Exit Zen
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Streaming API, parse input JSON incrementally
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Improved performance when parsing multiple JSON texts
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Ignore non-JSON content after the end of a JSON text
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The parser implementation uses temporary storage space to accumulate values during parsing. When using the <<ref_parse>> free functions, this storage is allocated and freed in each call. However, by declaring an instance of <<ref_parser>> or <<ref_stream_parser>>, this temporary storage can be reused when parsing more than one JSON text, reducing the total number of dynamic memory allocations.
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To use the <<ref_parser>>, declare an instance. Then call <<ref_parser_write>> once with the buffer containing representing the input JSON. Finally, call <<ref_parser_release>> to take ownership of the resulting <<ref_value>> upon success. This example persists the parser instance in a class member to reuse across calls:
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Sometimes a protocol may have a JSON text followed by data that is in a different format or specification. The JSON portion can still be parsed by using the function <<ref_parser_write_some>>. Upon success, the return value will indicate the number of characters consumed from the input, which will exclude the non-JSON characters:
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The parser instance may be constructed with parse options which allow some non-standard JSON extensions to be recognized:
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Streaming Parser
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The <<ref_stream_parser>> implements a https://en.wikipedia.org/wiki/Online_algorithm[__streaming algorithm__]; it allows incremental processing of large JSON inputs using one or more contiguous character buffers. The entire input JSON does not need to be loaded into memory at once. A network server can use the streaming interface to process incoming JSON in fixed-size amounts, providing these benefits:
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CPU consumption per I/O cycle is bounded
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Memory consumption per I/O cycle is bounded
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Jitter, unfairness, and latency is reduced
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Less total memory is required to process the full input
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To use the <<ref_stream_parser>>, declare an instance. Then call <<ref_stream_parser_write>> zero or more times with successive buffers representing the input JSON. When there are no more buffers, call <<ref_stream_parser_finish>>. The function <<ref_stream_parser_done>> returns `true` after a successful call to `write` or `finish` if parsing is complete.
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In the following example a JSON text is parsed from standard input a line at a time. Error codes are used instead. The function <<ref_stream_parser_finish>> is used to indicate the end of the input:
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This example will break, if comments are enabled, because of
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`std::getline` use (see the warning in <<non_standard_json>> section).
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We can complicate the example further by extracting _several_ JSON values from the sequence of lines.
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Controlling Memory
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After default construction, or after <<ref_stream_parser_reset>> is called with no arguments, the <<ref_value>> produced after a successful parse operation uses the default memory resource. To use a different memory resource, call `reset` with the resource to use. Here we use a <<ref_monotonic_resource>>, which is optimized for parsing but not subsequent modification:
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