diff --git a/doc/mkdocs/docs/features/parser_callbacks.md b/doc/mkdocs/docs/features/parser_callbacks.md new file mode 100644 index 000000000..dbf88849f --- /dev/null +++ b/doc/mkdocs/docs/features/parser_callbacks.md @@ -0,0 +1,79 @@ +# 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: + +```cpp +template +using parser_callback_t = + std::function; +``` + + +## 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" + ``` diff --git a/doc/mkdocs/mkdocs.yml b/doc/mkdocs/mkdocs.yml index 866a41a5d..a53dca2f3 100644 --- a/doc/mkdocs/mkdocs.yml +++ b/doc/mkdocs/mkdocs.yml @@ -49,6 +49,7 @@ nav: - features/json_patch.md - features/merge_patch.md - features/enum_conversion.md + - features/parser_callbacks.md - features/sax_interface.md - features/types.md - Integration: