parser_callback_t

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 = std::int64_t, class NumberUnsignedType = std::uint64_t, class NumberFloatType = double, template< typename U > class AllocatorType = std::allocator, template< typename T, typename SFINAE=void > class JSONSerializer = adl_serializer, class BinaryType = std::vector<std::uint8_t>>

using nlohmann::basic_json< ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType >::parser_callback_t = detail::parser_callback_t<basic_json>

per-element parser callback type

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.

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 certain parse events are triggered

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

see parse for examples

Since

version 1.0.0

Definition at line 19123 of file json.hpp.