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~basic_json
| template<template< typename U, typename V, typename...Args > class ObjectType = std::map, template< typename U, typename...Args > class ArrayType = std::vector, class StringType = std::string, class BooleanType = bool, class NumberIntegerType = int64_t, class NumberFloatType = double, template< typename U > class AllocatorType = std::allocator>
using nlohmann::basic_json::parser_callback_t = std::function<bool( int depth, parse_event_t event, basic_json& parsed)>
With a parser callback function, the result of parsing a JSON text can be influenced. When passed to parse(std::istream&, parser_callback_t) or parse(const string_t&, parser_callback_t), 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. 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
  Discarding a value (i.e., returning `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 `null`. This case happens if the top-level element is skipped. Parameters
  [in] | depth | the depth of the recursion during parsing
  [in] | event | an event of type parse_event_t indicating the context in the callback function has been called
  [in,out] | parsed | the current intermediate parse result; note that writing to this value has no effect for parse_event_t::key events
Returns Whether the JSON value which called the function during parsing should be kept (`true`) or not (`false`). In the latter case, it is either skipped completely or replaced by an empty discarded object. See also parse(std::istream&, parser_callback_t) or parse(const string_t&, parser_callback_t) for examples