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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

1.1 KiB

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nlohmann::basic_json_document::node_count

std::size_t node_count() const noexcept;

Returns the number of entries in the document's flat index.

Return value

The number of index entries: one per value (of any type, at any nesting depth) plus one per object key. 0 for a discarded document.

Exception safety

No-throw guarantee: this function never throws exceptions.

Complexity

Constant.

Notes

Each index entry is 16 bytes, so #!cpp node_count() * 16 is the size of the index itself (part, but not all, of memory_usage(), which also counts decoded strings and, for an owned document, the text).

Examples

??? example

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

Output:

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

Version history

Added in version 3.13.0.

1.1 KiB Raw Blame History