amorfo77/json

0 Fork 0

mirror of https://github.com/nlohmann/json.git synced 2026-10-12 17:37:15 +00:00

Files

T

Niels Lohmann 5bc36a6947 Add json_document and json_view: node index, parser, and document

Add json_document and json_view, a read-only, zero-copy index of a
JSON text, as the first public slice of the zero-copy view (#5295).

A parse produces a flat array of 16-byte nodes in document order,
one per value and one per object key. Strings stay in the source
text; escaped strings are decoded into an arena. Integers are
converted while their digits are in the cache; floats keep only
their digit layout and are converted on read. Containers store the
size of their subtree, so a reader can step over one in constant
time. A document makes a handful of allocations, however many
values it has.

The parser accepts exactly what json::parse accepts, with every
combination of ignore_comments and ignore_trailing_commas, with and
without a trailing NUL, and under JSON_STRICT_NUL_HANDLING. It is
portable C++11 and does not depend on byte order.

basic_json_document adds parse, parse_copy, accept, read (reuses a
document's memory), root, is_discarded, source, owns_source,
node_count, memory_usage, and shrink_to_fit. basic_json_view adds
type, the is_* queries, operator bool, size, empty, materialize,
and source_offset. A parse error throws the same exception
basic_json::parse would throw for the same input, message and
position included.

detail::abi_config keeps JSON_STRICT_NUL_HANDLING and
JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON readable after json.hpp
undefines them, in the ABI namespace so they always match the
basic_json in use.

A NUL byte that ends a // comment is the end of the input, as in
parse() since #5696.

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

2026-10-11 11:30:11 +02:00

2.3 KiB

Raw Blame History

nlohmann::basic_json_document::read

template<typename InputType>
void read(InputType&& input,
         const bool allow_exceptions = true,
         const bool ignore_comments = false,
         const bool ignore_trailing_commas = false);

(Re-)parses input into #!cpp *this, discarding the document's previous value and reusing its memory (the node index, the decoded-string buffer, and, if applicable, the owned copy of the text) rather than allocating a fresh document. parse() is implemented in terms of this function, applied to a default-constructed document.

Template parameters

InputType: A compatible input; see parse.

Parameters

input (in): Input to parse from.
allow_exceptions (in): whether to throw exceptions in case of a parse error (optional, #!cpp true by default)
ignore_comments (in): whether comments should be ignored and treated like whitespace (#!cpp true) or yield a parse error (#!cpp false); (optional, #!cpp false by default)
ignore_trailing_commas (in): whether trailing commas in arrays or objects should be ignored and treated like whitespace (#!cpp true) or yield a parse error (#!cpp false); (optional, #!cpp false by default)

Exceptions

Same as parse.

Complexity

Linear in the length of the input.

Notes

Every view taken from #!cpp *this before the call -- including the previous root() -- is invalidated, whether or not the new parse succeeds; take fresh views from root() afterward.

input is borrowed or owned by the same rules as parse(); a document can borrow on one call and own on the next, since ownership is decided freshly each time.

Examples

??? example

The example below parses a sequence of messages into the same document, reusing its memory instead of allocating
a new document for each one.

```cpp
--8<-- "examples/basic_json_document__read.cpp"
```

Output:

```json
--8<-- "examples/basic_json_document__read.output"
```

Version history

Added in version 3.13.0.

2.3 KiB Raw Blame History