api/basic_json/sax_parse.md

# <small>nlohmann::basic_json::</small>sax_parse

```cpp
// (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`](../json_sax/index.md).

## 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`](../json_sax/index.md)

## Parameters

`i` (in)
:   Input to parse from

`sax` (in)
:   SAX event listener (must not be null)

`format` (in)
:    the format to parse (JSON, CBOR, MessagePack, or UBJSON) (optional, `input_format_t::json` by default), see
     [`input_format_t`](input_format_t.md) for more information

`strict` (in)
:   whether the input has to be consumed completely (optional, `#!cpp true` by default)

`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

return value of the last processed SAX event

## Exception safety

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

## Exceptions

- Throws [`parse_error.101`](../../home/exceptions.md#jsonexceptionparse_error101) 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](parse.md) - deserialize from a compatible input
- [accept](accept.md) - 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.
- Extended overload (2) to accept heterogeneous iterator+sentinel pairs (C++20 ranges support) in version 3.13.0.

!!! 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);`.