mirror of https://github.com/nlohmann/json.git synced 2026-10-02 20:50:32 +00:00

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Niels Lohmann 061c30310e Document json_document and json_view

- API pages for basic_json_document and basic_json_view, one per member,
  and for the four aliases, each with an example
- features/json_view.md: the problem the view solves, ownership and
  lifetime, what matches basic_json::parse() and what differs, and when
  to choose json, ordered_json, SAX, or the view
- the examples show why one would use the view, not only how: borrowed
  vs. owned input, reading a few fields and materializing one subtree,
  reusing a document across many messages
- registered in the mkdocs navigation, llms.txt, the docset, the
  exceptions page (out_of_range.416), architecture.md, the integration
  page, and the README; the yyjson credit is added to the README and
  license.md

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

2026-09-30 20:58:44 +02:00

2.8 KiB

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nlohmann::basic_json_document

Defined in header <nlohmann/json_view.hpp>

template<typename BasicJsonType>
class basic_json_document;

A parsed JSON text, held as a flat index of its values (16 bytes per value) instead of a tree of BasicJsonType values. Strings and numbers stay in the source text; only strings that contain escapes are decoded, into one buffer owned by the document. basic_json_view is a read-only handle to one value of a basic_json_document; materialize() turns a subtree back into the BasicJsonType value that BasicJsonType::parse() would have produced for it.

A document may borrow the text it was parsed from (the caller's buffer must then outlive the document) or own it (a copy, or an rvalue #!cpp std::string that was moved in); see owns_source. basic_json_document is move-only: copying a document would either duplicate a potentially large index and text, or leave two documents claiming to borrow the same buffer, so it is disabled.

Template parameters

BasicJsonType: a specialization of basic_json, for instance json or ordered_json. Only 64-bit number_integer_t/number_unsigned_t types are supported; this is checked with a static_assert.

Specializations

json_document - documents of the default specialization json
ordered_json_document - documents of ordered_json

Member types

view_type - the type of view returned by root() (#!cpp basic_json_view<BasicJsonType>)
value_t - the JSON type enumeration, see basic_json::value_t

Member functions

(constructor)
parse (static) - deserialize from a compatible input, borrowing or owning it as appropriate
parse_copy (static) - deserialize a copy of a compatible input
accept (static) - check whether the input is valid JSON
read - (re-)parse into this document, reusing its memory
root - the view of the root value
is_discarded - return whether the last parse failed
source - the parsed text
owns_source - return whether the document holds its own copy of the text
node_count - the number of index entries (values plus object keys)
memory_usage - the number of bytes held by the document
shrink_to_fit - release unused index capacity

Version history

Added in version 3.13.0.

2.8 KiB Raw Blame History