json/docs/mkdocs/docs/features/parsing/error_recovery.md

Error Recovery

By default, parsing stops at the first error. With the SAX interface, you can instead ask the parser to recover: to repair the error and continue, so that you get as much as possible out of malformed input, for instance a file that was cut off, JSON edited by hand, or the output of a language model.

Recovering from errors

The SAX parser's parse_error function is called for every error. Its return value decides what happens next:

• #!cpp false stops parsing. This is what the SAX parsers of the library do, so parse and accept never recover.
• #!cpp true repairs the error and continues parsing.

When recovering, the SAX parser still receives well-formed events: every start_object or start_array is followed by the matching end_object or end_array, and every key is followed by exactly one value. A SAX parser that creates a JSON value, such as the one in the example below, therefore gets a complete value. Parsing always ends, and sax_parse returns #!cpp false for input that is not valid JSON, even if every error was repaired. Each token is reported at most once, and the SAX parser can stop at any error by returning #!cpp false.

!!! example

The example below derives a SAX parser from the library's parser for `json` values (`json_sax_dom_parser`),
and recovers from all errors.

```cpp
--8<-- "examples/sax_parse__error_recovery.cpp"
```

Output:

```
--8<-- "examples/sax_parse__error_recovery.output"
```

How errors are repaired

Each error is repaired with the smallest local edit: a missing separator is inserted, a stray token is removed, what can be read of a broken string or number is kept, and a value that cannot be read at all becomes #!json null.

Mistake	Repair	Example	Result
missing , or :	inserted	#!json [1 2], #!json {"a" 1}	[1,2], {"a":1}
missing value	#!json null for an object key or between commas in an array	#!json {"a":}, #!json [1,,2]	{"a":null}, [1,null,2]
trailing comma	removed	#!json [1,2,]	[1,2]
broken string	invalid escapes and bytes are replaced (see below); a line break ends the string	#!json ["a\qb"]	["aqb"]
broken number	the longest valid beginning is kept	#!json [1., 2e+]	[1,2]
unreadable value	#!json null	#!json [1, NaN, tru]	[1,null,null]
number too large	passed as infinity, together with its text	#!json [1e999]	infinity (see below)
stray :	removed	#!json ["a":1]	["a",1]
member without a key	skipped up to the next , or }	#!json {1:2, "b":3}	{"b":3}
wrong closing bracket	closes the innermost array or object	#!json {"a":[1,2}, "b":3}	{"a":[1,2],"b":3}
input ends too early	all open arrays and objects are closed	#!json {"a":[1,2	{"a":[1,2]}
text before the value	skipped	#!json )]}'{"a":1}	{"a":1}

In a string, an unknown escape like \q stands for the escaped character (q), as in JavaScript. An invalid \u escape, a lone surrogate, and ill-formed UTF-8 are each replaced by U+FFFD (REPLACEMENT CHARACTER), and control characters are kept. A string without its closing quote ends at the next line break or at the end of the input.

The input after the top-level value is not repaired: as without recovery, it is reported as an error, and parsing stops.

Binary formats

The binary formats (BJData, BON8, BSON, CBOR, MessagePack, and UBJSON) have no delimiters to find the next value by. So what can be repaired depends on whether the end of the item with the error is known, a distinction that RFC 8949, Section 5.3 makes for CBOR, too.

If the item is complete, but cannot be passed on as it is, it is replaced, and parsing continues after it:

Mistake	Formats	Repair
tag, if tag_handler is cbor_tag_handler_t::error (the default)	CBOR	ignored
simple value other than false, true, and null, like undefined	CBOR	#!json null
number too large for a custom number_float_t, like float	all	infinity
character (C) that is not ASCII	BJData, UBJSON	U+FFFD
invalid high-precision number (H)	BJData, UBJSON	the longest valid beginning is kept, as for JSON text, or #!json null
high-precision number too large	BJData, UBJSON	passed as infinity, together with its text
object key that is not a string	BON8, CBOR, MessagePack	the member is skipped
element of a type the library does not read, like ObjectId or date	BSON	#!json null
string without its terminator	BSON	kept
document whose size does not match its content	BSON	kept

CBOR tags and simple values are repaired as RFC 8949, Section 6.1 suggests for converting CBOR to JSON. By default, sax_parse reports every tag as an error; when recovering, tags are then ignored like with cbor_tag_handler_t::ignore. Strings that are not valid UTF-8 are no error: like from_cbor and the other functions by default, sax_parse passes them on as they are.

After any other error, the end of the item is unknown: the input ended, a byte is not a valid type marker, or a size cannot be right. Parsing then stops, and the value read so far is completed: a key that waits for its value gets #!json null, and all open arrays and objects are closed. This keeps everything before the error of an input that was cut off. The exception is BSON, which stores the size of every document: an element whose end is unknown gets #!json null, the rest of its document is skipped, and parsing continues after the document.

Limitations

• A repair is a guess. For example, #!json {"a" "b": 1} could be meant as #!json {"a": "b"} or as #!json {"a": null, "b": 1}; it is repaired to the former. Treat recovered values as a best effort, and check the reported errors.
• A closing bracket always closes the innermost array or object. If a bracket is missing rather than wrong, the repair differs from the intention: #!json {"a": {"b": [1, 2}, "c": 3} is repaired to #!json {"a": {"b": [1, 2], "c": 3}}, although #!json {"a": {"b": [1, 2]}, "c": 3} may have been meant.
• Keys without quotes, and strings in single quotes, are not supported; such members are skipped.
• In the binary formats, a member that is skipped because its key is not a string is lost, and so are the elements of a BSON document after one whose end is unknown.
• A number that is too large for number_float_t is passed as positive or negative infinity. The SAX parser's number_float also gets the number's text, but a JSON value cannot store it, and dump serializes infinity as #!json null.
• When parsing is not strict (see sax_parse), a repair may read parts of the input after the value, for instance of the next value in a stream of concatenated values.

See also

• SAX interface - implement a custom SAX handler
• parse_error - the SAX event for parse errors
• sax_parse - generate SAX events
• parsing and exceptions - control error handling