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80 lines
3.3 KiB
Markdown
80 lines
3.3 KiB
Markdown
# Parser Callbacks
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## Overview
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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
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(passed as `parse_event_t` via parameter `event`) with a set recursion depth `depth` and context JSON value `parsed`. The return value of the
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callback function is a boolean indicating whether the element that emitted the callback shall be kept or not.
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The type of the callback function is:
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```cpp
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template<typename BasicJsonType>
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using parser_callback_t =
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std::function<bool(int depth, parse_event_t event, BasicJsonType& parsed)>;
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```
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## Callback event types
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We distinguish six scenarios (determined by the event type) in which the callback function can be called. The following table describes the values
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of the parameters `depth`, `event`, and `parsed`.
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parameter `event` | description | parameter `depth` | parameter `parsed`
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------------------ | ----------- | ------------------ | -------------------
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`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
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`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
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`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
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`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
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`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
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`parse_event_t::value` | the parser finished reading a JSON value | depth of the value | the parsed JSON value
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??? example
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When parsing the following JSON text,
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```json
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{
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"name": "Berlin",
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"location": [
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52.519444,
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13.406667
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]
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}
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```
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these calls are made to the callback function:
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| event | depth | parsed |
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| -------------- | ----- | ------ |
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| `object_start` | 0 | *discarded* |
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| `key` | 1 | `#!json "name"` |
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| `value` | 1 | `#!json "Berlin"` |
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| `key` | 1 | `#!json "location"` |
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| `array_start` | 1 | *discarded* |
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| `value` | 2 | `#!json 52.519444` |
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| `value` | 2 | `#!json 13.406667` |
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| `array_end` | 1 | `#!json [52.519444,13.406667]` |
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| `object_end` | 0 | `#!json {"location":[52.519444,13.406667],"name":"Berlin"}` |
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## Return value
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Discarding a value (i.e., returning `#!c false`) has different effects depending on the context in which function was called:
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- Discarded values in structured types are skipped. That is, the parser will behave as if the discarded value was never read.
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- 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.
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??? example
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The example below demonstrates the `parse()` function with and without callback function.
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```cpp
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--8<-- "examples/parse__string__parser_callback_t.cpp"
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```
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Output:
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```json
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--8<-- "examples/parse__string__parser_callback_t.output"
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```
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