Niels Lohmann
a59d1f64e9
Add API pages for basic_json_document::save() and load(), including the image_check enumeration and its three levels. Add examples that show caching a parsed document as an image, loading it without parsing, the difference between a borrowed and an owned image, and a full check rejecting a damaged image that a bounds check still reads safely. Add an "Images" section to the json_view feature page, register the new pages in mkdocs.yml and docSet.sql, group basic_json_document's member list by parsing/access/images/edits, and document parse_error.116 and type_error.320 on the exceptions page, extending out_of_range.416 for images. Signed-off-by: Niels Lohmann <mail@nlohmann.me>
6.8 KiB
nlohmann::basic_json_document
Defined in header <nlohmann/json_view.hpp>
template<typename BasicJsonType, bool Editable = false>
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.
With #!cpp Editable == true, the document also offers set, push_back,
insert, and erase to change values in place, see Edits below. The source text
itself is never written; a read-only document (#!cpp Editable == false, the default) does not carry any of the
bookkeeping edits need, and calling any of them on one fails to compile (#!cpp static_assert).
Template parameters
BasicJsonType- a specialization of
basic_json, for instancejsonorordered_json. Only 64-bitnumber_integer_t/number_unsigned_ttypes are supported; this is checked with astatic_assert. Editable- whether the document supports
set,push_back,insert, anderase(optional,#!cpp falseby default). See Edits below.
Specializations
- json_document - read-only documents of the default specialization
json - ordered_json_document - read-only documents of
ordered_json - json_editable_document - editable documents of
json - ordered_json_editable_document - editable documents of
ordered_json
Member types
- view_type - the type of view returned by
root()(#!cpp basic_json_view<BasicJsonType, Editable>) - value_t - the JSON type enumeration, see
basic_json::value_t
Member functions
Parsing
- 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
Access
- 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
Images
- save - the document as an image that
load()reads without parsing - load (static) - read an image written by
save()
Edits
- set - replace a value, or set an object member, an array element, or the value a JSON pointer refers
to (
#!cpp Editabledocuments only) - push_back - append to an array (
#!cpp Editabledocuments only) - insert - insert an element into an array before a given position (
#!cpp Editabledocuments only) - erase - remove an object member, an array element, or the value a JSON pointer refers to
(
#!cpp Editabledocuments only)
Edits
An editable document (#!cpp Editable == true) can be changed after parsing, with set,
push_back, insert, and erase;
json_editable_document and
ordered_json_editable_document are the corresponding specializations. A few
points apply to every edit:
- The source text is never written, and the parsed index never moves: every value keeps the node it was parsed
into, so views taken before an edit stay valid, including
root(). New values (and the element sequences of an edited array/object) go to storage owned by the document, allocated on demand. - A view keeps referring to the same value. After
setreplaces the value a view refers to, that view sees the new value; a view of a value that a later edit replaces or drops keeps showing what it last held. An edit of an array or object, however, invalidates the iterators taken over it (its members may now live in a different sequence), and a string obtained withget_string()stays valid even as further edits happen (earlier buffers of edited text are kept alive, not overwritten). - Values are accepted three ways: a
basic_json_viewof any document (read-only or editable; it is copied, nothing is shared with the source document), aBasicJsonTypevalue, or anythingBasicJsonTypecan be constructed from (numbers, strings,#!cpp bool,#!cpp nullptr, containers, ...). dump()writes an edited document with members in document order, new members at the end, and, withnumber_format::source, keeps the spelling of every number that was not itself edited -- see Editing a document for why this matters.read()discards all edits,shrink_to_fit()does not move the node index once there are edits, andmemory_usage()includes the memory edits use.source_offset()of a value introduced by an edit is#!cpp static_cast<std::size_t>(-1), the same value it reports for a decoded string.
Version history
- Added in version 3.13.0.