json/docs/mkdocs/docs/api/basic_json/sax_parse.md

nlohmann::basic_json::sax_parse

// (1)
template <typename InputType, typename SAX>
static bool sax_parse(InputType&& i,
                      SAX* sax,
                      input_format_t format = input_format_t::json,
                      const bool strict = true,
                      const bool ignore_comments = false,
                      const bool ignore_trailing_commas = false);

// (2)
template<class IteratorType, class SAX, class SentinelType = IteratorType>
static bool sax_parse(IteratorType first, SentinelType last,
                      SAX* sax,
                      input_format_t format = input_format_t::json,
                      const bool strict = true,
                      const bool ignore_comments = false,
                      const bool ignore_trailing_commas = false);

Read from input and generate SAX events

1. Read from a compatible input.

2. Read from a pair of character iterators, or an iterator and a sentinel of a different type (C++20 ranges support)

   The value_type of the iterator must be an integral type with a size of 1, 2, or 4 bytes, which will be interpreted respectively as UTF-8, UTF-16, and UTF-32. If SentinelType differs from IteratorType, it must be comparable to the iterator type with operator!=.

The SAX event lister must follow the interface of json_sax.

Template parameters

InputType

A compatible input, for instance:

• an std::istream object
• a FILE pointer
• a C-style array of characters
• a pointer to a null-terminated string of single byte characters
• a container obj for which begin(obj) and end(obj) produce a valid pair of iterators (as found via ADL or member functions, with semantics compatible to std::begin and std::end)

IteratorType

a compatible iterator type for overload (2); a pair of character iterators whose value_type is an integral type with a size of 1, 2, or 4 bytes (interpreted respectively as UTF-8, UTF-16, and UTF-32)

SentinelType

defaults to IteratorType; may be a different type comparable to IteratorType via operator!=, for overload (2), for instance.

• a custom sentinel type for C++20 ranges
• std::default_sentinel_t, when IteratorType is std::counted_iterator

SAX

a class fulfilling the SAX event listener interface; see json_sax

Parameters

i (in)

Input to parse from

sax (in)

SAX event listener (must not be null)

format (in)

the format to parse (JSON, BJData, BON8, BSON, CBOR, MessagePack, or UBJSON) (optional, input_format_t::json by default), see input_format_t for more information

strict (in)

whether the input has to be consumed completely (optional, #!cpp true by default); when #!cpp false and the input is a #!cpp std::istream, the character that terminates a number is consumed unless JSON_PRECISE_STREAM_POSITION is defined to 1; see operator>>

ignore_comments (in)

whether comments should be ignored and treated like whitespace (#!cpp true) or yield a parse error (#!cpp false); (optional, #!cpp false by default)

ignore_trailing_commas (in)

whether trailing commas in arrays or objects should be ignored and treated like whitespace (#!cpp true) or yield a parse error (#!cpp false); (optional, #!cpp false by default)

first (in)

iterator to the start of a character range

last (in)

iterator to the end of a character range, or a sentinel value that compares equal to the end iterator with operator!=

Return value

#!cpp true if the input was parsed without errors and no SAX event returned #!cpp false; #!cpp false otherwise. In particular, the result is #!cpp false for input with errors, even if the SAX parser recovered from all of them (see error recovery).

Exception safety

Strong guarantee: if an exception is thrown, there are no changes in the JSON value.

Exceptions

• Throws parse_error.101 in case of an unexpected token, or empty input like a null FILE* or char* pointer.

Complexity

Linear in the length of the input. The parser is a predictive LL(1) parser. The complexity can be higher if the SAX consumer sax has a super-linear complexity.

Notes

A UTF-8 byte order mark is silently ignored.

Examples

??? example

The example below demonstrates the `sax_parse()` function reading from string and processing the events with a
user-defined SAX event consumer.

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

Output:

```json
--8<-- "examples/sax_parse.output"
```

See also

• parse - deserialize from a compatible input
• accept - check if the input is valid JSON

Version history

• Added in version 3.2.0.
• Ignoring comments via ignore_comments added in version 3.9.0.
• Added ignore_trailing_commas in version 3.13.0.
• Extended container support (1) to include types with lvalue-only ADL begin/end (matching std::begin/std::end semantics) in version 3.13.0.
• Recovering from parse errors (see parse_error) added in version 3.13.0.
• Extended overload (2) to accept heterogeneous iterator+sentinel pairs (C++20 ranges support) in version 3.13.0.
• JSON_PRECISE_STREAM_POSITION added in version 3.13.0 to optionally leave a #!cpp std::istream positioned right after the parsed value when strict is #!cpp false.

!!! warning "Deprecation"

Overload (2) replaces calls to `sax_parse` with a pair of iterators as their first parameter which has been
deprecated in version 3.8.0. This overload will be removed in version 4.0.0. Please replace all calls like
`#!cpp sax_parse({ptr, ptr+len});` with `#!cpp sax_parse(ptr, ptr+len);`.