latter goal it was written in C.	38	I<fast>. To reach the latter goal it was written in C.
		39
		40	Beginning with version 2.0 of the JSON module, when both JSON and
		41	JSON::XS are installed, then JSON will fall back on JSON::XS (this can be
		42	overridden) with no overhead due to emulation (by inheriting constructor
		43	and methods). If JSON::XS is not available, it will fall back to the
		44	compatible JSON::PP module as backend, so using JSON instead of JSON::XS
		45	gives you a portable JSON API that can be fast when you need and doesn't
		46	require a C compiler when that is a problem.
14		47
15	As this is the n-th-something JSON module on CPAN, what was the reason	48	As this is the n-th-something JSON module on CPAN, what was the reason
16	to write yet another JSON module? While it seems there are many JSON	49	to write yet another JSON module? While it seems there are many JSON
17	modules, none of them correctly handle all corner cases, and in most cases	50	modules, none of them correctly handle all corner cases, and in most cases
18	their maintainers are unresponsive, gone missing, or not listening to bug	51	their maintainers are unresponsive, gone missing, or not listening to bug
19	reports for other reasons.	52	reports for other reasons.
20		53
21	See COMPARISON, below, for a comparison to some other JSON modules.	54	See MAPPING, below, on how JSON::XS maps perl values to JSON values and
		55	vice versa.
22		56
23	=head2 FEATURES	57	=head2 FEATURES
24		58
25	=over 4	59	=over 4
26		60
27	=item * correct handling of unicode issues	61	=item * correct Unicode handling
28		62
29	This module knows how to handle Unicode, and even documents how it does so.	63	This module knows how to handle Unicode, documents how and when it does
		64	so, and even documents what "correct" means.
30		65
31	=item * round-trip integrity	66	=item * round-trip integrity
32		67
33	When you serialise a perl data structure using only datatypes supported	68	When you serialise a perl data structure using only data types supported
34	by JSON, the deserialised data structure is identical on the Perl level.	69	by JSON and Perl, the deserialised data structure is identical on the Perl
35	(e.g. the string "2.0" doesn't suddenly become "2").	70	level. (e.g. the string "2.0" doesn't suddenly become "2" just because
		71	it looks like a number). There I<are> minor exceptions to this, read the
		72	MAPPING section below to learn about those.
36		73
37	=item * strict checking of JSON correctness	74	=item * strict checking of JSON correctness
38		75
39	There is no guessing, no generating of illegal JSON strings by default,	76	There is no guessing, no generating of illegal JSON texts by default,
40	and only JSON is accepted as input (the latter is a security feature).	77	and only JSON is accepted as input by default (the latter is a security
		78	feature).
41		79
42	=item * fast	80	=item * fast
43		81
44	compared to other JSON modules, this module compares favourably.	82	Compared to other JSON modules and other serialisers such as Storable,
		83	this module usually compares favourably in terms of speed, too.
45		84
46	=item * simple to use	85	=item * simple to use
47		86
48	This module has both a simple functional interface as well as an OO	87	This module has both a simple functional interface as well as an object
49	interface.	88	oriented interface.
50		89
51	=item * reasonably versatile output formats	90	=item * reasonably versatile output formats
52		91
53	You can choose between the most compact format possible, a pure-ascii	92	You can choose between the most compact guaranteed-single-line format
54	format, or a pretty-printed format. Or you can combine those features in	93	possible (nice for simple line-based protocols), a pure-ASCII format
55	whatever way you like.	94	(for when your transport is not 8-bit clean, still supports the whole
		95	Unicode range), or a pretty-printed format (for when you want to read that
		96	stuff). Or you can combine those features in whatever way you like.
56		97
57	=back	98	=back
58		99
59	=cut	100	=cut
60		101
61	package JSON::XS;	102	package JSON::XS;
62		103
63	BEGIN {	104	use common::sense;
64	$VERSION = '0.1';	105
		106	our $VERSION = 2.34;
65	@ISA = qw(Exporter);	107	our @ISA = qw(Exporter);
66		108
67	@EXPORT = qw(to_json from_json);	109	our @EXPORT = qw(encode_json decode_json);
68	require Exporter;
69		110
70	require XSLoader;	111	use Exporter;
71	XSLoader::load JSON::XS::, $VERSION;	112	use XSLoader;
72	}
73		113
74	=head1 FUNCTIONAL INTERFACE	114	=head1 FUNCTIONAL INTERFACE
75		115
76	The following convinience methods are provided by this module. They are	116	The following convenience methods are provided by this module. They are
77	exported by default:	117	exported by default:
78		118
79	=over 4	119	=over 4
80		120
81	=item $json_string = to_json $perl_scalar	121	=item $json_text = encode_json $perl_scalar
82		122
83	Converts the given Perl data structure (a simple scalar or a reference to	123	Converts the given Perl data structure to a UTF-8 encoded, binary string
84	a hash or array) to a UTF-8 encoded, binary string (that is, the string contains	124	(that is, the string contains octets only). Croaks on error.
85	octets only). Croaks on error.
86		125
87	This function call is functionally identical to C<< JSON::XS->new->utf8	126	This function call is functionally identical to:
88	(1)->encode ($perl_scalar) >>.
89		127
		128	$json_text = JSON::XS->new->utf8->encode ($perl_scalar)
		129
		130	Except being faster.
		131
90	=item $perl_scalar = from_json $json_string	132	=item $perl_scalar = decode_json $json_text
91		133
92	The opposite of C<to_json>: expects an UTF-8 (binary) string and tries to	134	The opposite of C<encode_json>: expects an UTF-8 (binary) string and tries
93	parse that as an UTF-8 encoded JSON string, returning the resulting simple	135	to parse that as an UTF-8 encoded JSON text, returning the resulting
94	scalar or reference. Croaks on error.	136	reference. Croaks on error.
95		137
96	This function call is functionally identical to C<< JSON::XS->new->utf8	138	This function call is functionally identical to:
97	(1)->decode ($json_string) >>.	139
		140	$perl_scalar = JSON::XS->new->utf8->decode ($json_text)
		141
		142	Except being faster.
		143
		144	=item $is_boolean = JSON::XS::is_bool $scalar
		145
		146	Returns true if the passed scalar represents either JSON::XS::true or
		147	JSON::XS::false, two constants that act like C<1> and C<0>, respectively
		148	and are used to represent JSON C<true> and C<false> values in Perl.
		149
		150	See MAPPING, below, for more information on how JSON values are mapped to
		151	Perl.
98		152
99	=back	153	=back
		154
		155
		156	=head1 A FEW NOTES ON UNICODE AND PERL
		157
		158	Since this often leads to confusion, here are a few very clear words on
		159	how Unicode works in Perl, modulo bugs.
		160
		161	=over 4
		162
		163	=item 1. Perl strings can store characters with ordinal values > 255.
		164
		165	This enables you to store Unicode characters as single characters in a
		166	Perl string - very natural.
		167
		168	=item 2. Perl does I<not> associate an encoding with your strings.
		169
		170	... until you force it to, e.g. when matching it against a regex, or
		171	printing the scalar to a file, in which case Perl either interprets your
		172	string as locale-encoded text, octets/binary, or as Unicode, depending
		173	on various settings. In no case is an encoding stored together with your
		174	data, it is I<use> that decides encoding, not any magical meta data.
		175
		176	=item 3. The internal utf-8 flag has no meaning with regards to the
		177	encoding of your string.
		178
		179	Just ignore that flag unless you debug a Perl bug, a module written in
		180	XS or want to dive into the internals of perl. Otherwise it will only
		181	confuse you, as, despite the name, it says nothing about how your string
		182	is encoded. You can have Unicode strings with that flag set, with that
		183	flag clear, and you can have binary data with that flag set and that flag
		184	clear. Other possibilities exist, too.
		185
		186	If you didn't know about that flag, just the better, pretend it doesn't
		187	exist.
		188
		189	=item 4. A "Unicode String" is simply a string where each character can be
		190	validly interpreted as a Unicode code point.
		191
		192	If you have UTF-8 encoded data, it is no longer a Unicode string, but a
		193	Unicode string encoded in UTF-8, giving you a binary string.
		194
		195	=item 5. A string containing "high" (> 255) character values is I<not> a UTF-8 string.
		196
		197	It's a fact. Learn to live with it.
		198
		199	=back
		200
		201	I hope this helps :)
		202
100		203
101	=head1 OBJECT-ORIENTED INTERFACE	204	=head1 OBJECT-ORIENTED INTERFACE
102		205
103	The object oriented interface lets you configure your own encoding or	206	The object oriented interface lets you configure your own encoding or
104	decoding style, within the limits of supported formats.	207	decoding style, within the limits of supported formats.
…		…
111	strings. All boolean flags described below are by default I<disabled>.	214	strings. All boolean flags described below are by default I<disabled>.
112		215
113	The mutators for flags all return the JSON object again and thus calls can	216	The mutators for flags all return the JSON object again and thus calls can
114	be chained:	217	be chained:
115		218
116	my $json = JSON::XS->new->utf8(1)->pretty(1)->encode ({a => [1,2]})	219	my $json = JSON::XS->new->utf8->space_after->encode ({a => [1,2]})
117	=> {"a" : [1, 2]}	220	=> {"a": [1, 2]}
118		221
119	=item $json = $json->ascii ($enable)	222	=item $json = $json->ascii ([$enable])
120		223
		224	=item $enabled = $json->get_ascii
		225
121	If C<$enable> is true, then the C<encode> method will not generate	226	If C<$enable> is true (or missing), then the C<encode> method will not
122	characters outside the code range C<0..127>. Any unicode characters	227	generate characters outside the code range C<0..127> (which is ASCII). Any
123	outside that range will be escaped using either a single \uXXXX (BMP	228	Unicode characters outside that range will be escaped using either a
124	characters) or a double \uHHHH\uLLLLL escape sequence, as per RFC4627.	229	single \uXXXX (BMP characters) or a double \uHHHH\uLLLLL escape sequence,
		230	as per RFC4627. The resulting encoded JSON text can be treated as a native
		231	Unicode string, an ascii-encoded, latin1-encoded or UTF-8 encoded string,
		232	or any other superset of ASCII.
125		233
126	If C<$enable> is false, then the C<encode> method will not escape Unicode	234	If C<$enable> is false, then the C<encode> method will not escape Unicode
127	characters unless necessary.	235	characters unless required by the JSON syntax or other flags. This results
		236	in a faster and more compact format.
128		237
		238	See also the section I<ENCODING/CODESET FLAG NOTES> later in this
		239	document.
		240
		241	The main use for this flag is to produce JSON texts that can be
		242	transmitted over a 7-bit channel, as the encoded JSON texts will not
		243	contain any 8 bit characters.
		244
		245	JSON::XS->new->ascii (1)->encode ([chr 0x10401])
		246	=> ["\ud801\udc01"]
		247
		248	=item $json = $json->latin1 ([$enable])
		249
		250	=item $enabled = $json->get_latin1
		251
		252	If C<$enable> is true (or missing), then the C<encode> method will encode
		253	the resulting JSON text as latin1 (or iso-8859-1), escaping any characters
		254	outside the code range C<0..255>. The resulting string can be treated as a
		255	latin1-encoded JSON text or a native Unicode string. The C<decode> method
		256	will not be affected in any way by this flag, as C<decode> by default
		257	expects Unicode, which is a strict superset of latin1.
		258
		259	If C<$enable> is false, then the C<encode> method will not escape Unicode
		260	characters unless required by the JSON syntax or other flags.
		261
		262	See also the section I<ENCODING/CODESET FLAG NOTES> later in this
		263	document.
		264
		265	The main use for this flag is efficiently encoding binary data as JSON
		266	text, as most octets will not be escaped, resulting in a smaller encoded
		267	size. The disadvantage is that the resulting JSON text is encoded
		268	in latin1 (and must correctly be treated as such when storing and
		269	transferring), a rare encoding for JSON. It is therefore most useful when
		270	you want to store data structures known to contain binary data efficiently
		271	in files or databases, not when talking to other JSON encoders/decoders.
		272
		273	JSON::XS->new->latin1->encode (["\x{89}\x{abc}"]
		274	=> ["\x{89}\\u0abc"] # (perl syntax, U+abc escaped, U+89 not)
		275
129	=item $json = $json->utf8 ($enable)	276	=item $json = $json->utf8 ([$enable])
130		277
		278	=item $enabled = $json->get_utf8
		279
131	If C<$enable> is true, then the C<encode> method will encode the JSON	280	If C<$enable> is true (or missing), then the C<encode> method will encode
132	string into UTF-8, as required by many protocols, while the C<decode>	281	the JSON result into UTF-8, as required by many protocols, while the
133	method expects to be handled an UTF-8-encoded string. Please note that	282	C<decode> method expects to be handled an UTF-8-encoded string. Please
134	UTF-8-encoded strings do not contain any characters outside the range	283	note that UTF-8-encoded strings do not contain any characters outside the
135	C<0..255>, they are thus useful for bytewise/binary I/O.	284	range C<0..255>, they are thus useful for bytewise/binary I/O. In future
		285	versions, enabling this option might enable autodetection of the UTF-16
		286	and UTF-32 encoding families, as described in RFC4627.
136		287
137	If C<$enable> is false, then the C<encode> method will return the JSON	288	If C<$enable> is false, then the C<encode> method will return the JSON
138	string as a (non-encoded) unicode string, while C<decode> expects thus a	289	string as a (non-encoded) Unicode string, while C<decode> expects thus a
139	unicode string. Any decoding or encoding (e.g. to UTF-8 or UTF-16) needs	290	Unicode string. Any decoding or encoding (e.g. to UTF-8 or UTF-16) needs
140	to be done yourself, e.g. using the Encode module.	291	to be done yourself, e.g. using the Encode module.
141		292
		293	See also the section I<ENCODING/CODESET FLAG NOTES> later in this
		294	document.
		295
		296	Example, output UTF-16BE-encoded JSON:
		297
		298	use Encode;
		299	$jsontext = encode "UTF-16BE", JSON::XS->new->encode ($object);
		300
		301	Example, decode UTF-32LE-encoded JSON:
		302
		303	use Encode;
		304	$object = JSON::XS->new->decode (decode "UTF-32LE", $jsontext);
		305
142	=item $json = $json->pretty ($enabla)	306	=item $json = $json->pretty ([$enable])
143		307
144	This enables (or disables) all of the C<indent>, C<space_before> and	308	This enables (or disables) all of the C<indent>, C<space_before> and
145	C<space_after> (and in the future possibly more) settings in one call to	309	C<space_after> (and in the future possibly more) flags in one call to
146	generate the most readable (or most compact) form possible.	310	generate the most readable (or most compact) form possible.
147		311
		312	Example, pretty-print some simple structure:
		313
		314	my $json = JSON::XS->new->pretty(1)->encode ({a => [1,2]})
		315	=>
		316	{
		317	"a" : [
		318	1,
		319	2
		320	]
		321	}
		322
148	=item $json = $json->indent ($enable)	323	=item $json = $json->indent ([$enable])
149		324
		325	=item $enabled = $json->get_indent
		326
150	If C<$enable> is true, then the C<encode> method will use a multiline	327	If C<$enable> is true (or missing), then the C<encode> method will use a multiline
151	format as output, putting every array member or object/hash key-value pair	328	format as output, putting every array member or object/hash key-value pair
152	into its own line, identing them properly.	329	into its own line, indenting them properly.
153		330
154	If C<$enable> is false, no newlines or indenting will be produced, and the	331	If C<$enable> is false, no newlines or indenting will be produced, and the
155	resulting JSON strings is guarenteed not to contain any C<newlines>.	332	resulting JSON text is guaranteed not to contain any C<newlines>.
156		333
157	This setting has no effect when decoding JSON strings.	334	This setting has no effect when decoding JSON texts.
158		335
159	=item $json = $json->space_before ($enable)	336	=item $json = $json->space_before ([$enable])
160		337
		338	=item $enabled = $json->get_space_before
		339
161	If C<$enable> is true, then the C<encode> method will add an extra	340	If C<$enable> is true (or missing), then the C<encode> method will add an extra
162	optional space before the C<:> separating keys from values in JSON objects.	341	optional space before the C<:> separating keys from values in JSON objects.
163		342
164	If C<$enable> is false, then the C<encode> method will not add any extra	343	If C<$enable> is false, then the C<encode> method will not add any extra
165	space at those places.	344	space at those places.
166		345
167	This setting has no effect when decoding JSON strings. You will also most	346	This setting has no effect when decoding JSON texts. You will also
168	likely combine this setting with C<space_after>.	347	most likely combine this setting with C<space_after>.
169		348
		349	Example, space_before enabled, space_after and indent disabled:
		350
		351	{"key" :"value"}
		352
170	=item $json = $json->space_after ($enable)	353	=item $json = $json->space_after ([$enable])
171		354
		355	=item $enabled = $json->get_space_after
		356
172	If C<$enable> is true, then the C<encode> method will add an extra	357	If C<$enable> is true (or missing), then the C<encode> method will add an extra
173	optional space after the C<:> separating keys from values in JSON objects	358	optional space after the C<:> separating keys from values in JSON objects
174	and extra whitespace after the C<,> separating key-value pairs and array	359	and extra whitespace after the C<,> separating key-value pairs and array
175	members.	360	members.
176		361
177	If C<$enable> is false, then the C<encode> method will not add any extra	362	If C<$enable> is false, then the C<encode> method will not add any extra
178	space at those places.	363	space at those places.
179		364
180	This setting has no effect when decoding JSON strings.	365	This setting has no effect when decoding JSON texts.
181		366
		367	Example, space_before and indent disabled, space_after enabled:
		368
		369	{"key": "value"}
		370
		371	=item $json = $json->relaxed ([$enable])
		372
		373	=item $enabled = $json->get_relaxed
		374
		375	If C<$enable> is true (or missing), then C<decode> will accept some
		376	extensions to normal JSON syntax (see below). C<encode> will not be
		377	affected in anyway. I<Be aware that this option makes you accept invalid
		378	JSON texts as if they were valid!>. I suggest only to use this option to
		379	parse application-specific files written by humans (configuration files,
		380	resource files etc.)
		381
		382	If C<$enable> is false (the default), then C<decode> will only accept
		383	valid JSON texts.
		384
		385	Currently accepted extensions are:
		386
		387	=over 4
		388
		389	=item * list items can have an end-comma
		390
		391	JSON I<separates> array elements and key-value pairs with commas. This
		392	can be annoying if you write JSON texts manually and want to be able to
		393	quickly append elements, so this extension accepts comma at the end of
		394	such items not just between them:
		395
		396	[
		397	1,
		398	2, <- this comma not normally allowed
		399	]
		400	{
		401	"k1": "v1",
		402	"k2": "v2", <- this comma not normally allowed
		403	}
		404
		405	=item * shell-style '#'-comments
		406
		407	Whenever JSON allows whitespace, shell-style comments are additionally
		408	allowed. They are terminated by the first carriage-return or line-feed
		409	character, after which more white-space and comments are allowed.
		410
		411	[
		412	1, # this comment not allowed in JSON
		413	# neither this one...
		414	]
		415
		416	=back
		417
182	=item $json = $json->canonical ($enable)	418	=item $json = $json->canonical ([$enable])
183		419
		420	=item $enabled = $json->get_canonical
		421
184	If C<$enable> is true, then the C<encode> method will output JSON objects	422	If C<$enable> is true (or missing), then the C<encode> method will output JSON objects
185	by sorting their keys. This is adding a comparatively high overhead.	423	by sorting their keys. This is adding a comparatively high overhead.
186		424
187	If C<$enable> is false, then the C<encode> method will output key-value	425	If C<$enable> is false, then the C<encode> method will output key-value
188	pairs in the order Perl stores them (which will likely change between runs	426	pairs in the order Perl stores them (which will likely change between runs
189	of the same script).	427	of the same script, and can change even within the same run from 5.18
		428	onwards).
190		429
191	This option is useful if you want the same data structure to be encoded as	430	This option is useful if you want the same data structure to be encoded as
192	the same JSON string (given the same overall settings). If it is disabled,	431	the same JSON text (given the same overall settings). If it is disabled,
193	the same hash migh be encoded differently even if contains the same data,	432	the same hash might be encoded differently even if contains the same data,
194	as key-value pairs have no inherent ordering in Perl.	433	as key-value pairs have no inherent ordering in Perl.
195		434
196	This setting has no effect when decoding JSON strings.	435	This setting has no effect when decoding JSON texts.
197		436
		437	This setting has currently no effect on tied hashes.
		438
		439	=item $json = $json->allow_nonref ([$enable])
		440
		441	=item $enabled = $json->get_allow_nonref
		442
		443	If C<$enable> is true (or missing), then the C<encode> method can convert a
		444	non-reference into its corresponding string, number or null JSON value,
		445	which is an extension to RFC4627. Likewise, C<decode> will accept those JSON
		446	values instead of croaking.
		447
		448	If C<$enable> is false, then the C<encode> method will croak if it isn't
		449	passed an arrayref or hashref, as JSON texts must either be an object
		450	or array. Likewise, C<decode> will croak if given something that is not a
		451	JSON object or array.
		452
		453	Example, encode a Perl scalar as JSON value with enabled C<allow_nonref>,
		454	resulting in an invalid JSON text:
		455
		456	JSON::XS->new->allow_nonref->encode ("Hello, World!")
		457	=> "Hello, World!"
		458
		459	=item $json = $json->allow_unknown ([$enable])
		460
		461	=item $enabled = $json->get_allow_unknown
		462
		463	If C<$enable> is true (or missing), then C<encode> will I<not> throw an
		464	exception when it encounters values it cannot represent in JSON (for
		465	example, filehandles) but instead will encode a JSON C<null> value. Note
		466	that blessed objects are not included here and are handled separately by
		467	c<allow_nonref>.
		468
		469	If C<$enable> is false (the default), then C<encode> will throw an
		470	exception when it encounters anything it cannot encode as JSON.
		471
		472	This option does not affect C<decode> in any way, and it is recommended to
		473	leave it off unless you know your communications partner.
		474
		475	=item $json = $json->allow_blessed ([$enable])
		476
		477	=item $enabled = $json->get_allow_blessed
		478
		479	If C<$enable> is true (or missing), then the C<encode> method will not
		480	barf when it encounters a blessed reference. Instead, the value of the
		481	B<convert_blessed> option will decide whether C<null> (C<convert_blessed>
		482	disabled or no C<TO_JSON> method found) or a representation of the
		483	object (C<convert_blessed> enabled and C<TO_JSON> method found) is being
		484	encoded. Has no effect on C<decode>.
		485
		486	If C<$enable> is false (the default), then C<encode> will throw an
		487	exception when it encounters a blessed object.
		488
		489	=item $json = $json->convert_blessed ([$enable])
		490
		491	=item $enabled = $json->get_convert_blessed
		492
		493	If C<$enable> is true (or missing), then C<encode>, upon encountering a
		494	blessed object, will check for the availability of the C<TO_JSON> method
		495	on the object's class. If found, it will be called in scalar context
		496	and the resulting scalar will be encoded instead of the object. If no
		497	C<TO_JSON> method is found, the value of C<allow_blessed> will decide what
		498	to do.
		499
		500	The C<TO_JSON> method may safely call die if it wants. If C<TO_JSON>
		501	returns other blessed objects, those will be handled in the same
		502	way. C<TO_JSON> must take care of not causing an endless recursion cycle
		503	(== crash) in this case. The name of C<TO_JSON> was chosen because other
		504	methods called by the Perl core (== not by the user of the object) are
		505	usually in upper case letters and to avoid collisions with any C<to_json>
		506	function or method.
		507
		508	This setting does not yet influence C<decode> in any way, but in the
		509	future, global hooks might get installed that influence C<decode> and are
		510	enabled by this setting.
		511
		512	If C<$enable> is false, then the C<allow_blessed> setting will decide what
		513	to do when a blessed object is found.
		514
		515	=item $json = $json->filter_json_object ([$coderef->($hashref)])
		516
		517	When C<$coderef> is specified, it will be called from C<decode> each
		518	time it decodes a JSON object. The only argument is a reference to the
		519	newly-created hash. If the code references returns a single scalar (which
		520	need not be a reference), this value (i.e. a copy of that scalar to avoid
		521	aliasing) is inserted into the deserialised data structure. If it returns
		522	an empty list (NOTE: I<not> C<undef>, which is a valid scalar), the
		523	original deserialised hash will be inserted. This setting can slow down
		524	decoding considerably.
		525
		526	When C<$coderef> is omitted or undefined, any existing callback will
		527	be removed and C<decode> will not change the deserialised hash in any
		528	way.
		529
		530	Example, convert all JSON objects into the integer 5:
		531
		532	my $js = JSON::XS->new->filter_json_object (sub { 5 });
		533	# returns [5]
		534	$js->decode ('[{}]')
		535	# throw an exception because allow_nonref is not enabled
		536	# so a lone 5 is not allowed.
		537	$js->decode ('{"a":1, "b":2}');
		538
		539	=item $json = $json->filter_json_single_key_object ($key [=> $coderef->($value)])
		540
		541	Works remotely similar to C<filter_json_object>, but is only called for
		542	JSON objects having a single key named C<$key>.
		543
		544	This C<$coderef> is called before the one specified via
		545	C<filter_json_object>, if any. It gets passed the single value in the JSON
		546	object. If it returns a single value, it will be inserted into the data
		547	structure. If it returns nothing (not even C<undef> but the empty list),
		548	the callback from C<filter_json_object> will be called next, as if no
		549	single-key callback were specified.
		550
		551	If C<$coderef> is omitted or undefined, the corresponding callback will be
		552	disabled. There can only ever be one callback for a given key.
		553
		554	As this callback gets called less often then the C<filter_json_object>
		555	one, decoding speed will not usually suffer as much. Therefore, single-key
		556	objects make excellent targets to serialise Perl objects into, especially
		557	as single-key JSON objects are as close to the type-tagged value concept
		558	as JSON gets (it's basically an ID/VALUE tuple). Of course, JSON does not
		559	support this in any way, so you need to make sure your data never looks
		560	like a serialised Perl hash.
		561
		562	Typical names for the single object key are C<__class_whatever__>, or
		563	C<$__dollars_are_rarely_used__$> or C<}ugly_brace_placement>, or even
		564	things like C<__class_md5sum(classname)__>, to reduce the risk of clashing
		565	with real hashes.
		566
		567	Example, decode JSON objects of the form C<< { "__widget__" => <id> } >>
		568	into the corresponding C<< $WIDGET{<id>} >> object:
		569
		570	# return whatever is in $WIDGET{5}:
		571	JSON::XS
		572	->new
		573	->filter_json_single_key_object (__widget__ => sub {
		574	$WIDGET{ $_[0] }
		575	})
		576	->decode ('{"__widget__": 5')
		577
		578	# this can be used with a TO_JSON method in some "widget" class
		579	# for serialisation to json:
		580	sub WidgetBase::TO_JSON {
		581	my ($self) = @_;
		582
		583	unless ($self->{id}) {
		584	$self->{id} = ..get..some..id..;
		585	$WIDGET{$self->{id}} = $self;
		586	}
		587
		588	{ __widget__ => $self->{id} }
		589	}
		590
		591	=item $json = $json->shrink ([$enable])
		592
		593	=item $enabled = $json->get_shrink
		594
		595	Perl usually over-allocates memory a bit when allocating space for
		596	strings. This flag optionally resizes strings generated by either
		597	C<encode> or C<decode> to their minimum size possible. This can save
		598	memory when your JSON texts are either very very long or you have many
		599	short strings. It will also try to downgrade any strings to octet-form
		600	if possible: perl stores strings internally either in an encoding called
		601	UTF-X or in octet-form. The latter cannot store everything but uses less
		602	space in general (and some buggy Perl or C code might even rely on that
		603	internal representation being used).
		604
		605	The actual definition of what shrink does might change in future versions,
		606	but it will always try to save space at the expense of time.
		607
		608	If C<$enable> is true (or missing), the string returned by C<encode> will
		609	be shrunk-to-fit, while all strings generated by C<decode> will also be
		610	shrunk-to-fit.
		611
		612	If C<$enable> is false, then the normal perl allocation algorithms are used.
		613	If you work with your data, then this is likely to be faster.
		614
		615	In the future, this setting might control other things, such as converting
		616	strings that look like integers or floats into integers or floats
		617	internally (there is no difference on the Perl level), saving space.
		618
		619	=item $json = $json->max_depth ([$maximum_nesting_depth])
		620
		621	=item $max_depth = $json->get_max_depth
		622
		623	Sets the maximum nesting level (default C<512>) accepted while encoding
		624	or decoding. If a higher nesting level is detected in JSON text or a Perl
		625	data structure, then the encoder and decoder will stop and croak at that
		626	point.
		627
		628	Nesting level is defined by number of hash- or arrayrefs that the encoder
		629	needs to traverse to reach a given point or the number of C<{> or C<[>
		630	characters without their matching closing parenthesis crossed to reach a
		631	given character in a string.
		632
		633	Setting the maximum depth to one disallows any nesting, so that ensures
		634	that the object is only a single hash/object or array.
		635
		636	If no argument is given, the highest possible setting will be used, which
		637	is rarely useful.
		638
		639	Note that nesting is implemented by recursion in C. The default value has
		640	been chosen to be as large as typical operating systems allow without
		641	crashing.
		642
		643	See SECURITY CONSIDERATIONS, below, for more info on why this is useful.
		644
		645	=item $json = $json->max_size ([$maximum_string_size])
		646
		647	=item $max_size = $json->get_max_size
		648
		649	Set the maximum length a JSON text may have (in bytes) where decoding is
		650	being attempted. The default is C<0>, meaning no limit. When C<decode>
		651	is called on a string that is longer then this many bytes, it will not
		652	attempt to decode the string but throw an exception. This setting has no
		653	effect on C<encode> (yet).
		654
		655	If no argument is given, the limit check will be deactivated (same as when
		656	C<0> is specified).
		657
		658	See SECURITY CONSIDERATIONS, below, for more info on why this is useful.
		659
198	=item $json_string = $json->encode ($perl_scalar)	660	=item $json_text = $json->encode ($perl_scalar)
199		661
200	Converts the given Perl data structure (a simple scalar or a reference	662	Converts the given Perl value or data structure to its JSON
201	to a hash or array) to its JSON representation. Simple scalars will be	663	representation. Croaks on error.
202	converted into JSON string or number sequences, while references to arrays
203	become JSON arrays and references to hashes become JSON objects. Undefined
204	Perl values (e.g. C<undef>) become JSON C<null> values. Neither C<true>
205	nor C<false> values will be generated.
206		664
207	=item $perl_scalar = $json->decode ($json_string)	665	=item $perl_scalar = $json->decode ($json_text)
208		666
209	The opposite of C<encode>: expects a JSON string and tries to parse it,	667	The opposite of C<encode>: expects a JSON text and tries to parse it,
210	returning the resulting simple scalar or reference. Croaks on error.	668	returning the resulting simple scalar or reference. Croaks on error.
211		669
212	JSON numbers and strings become simple Perl scalars. JSON arrays become	670	=item ($perl_scalar, $characters) = $json->decode_prefix ($json_text)
213	Perl arrayrefs and JSON objects become Perl hashrefs. C<true> becomes	671
214	C<1>, C<false> becomes C<0> and C<null> becomes C<undef>.	672	This works like the C<decode> method, but instead of raising an exception
		673	when there is trailing garbage after the first JSON object, it will
		674	silently stop parsing there and return the number of characters consumed
		675	so far.
		676
		677	This is useful if your JSON texts are not delimited by an outer protocol
		678	(which is not the brightest thing to do in the first place) and you need
		679	to know where the JSON text ends.
		680
		681	JSON::XS->new->decode_prefix ("[1] the tail")
		682	=> ([], 3)
215		683
216	=back	684	=back
217		685
		686
		687	=head1 INCREMENTAL PARSING
		688
		689	In some cases, there is the need for incremental parsing of JSON
		690	texts. While this module always has to keep both JSON text and resulting
		691	Perl data structure in memory at one time, it does allow you to parse a
		692	JSON stream incrementally. It does so by accumulating text until it has
		693	a full JSON object, which it then can decode. This process is similar to
		694	using C<decode_prefix> to see if a full JSON object is available, but
		695	is much more efficient (and can be implemented with a minimum of method
		696	calls).
		697
		698	JSON::XS will only attempt to parse the JSON text once it is sure it
		699	has enough text to get a decisive result, using a very simple but
		700	truly incremental parser. This means that it sometimes won't stop as
		701	early as the full parser, for example, it doesn't detect mismatched
		702	parentheses. The only thing it guarantees is that it starts decoding as
		703	soon as a syntactically valid JSON text has been seen. This means you need
		704	to set resource limits (e.g. C<max_size>) to ensure the parser will stop
		705	parsing in the presence if syntax errors.
		706
		707	The following methods implement this incremental parser.
		708
		709	=over 4
		710
		711	=item [void, scalar or list context] = $json->incr_parse ([$string])
		712
		713	This is the central parsing function. It can both append new text and
		714	extract objects from the stream accumulated so far (both of these
		715	functions are optional).
		716
		717	If C<$string> is given, then this string is appended to the already
		718	existing JSON fragment stored in the C<$json> object.
		719
		720	After that, if the function is called in void context, it will simply
		721	return without doing anything further. This can be used to add more text
		722	in as many chunks as you want.
		723
		724	If the method is called in scalar context, then it will try to extract
		725	exactly I<one> JSON object. If that is successful, it will return this
		726	object, otherwise it will return C<undef>. If there is a parse error,
		727	this method will croak just as C<decode> would do (one can then use
		728	C<incr_skip> to skip the erroneous part). This is the most common way of
		729	using the method.
		730
		731	And finally, in list context, it will try to extract as many objects
		732	from the stream as it can find and return them, or the empty list
		733	otherwise. For this to work, there must be no separators between the JSON
		734	objects or arrays, instead they must be concatenated back-to-back. If
		735	an error occurs, an exception will be raised as in the scalar context
		736	case. Note that in this case, any previously-parsed JSON texts will be
		737	lost.
		738
		739	Example: Parse some JSON arrays/objects in a given string and return
		740	them.
		741
		742	my @objs = JSON::XS->new->incr_parse ("[5][7][1,2]");
		743
		744	=item $lvalue_string = $json->incr_text
		745
		746	This method returns the currently stored JSON fragment as an lvalue, that
		747	is, you can manipulate it. This I<only> works when a preceding call to
		748	C<incr_parse> in I<scalar context> successfully returned an object. Under
		749	all other circumstances you must not call this function (I mean it.
		750	although in simple tests it might actually work, it I<will> fail under
		751	real world conditions). As a special exception, you can also call this
		752	method before having parsed anything.
		753
		754	This function is useful in two cases: a) finding the trailing text after a
		755	JSON object or b) parsing multiple JSON objects separated by non-JSON text
		756	(such as commas).
		757
		758	=item $json->incr_skip
		759
		760	This will reset the state of the incremental parser and will remove
		761	the parsed text from the input buffer so far. This is useful after
		762	C<incr_parse> died, in which case the input buffer and incremental parser
		763	state is left unchanged, to skip the text parsed so far and to reset the
		764	parse state.
		765
		766	The difference to C<incr_reset> is that only text until the parse error
		767	occurred is removed.
		768
		769	=item $json->incr_reset
		770
		771	This completely resets the incremental parser, that is, after this call,
		772	it will be as if the parser had never parsed anything.
		773
		774	This is useful if you want to repeatedly parse JSON objects and want to
		775	ignore any trailing data, which means you have to reset the parser after
		776	each successful decode.
		777
		778	=back
		779
		780	=head2 LIMITATIONS
		781
		782	All options that affect decoding are supported, except
		783	C<allow_nonref>. The reason for this is that it cannot be made to
		784	work sensibly: JSON objects and arrays are self-delimited, i.e. you can concatenate
		785	them back to back and still decode them perfectly. This does not hold true
		786	for JSON numbers, however.
		787
		788	For example, is the string C<1> a single JSON number, or is it simply the
		789	start of C<12>? Or is C<12> a single JSON number, or the concatenation
		790	of C<1> and C<2>? In neither case you can tell, and this is why JSON::XS
		791	takes the conservative route and disallows this case.
		792
		793	=head2 EXAMPLES
		794
		795	Some examples will make all this clearer. First, a simple example that
		796	works similarly to C<decode_prefix>: We want to decode the JSON object at
		797	the start of a string and identify the portion after the JSON object:
		798
		799	my $text = "[1,2,3] hello";
		800
		801	my $json = new JSON::XS;
		802
		803	my $obj = $json->incr_parse ($text)
		804	or die "expected JSON object or array at beginning of string";
		805
		806	my $tail = $json->incr_text;
		807	# $tail now contains " hello"
		808
		809	Easy, isn't it?
		810
		811	Now for a more complicated example: Imagine a hypothetical protocol where
		812	you read some requests from a TCP stream, and each request is a JSON
		813	array, without any separation between them (in fact, it is often useful to
		814	use newlines as "separators", as these get interpreted as whitespace at
		815	the start of the JSON text, which makes it possible to test said protocol
		816	with C<telnet>...).
		817
		818	Here is how you'd do it (it is trivial to write this in an event-based
		819	manner):
		820
		821	my $json = new JSON::XS;
		822
		823	# read some data from the socket
		824	while (sysread $socket, my $buf, 4096) {
		825
		826	# split and decode as many requests as possible
		827	for my $request ($json->incr_parse ($buf)) {
		828	# act on the $request
		829	}
		830	}
		831
		832	Another complicated example: Assume you have a string with JSON objects
		833	or arrays, all separated by (optional) comma characters (e.g. C<[1],[2],
		834	[3]>). To parse them, we have to skip the commas between the JSON texts,
		835	and here is where the lvalue-ness of C<incr_text> comes in useful:
		836
		837	my $text = "[1],[2], [3]";
		838	my $json = new JSON::XS;
		839
		840	# void context, so no parsing done
		841	$json->incr_parse ($text);
		842
		843	# now extract as many objects as possible. note the
		844	# use of scalar context so incr_text can be called.
		845	while (my $obj = $json->incr_parse) {
		846	# do something with $obj
		847
		848	# now skip the optional comma
		849	$json->incr_text =~ s/^ \s* , //x;
		850	}
		851
		852	Now lets go for a very complex example: Assume that you have a gigantic
		853	JSON array-of-objects, many gigabytes in size, and you want to parse it,
		854	but you cannot load it into memory fully (this has actually happened in
		855	the real world :).
		856
		857	Well, you lost, you have to implement your own JSON parser. But JSON::XS
		858	can still help you: You implement a (very simple) array parser and let
		859	JSON decode the array elements, which are all full JSON objects on their
		860	own (this wouldn't work if the array elements could be JSON numbers, for
		861	example):
		862
		863	my $json = new JSON::XS;
		864
		865	# open the monster
		866	open my $fh, "<bigfile.json"
		867	or die "bigfile: $!";
		868
		869	# first parse the initial "["
		870	for (;;) {
		871	sysread $fh, my $buf, 65536
		872	or die "read error: $!";
		873	$json->incr_parse ($buf); # void context, so no parsing
		874
		875	# Exit the loop once we found and removed(!) the initial "[".
		876	# In essence, we are (ab-)using the $json object as a simple scalar
		877	# we append data to.
		878	last if $json->incr_text =~ s/^ \s* \[ //x;
		879	}
		880
		881	# now we have the skipped the initial "[", so continue
		882	# parsing all the elements.
		883	for (;;) {
		884	# in this loop we read data until we got a single JSON object
		885	for (;;) {
		886	if (my $obj = $json->incr_parse) {
		887	# do something with $obj
		888	last;
		889	}
		890
		891	# add more data
		892	sysread $fh, my $buf, 65536
		893	or die "read error: $!";
		894	$json->incr_parse ($buf); # void context, so no parsing
		895	}
		896
		897	# in this loop we read data until we either found and parsed the
		898	# separating "," between elements, or the final "]"
		899	for (;;) {
		900	# first skip whitespace
		901	$json->incr_text =~ s/^\s*//;
		902
		903	# if we find "]", we are done
		904	if ($json->incr_text =~ s/^\]//) {
		905	print "finished.\n";
		906	exit;
		907	}
		908
		909	# if we find ",", we can continue with the next element
		910	if ($json->incr_text =~ s/^,//) {
		911	last;
		912	}
		913
		914	# if we find anything else, we have a parse error!
		915	if (length $json->incr_text) {
		916	die "parse error near ", $json->incr_text;
		917	}
		918
		919	# else add more data
		920	sysread $fh, my $buf, 65536
		921	or die "read error: $!";
		922	$json->incr_parse ($buf); # void context, so no parsing
		923	}
		924
		925	This is a complex example, but most of the complexity comes from the fact
		926	that we are trying to be correct (bear with me if I am wrong, I never ran
		927	the above example :).
		928
		929
		930
		931	=head1 MAPPING
		932
		933	This section describes how JSON::XS maps Perl values to JSON values and
		934	vice versa. These mappings are designed to "do the right thing" in most
		935	circumstances automatically, preserving round-tripping characteristics
		936	(what you put in comes out as something equivalent).
		937
		938	For the more enlightened: note that in the following descriptions,
		939	lowercase I<perl> refers to the Perl interpreter, while uppercase I<Perl>
		940	refers to the abstract Perl language itself.
		941
		942
		943	=head2 JSON -> PERL
		944
		945	=over 4
		946
		947	=item object
		948
		949	A JSON object becomes a reference to a hash in Perl. No ordering of object
		950	keys is preserved (JSON does not preserve object key ordering itself).
		951
		952	=item array
		953
		954	A JSON array becomes a reference to an array in Perl.
		955
		956	=item string
		957
		958	A JSON string becomes a string scalar in Perl - Unicode codepoints in JSON
		959	are represented by the same codepoints in the Perl string, so no manual
		960	decoding is necessary.
		961
		962	=item number
		963
		964	A JSON number becomes either an integer, numeric (floating point) or
		965	string scalar in perl, depending on its range and any fractional parts. On
		966	the Perl level, there is no difference between those as Perl handles all
		967	the conversion details, but an integer may take slightly less memory and
		968	might represent more values exactly than floating point numbers.
		969
		970	If the number consists of digits only, JSON::XS will try to represent
		971	it as an integer value. If that fails, it will try to represent it as
		972	a numeric (floating point) value if that is possible without loss of
		973	precision. Otherwise it will preserve the number as a string value (in
		974	which case you lose roundtripping ability, as the JSON number will be
		975	re-encoded to a JSON string).
		976
		977	Numbers containing a fractional or exponential part will always be
		978	represented as numeric (floating point) values, possibly at a loss of
		979	precision (in which case you might lose perfect roundtripping ability, but
		980	the JSON number will still be re-encoded as a JSON number).
		981
		982	Note that precision is not accuracy - binary floating point values cannot
		983	represent most decimal fractions exactly, and when converting from and to
		984	floating point, JSON::XS only guarantees precision up to but not including
		985	the least significant bit.
		986
		987	=item true, false
		988
		989	These JSON atoms become C<JSON::XS::true> and C<JSON::XS::false>,
		990	respectively. They are overloaded to act almost exactly like the numbers
		991	C<1> and C<0>. You can check whether a scalar is a JSON boolean by using
		992	the C<JSON::XS::is_bool> function.
		993
		994	=item null
		995
		996	A JSON null atom becomes C<undef> in Perl.
		997
		998	=back
		999
		1000
		1001	=head2 PERL -> JSON
		1002
		1003	The mapping from Perl to JSON is slightly more difficult, as Perl is a
		1004	truly typeless language, so we can only guess which JSON type is meant by
		1005	a Perl value.
		1006
		1007	=over 4
		1008
		1009	=item hash references
		1010
		1011	Perl hash references become JSON objects. As there is no inherent ordering
		1012	in hash keys (or JSON objects), they will usually be encoded in a
		1013	pseudo-random order that can change between runs of the same program but
		1014	stays generally the same within a single run of a program. JSON::XS can
		1015	optionally sort the hash keys (determined by the I<canonical> flag), so
		1016	the same datastructure will serialise to the same JSON text (given same
		1017	settings and version of JSON::XS), but this incurs a runtime overhead
		1018	and is only rarely useful, e.g. when you want to compare some JSON text
		1019	against another for equality.
		1020
		1021	=item array references
		1022
		1023	Perl array references become JSON arrays.
		1024
		1025	=item other references
		1026
		1027	Other unblessed references are generally not allowed and will cause an
		1028	exception to be thrown, except for references to the integers C<0> and
		1029	C<1>, which get turned into C<false> and C<true> atoms in JSON. You can
		1030	also use C<JSON::XS::false> and C<JSON::XS::true> to improve readability.
		1031
		1032	encode_json [\0, JSON::XS::true] # yields [false,true]
		1033
		1034	=item JSON::XS::true, JSON::XS::false
		1035
		1036	These special values become JSON true and JSON false values,
		1037	respectively. You can also use C<\1> and C<\0> directly if you want.
		1038
		1039	=item blessed objects
		1040
		1041	Blessed objects are not directly representable in JSON. See the
		1042	C<allow_blessed> and C<convert_blessed> methods on various options on
		1043	how to deal with this: basically, you can choose between throwing an
		1044	exception, encoding the reference as if it weren't blessed, or provide
		1045	your own serialiser method.
		1046
		1047	=item simple scalars
		1048
		1049	Simple Perl scalars (any scalar that is not a reference) are the most
		1050	difficult objects to encode: JSON::XS will encode undefined scalars as
		1051	JSON C<null> values, scalars that have last been used in a string context
		1052	before encoding as JSON strings, and anything else as number value:
		1053
		1054	# dump as number
		1055	encode_json [2] # yields [2]
		1056	encode_json [-3.0e17] # yields [-3e+17]
		1057	my $value = 5; encode_json [$value] # yields [5]
		1058
		1059	# used as string, so dump as string
		1060	print $value;
		1061	encode_json [$value] # yields ["5"]
		1062
		1063	# undef becomes null
		1064	encode_json [undef] # yields [null]
		1065
		1066	You can force the type to be a JSON string by stringifying it:
		1067
		1068	my $x = 3.1; # some variable containing a number
		1069	"$x"; # stringified
		1070	$x .= ""; # another, more awkward way to stringify
		1071	print $x; # perl does it for you, too, quite often
		1072
		1073	You can force the type to be a JSON number by numifying it:
		1074
		1075	my $x = "3"; # some variable containing a string
		1076	$x += 0; # numify it, ensuring it will be dumped as a number
		1077	$x *= 1; # same thing, the choice is yours.
		1078
		1079	You can not currently force the type in other, less obscure, ways. Tell me
		1080	if you need this capability (but don't forget to explain why it's needed
		1081	:).
		1082
		1083	Note that numerical precision has the same meaning as under Perl (so
		1084	binary to decimal conversion follows the same rules as in Perl, which
		1085	can differ to other languages). Also, your perl interpreter might expose
		1086	extensions to the floating point numbers of your platform, such as
		1087	infinities or NaN's - these cannot be represented in JSON, and it is an
		1088	error to pass those in.
		1089
		1090	=back
		1091
		1092
		1093	=head1 ENCODING/CODESET FLAG NOTES
		1094
		1095	The interested reader might have seen a number of flags that signify
		1096	encodings or codesets - C<utf8>, C<latin1> and C<ascii>. There seems to be
		1097	some confusion on what these do, so here is a short comparison:
		1098
		1099	C<utf8> controls whether the JSON text created by C<encode> (and expected
		1100	by C<decode>) is UTF-8 encoded or not, while C<latin1> and C<ascii> only
		1101	control whether C<encode> escapes character values outside their respective
		1102	codeset range. Neither of these flags conflict with each other, although
		1103	some combinations make less sense than others.
		1104
		1105	Care has been taken to make all flags symmetrical with respect to
		1106	C<encode> and C<decode>, that is, texts encoded with any combination of
		1107	these flag values will be correctly decoded when the same flags are used
		1108	- in general, if you use different flag settings while encoding vs. when
		1109	decoding you likely have a bug somewhere.
		1110
		1111	Below comes a verbose discussion of these flags. Note that a "codeset" is
		1112	simply an abstract set of character-codepoint pairs, while an encoding
		1113	takes those codepoint numbers and I<encodes> them, in our case into
		1114	octets. Unicode is (among other things) a codeset, UTF-8 is an encoding,
		1115	and ISO-8859-1 (= latin 1) and ASCII are both codesets I<and> encodings at
		1116	the same time, which can be confusing.
		1117
		1118	=over 4
		1119
		1120	=item C<utf8> flag disabled
		1121
		1122	When C<utf8> is disabled (the default), then C<encode>/C<decode> generate
		1123	and expect Unicode strings, that is, characters with high ordinal Unicode
		1124	values (> 255) will be encoded as such characters, and likewise such
		1125	characters are decoded as-is, no changes to them will be done, except
		1126	"(re-)interpreting" them as Unicode codepoints or Unicode characters,
		1127	respectively (to Perl, these are the same thing in strings unless you do
		1128	funny/weird/dumb stuff).
		1129
		1130	This is useful when you want to do the encoding yourself (e.g. when you
		1131	want to have UTF-16 encoded JSON texts) or when some other layer does
		1132	the encoding for you (for example, when printing to a terminal using a
		1133	filehandle that transparently encodes to UTF-8 you certainly do NOT want
		1134	to UTF-8 encode your data first and have Perl encode it another time).
		1135
		1136	=item C<utf8> flag enabled
		1137
		1138	If the C<utf8>-flag is enabled, C<encode>/C<decode> will encode all
		1139	characters using the corresponding UTF-8 multi-byte sequence, and will
		1140	expect your input strings to be encoded as UTF-8, that is, no "character"
		1141	of the input string must have any value > 255, as UTF-8 does not allow
		1142	that.
		1143
		1144	The C<utf8> flag therefore switches between two modes: disabled means you
		1145	will get a Unicode string in Perl, enabled means you get an UTF-8 encoded
		1146	octet/binary string in Perl.
		1147
		1148	=item C<latin1> or C<ascii> flags enabled
		1149
		1150	With C<latin1> (or C<ascii>) enabled, C<encode> will escape characters
		1151	with ordinal values > 255 (> 127 with C<ascii>) and encode the remaining
		1152	characters as specified by the C<utf8> flag.
		1153
		1154	If C<utf8> is disabled, then the result is also correctly encoded in those
		1155	character sets (as both are proper subsets of Unicode, meaning that a
		1156	Unicode string with all character values < 256 is the same thing as a
		1157	ISO-8859-1 string, and a Unicode string with all character values < 128 is
		1158	the same thing as an ASCII string in Perl).
		1159
		1160	If C<utf8> is enabled, you still get a correct UTF-8-encoded string,
		1161	regardless of these flags, just some more characters will be escaped using
		1162	C<\uXXXX> then before.
		1163
		1164	Note that ISO-8859-1-I<encoded> strings are not compatible with UTF-8
		1165	encoding, while ASCII-encoded strings are. That is because the ISO-8859-1
		1166	encoding is NOT a subset of UTF-8 (despite the ISO-8859-1 I<codeset> being
		1167	a subset of Unicode), while ASCII is.
		1168
		1169	Surprisingly, C<decode> will ignore these flags and so treat all input
		1170	values as governed by the C<utf8> flag. If it is disabled, this allows you
		1171	to decode ISO-8859-1- and ASCII-encoded strings, as both strict subsets of
		1172	Unicode. If it is enabled, you can correctly decode UTF-8 encoded strings.
		1173
		1174	So neither C<latin1> nor C<ascii> are incompatible with the C<utf8> flag -
		1175	they only govern when the JSON output engine escapes a character or not.
		1176
		1177	The main use for C<latin1> is to relatively efficiently store binary data
		1178	as JSON, at the expense of breaking compatibility with most JSON decoders.
		1179
		1180	The main use for C<ascii> is to force the output to not contain characters
		1181	with values > 127, which means you can interpret the resulting string
		1182	as UTF-8, ISO-8859-1, ASCII, KOI8-R or most about any character set and
		1183	8-bit-encoding, and still get the same data structure back. This is useful
		1184	when your channel for JSON transfer is not 8-bit clean or the encoding
		1185	might be mangled in between (e.g. in mail), and works because ASCII is a
		1186	proper subset of most 8-bit and multibyte encodings in use in the world.
		1187
		1188	=back
		1189
		1190
		1191	=head2 JSON and ECMAscript
		1192
		1193	JSON syntax is based on how literals are represented in javascript (the
		1194	not-standardised predecessor of ECMAscript) which is presumably why it is
		1195	called "JavaScript Object Notation".
		1196
		1197	However, JSON is not a subset (and also not a superset of course) of
		1198	ECMAscript (the standard) or javascript (whatever browsers actually
		1199	implement).
		1200
		1201	If you want to use javascript's C<eval> function to "parse" JSON, you
		1202	might run into parse errors for valid JSON texts, or the resulting data
		1203	structure might not be queryable:
		1204
		1205	One of the problems is that U+2028 and U+2029 are valid characters inside
		1206	JSON strings, but are not allowed in ECMAscript string literals, so the
		1207	following Perl fragment will not output something that can be guaranteed
		1208	to be parsable by javascript's C<eval>:
		1209
		1210	use JSON::XS;
		1211
		1212	print encode_json [chr 0x2028];
		1213
		1214	The right fix for this is to use a proper JSON parser in your javascript
		1215	programs, and not rely on C<eval> (see for example Douglas Crockford's
		1216	F<json2.js> parser).
		1217
		1218	If this is not an option, you can, as a stop-gap measure, simply encode to
		1219	ASCII-only JSON:
		1220
		1221	use JSON::XS;
		1222
		1223	print JSON::XS->new->ascii->encode ([chr 0x2028]);
		1224
		1225	Note that this will enlarge the resulting JSON text quite a bit if you
		1226	have many non-ASCII characters. You might be tempted to run some regexes
		1227	to only escape U+2028 and U+2029, e.g.:
		1228
		1229	# DO NOT USE THIS!
		1230	my $json = JSON::XS->new->utf8->encode ([chr 0x2028]);
		1231	$json =~ s/\xe2\x80\xa8/\\u2028/g; # escape U+2028
		1232	$json =~ s/\xe2\x80\xa9/\\u2029/g; # escape U+2029
		1233	print $json;
		1234
		1235	Note that I<this is a bad idea>: the above only works for U+2028 and
		1236	U+2029 and thus only for fully ECMAscript-compliant parsers. Many existing
		1237	javascript implementations, however, have issues with other characters as
		1238	well - using C<eval> naively simply I<will> cause problems.
		1239
		1240	Another problem is that some javascript implementations reserve
		1241	some property names for their own purposes (which probably makes
		1242	them non-ECMAscript-compliant). For example, Iceweasel reserves the
		1243	C<__proto__> property name for its own purposes.
		1244
		1245	If that is a problem, you could parse try to filter the resulting JSON
		1246	output for these property strings, e.g.:
		1247
		1248	$json =~ s/"__proto__"\s*:/"__proto__renamed":/g;
		1249
		1250	This works because C<__proto__> is not valid outside of strings, so every
		1251	occurrence of C<"__proto__"\s*:> must be a string used as property name.
		1252
		1253	If you know of other incompatibilities, please let me know.
		1254
		1255
		1256	=head2 JSON and YAML
		1257
		1258	You often hear that JSON is a subset of YAML. This is, however, a mass
		1259	hysteria(*) and very far from the truth (as of the time of this writing),
		1260	so let me state it clearly: I<in general, there is no way to configure
		1261	JSON::XS to output a data structure as valid YAML> that works in all
		1262	cases.
		1263
		1264	If you really must use JSON::XS to generate YAML, you should use this
		1265	algorithm (subject to change in future versions):
		1266
		1267	my $to_yaml = JSON::XS->new->utf8->space_after (1);
		1268	my $yaml = $to_yaml->encode ($ref) . "\n";
		1269
		1270	This will I<usually> generate JSON texts that also parse as valid
		1271	YAML. Please note that YAML has hardcoded limits on (simple) object key
		1272	lengths that JSON doesn't have and also has different and incompatible
		1273	unicode character escape syntax, so you should make sure that your hash
		1274	keys are noticeably shorter than the 1024 "stream characters" YAML allows
		1275	and that you do not have characters with codepoint values outside the
		1276	Unicode BMP (basic multilingual page). YAML also does not allow C<\/>
		1277	sequences in strings (which JSON::XS does not I<currently> generate, but
		1278	other JSON generators might).
		1279
		1280	There might be other incompatibilities that I am not aware of (or the YAML
		1281	specification has been changed yet again - it does so quite often). In
		1282	general you should not try to generate YAML with a JSON generator or vice
		1283	versa, or try to parse JSON with a YAML parser or vice versa: chances are
		1284	high that you will run into severe interoperability problems when you
		1285	least expect it.
		1286
		1287	=over 4
		1288
		1289	=item (*)
		1290
		1291	I have been pressured multiple times by Brian Ingerson (one of the
		1292	authors of the YAML specification) to remove this paragraph, despite him
		1293	acknowledging that the actual incompatibilities exist. As I was personally
		1294	bitten by this "JSON is YAML" lie, I refused and said I will continue to
		1295	educate people about these issues, so others do not run into the same
		1296	problem again and again. After this, Brian called me a (quote)I<complete
		1297	and worthless idiot>(unquote).
		1298
		1299	In my opinion, instead of pressuring and insulting people who actually
		1300	clarify issues with YAML and the wrong statements of some of its
		1301	proponents, I would kindly suggest reading the JSON spec (which is not
		1302	that difficult or long) and finally make YAML compatible to it, and
		1303	educating users about the changes, instead of spreading lies about the
		1304	real compatibility for many I<years> and trying to silence people who
		1305	point out that it isn't true.
		1306
		1307	Addendum/2009: the YAML 1.2 spec is still incompatible with JSON, even
		1308	though the incompatibilities have been documented (and are known to Brian)
		1309	for many years and the spec makes explicit claims that YAML is a superset
		1310	of JSON. It would be so easy to fix, but apparently, bullying people and
		1311	corrupting userdata is so much easier.
		1312
		1313	=back
		1314
		1315
		1316	=head2 SPEED
		1317
		1318	It seems that JSON::XS is surprisingly fast, as shown in the following
		1319	tables. They have been generated with the help of the C<eg/bench> program
		1320	in the JSON::XS distribution, to make it easy to compare on your own
		1321	system.
		1322
		1323	First comes a comparison between various modules using
		1324	a very short single-line JSON string (also available at
		1325	L<http://dist.schmorp.de/misc/json/short.json>).
		1326
		1327	{"method": "handleMessage", "params": ["user1",
		1328	"we were just talking"], "id": null, "array":[1,11,234,-5,1e5,1e7,
		1329	1, 0]}
		1330
		1331	It shows the number of encodes/decodes per second (JSON::XS uses
		1332	the functional interface, while JSON::XS/2 uses the OO interface
		1333	with pretty-printing and hashkey sorting enabled, JSON::XS/3 enables
		1334	shrink. JSON::DWIW/DS uses the deserialise function, while JSON::DWIW::FJ
		1335	uses the from_json method). Higher is better:
		1336
		1337	module | encode | decode |
		1338	--------------|------------|------------|
		1339	JSON::DWIW/DS | 86302.551 | 102300.098 |
		1340	JSON::DWIW/FJ | 86302.551 | 75983.768 |
		1341	JSON::PP | 15827.562 | 6638.658 |
		1342	JSON::Syck | 63358.066 | 47662.545 |
		1343	JSON::XS | 511500.488 | 511500.488 |
		1344	JSON::XS/2 | 291271.111 | 388361.481 |
		1345	JSON::XS/3 | 361577.931 | 361577.931 |
		1346	Storable | 66788.280 | 265462.278 |
		1347	--------------+------------+------------+
		1348
		1349	That is, JSON::XS is almost six times faster than JSON::DWIW on encoding,
		1350	about five times faster on decoding, and over thirty to seventy times
		1351	faster than JSON's pure perl implementation. It also compares favourably
		1352	to Storable for small amounts of data.
		1353
		1354	Using a longer test string (roughly 18KB, generated from Yahoo! Locals
		1355	search API (L<http://dist.schmorp.de/misc/json/long.json>).
		1356
		1357	module | encode | decode |
		1358	--------------|------------|------------|
		1359	JSON::DWIW/DS | 1647.927 | 2673.916 |
		1360	JSON::DWIW/FJ | 1630.249 | 2596.128 |
		1361	JSON::PP | 400.640 | 62.311 |
		1362	JSON::Syck | 1481.040 | 1524.869 |
		1363	JSON::XS | 20661.596 | 9541.183 |
		1364	JSON::XS/2 | 10683.403 | 9416.938 |
		1365	JSON::XS/3 | 20661.596 | 9400.054 |
		1366	Storable | 19765.806 | 10000.725 |
		1367	--------------+------------+------------+
		1368
		1369	Again, JSON::XS leads by far (except for Storable which non-surprisingly
		1370	decodes a bit faster).
		1371
		1372	On large strings containing lots of high Unicode characters, some modules
		1373	(such as JSON::PC) seem to decode faster than JSON::XS, but the result
		1374	will be broken due to missing (or wrong) Unicode handling. Others refuse
		1375	to decode or encode properly, so it was impossible to prepare a fair
		1376	comparison table for that case.
		1377
		1378
		1379	=head1 SECURITY CONSIDERATIONS
		1380
		1381	When you are using JSON in a protocol, talking to untrusted potentially
		1382	hostile creatures requires relatively few measures.
		1383
		1384	First of all, your JSON decoder should be secure, that is, should not have
		1385	any buffer overflows. Obviously, this module should ensure that and I am
		1386	trying hard on making that true, but you never know.
		1387
		1388	Second, you need to avoid resource-starving attacks. That means you should
		1389	limit the size of JSON texts you accept, or make sure then when your
		1390	resources run out, that's just fine (e.g. by using a separate process that
		1391	can crash safely). The size of a JSON text in octets or characters is
		1392	usually a good indication of the size of the resources required to decode
		1393	it into a Perl structure. While JSON::XS can check the size of the JSON
		1394	text, it might be too late when you already have it in memory, so you
		1395	might want to check the size before you accept the string.
		1396
		1397	Third, JSON::XS recurses using the C stack when decoding objects and
		1398	arrays. The C stack is a limited resource: for instance, on my amd64
		1399	machine with 8MB of stack size I can decode around 180k nested arrays but
		1400	only 14k nested JSON objects (due to perl itself recursing deeply on croak
		1401	to free the temporary). If that is exceeded, the program crashes. To be
		1402	conservative, the default nesting limit is set to 512. If your process
		1403	has a smaller stack, you should adjust this setting accordingly with the
		1404	C<max_depth> method.
		1405
		1406	Something else could bomb you, too, that I forgot to think of. In that
		1407	case, you get to keep the pieces. I am always open for hints, though...
		1408
		1409	Also keep in mind that JSON::XS might leak contents of your Perl data
		1410	structures in its error messages, so when you serialise sensitive
		1411	information you might want to make sure that exceptions thrown by JSON::XS
		1412	will not end up in front of untrusted eyes.
		1413
		1414	If you are using JSON::XS to return packets to consumption
		1415	by JavaScript scripts in a browser you should have a look at
		1416	L<http://blog.archive.jpsykes.com/47/practical-csrf-and-json-security/> to
		1417	see whether you are vulnerable to some common attack vectors (which really
		1418	are browser design bugs, but it is still you who will have to deal with
		1419	it, as major browser developers care only for features, not about getting
		1420	security right).
		1421
		1422
		1423	=head1 THREADS
		1424
		1425	This module is I<not> guaranteed to be thread safe and there are no
		1426	plans to change this until Perl gets thread support (as opposed to the
		1427	horribly slow so-called "threads" which are simply slow and bloated
		1428	process simulations - use fork, it's I<much> faster, cheaper, better).
		1429
		1430	(It might actually work, but you have been warned).
		1431
		1432
		1433	=head1 THE PERILS OF SETLOCALE
		1434
		1435	Sometimes people avoid the Perl locale support and directly call the
		1436	system's setlocale function with C<LC_ALL>.
		1437
		1438	This breaks both perl and modules such as JSON::XS, as stringification of
		1439	numbers no longer works correctly (e.g. C<$x = 0.1; print "$x"+1> might
		1440	print C<1>, and JSON::XS might output illegal JSON as JSON::XS relies on
		1441	perl to stringify numbers).
		1442
		1443	The solution is simple: don't call C<setlocale>, or use it for only those
		1444	categories you need, such as C<LC_MESSAGES> or C<LC_CTYPE>.
		1445
		1446	If you need C<LC_NUMERIC>, you should enable it only around the code that
		1447	actually needs it (avoiding stringification of numbers), and restore it
		1448	afterwards.
		1449
		1450
		1451	=head1 BUGS
		1452
		1453	While the goal of this module is to be correct, that unfortunately does
		1454	not mean it's bug-free, only that I think its design is bug-free. If you
		1455	keep reporting bugs they will be fixed swiftly, though.
		1456
		1457	Please refrain from using rt.cpan.org or any other bug reporting
		1458	service. I put the contact address into my modules for a reason.
		1459
218	=cut	1460	=cut
219		1461
		1462	our $true = do { bless \(my $dummy = 1), "JSON::XS::Boolean" };
		1463	our $false = do { bless \(my $dummy = 0), "JSON::XS::Boolean" };
		1464
		1465	sub true() { $true }
		1466	sub false() { $false }
		1467
		1468	sub is_bool($) {
		1469	UNIVERSAL::isa $_[0], "JSON::XS::Boolean"
		1470	# or UNIVERSAL::isa $_[0], "JSON::Literal"
		1471	}
		1472
		1473	XSLoader::load "JSON::XS", $VERSION;
		1474
		1475	package JSON::XS::Boolean;
		1476
		1477	use overload
		1478	"0+" => sub { ${$_[0]} },
		1479	"++" => sub { $_[0] = ${$_[0]} + 1 },
		1480	"--" => sub { $_[0] = ${$_[0]} - 1 },
		1481	fallback => 1;
		1482
220	1;	1483	1;
		1484
		1485	=head1 SEE ALSO
		1486
		1487	The F<json_xs> command line utility for quick experiments.
221		1488
222	=head1 AUTHOR	1489	=head1 AUTHOR
223		1490
224	Marc Lehmann <schmorp@schmorp.de>	1491	Marc Lehmann <schmorp@schmorp.de>
225	http://home.schmorp.de/	1492	http://home.schmorp.de/

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