json/doc/mkdocs/docs/features/parsing/parser_callbacks.md

Parser Callbacks

Overview

With a parser callback function, the result of parsing a JSON text can be influenced. When passed to parse, it is called on certain events (passed as parse_event_t via parameter event) with a set recursion depth depth and context JSON value parsed. The return value of the callback function is a boolean indicating whether the element that emitted the callback shall be kept or not.

The type of the callback function is:

template<typename BasicJsonType>
using parser_callback_t =
    std::function<bool(int depth, parse_event_t event, BasicJsonType& parsed)>;

Callback event types

We distinguish six scenarios (determined by the event type) in which the callback function can be called. The following table describes the values of the parameters depth, event, and parsed.

parameter event	description	parameter depth	parameter parsed
parse_event_t::object_start	the parser read { and started to process a JSON object	depth of the parent of the JSON object	a JSON value with type discarded
parse_event_t::key	the parser read a key of a value in an object	depth of the currently parsed JSON object	a JSON string containing the key
parse_event_t::object_end	the parser read } and finished processing a JSON object	depth of the parent of the JSON object	the parsed JSON object
parse_event_t::array_start	the parser read [ and started to process a JSON array	depth of the parent of the JSON array	a JSON value with type discarded
parse_event_t::array_end	the parser read ] and finished processing a JSON array	depth of the parent of the JSON array	the parsed JSON array
parse_event_t::value	the parser finished reading a JSON value	depth of the value	the parsed JSON value

??? example

When parsing the following JSON text,

```json
{
    "name": "Berlin",
    "location": [
        52.519444,
        13.406667
    ]
}
```

these calls are made to the callback function:

| event          | depth | parsed |
| -------------- | ----- | ------ |
| `object_start` | 0     | *discarded* |
| `key`          | 1     | `#!json "name"` |
| `value`        | 1     | `#!json "Berlin"` |
| `key`          | 1     | `#!json "location"` |
| `array_start`  | 1     | *discarded* |
| `value`        | 2     | `#!json 52.519444` |
| `value`        | 2     | `#!json 13.406667` |
| `array_end`    | 1     | `#!json [52.519444,13.406667]` |
| `object_end`   | 0     | `#!json {"location":[52.519444,13.406667],"name":"Berlin"}` |

Return value

Discarding a value (i.e., returning #!c false) has different effects depending on the context in which function was called:

• Discarded values in structured types are skipped. That is, the parser will behave as if the discarded value was never read.
• In case a value outside a structured type is skipped, it is replaced with #!json null. This case happens if the top-level element is skipped.

??? example

The example below demonstrates the `parse()` function with and without callback function.

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

Output:

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