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Niels Lohmann 4303ba674d 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>

2026-10-06 10:48:06 +02:00

1.8 KiB

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nlohmann::basic_json_document::accept

template<typename InputType>
static bool accept(InputType&& input,
                   const bool ignore_comments = false,
                   const bool ignore_trailing_commas = false);

Checks whether the input is valid JSON, accepting and rejecting exactly what BasicJsonType::accept() does, with the same options. Unlike parse(), this function never throws an exception for invalid input, and the returned #!cpp bool is the only result -- no document is returned.

Template parameters

InputType: A compatible input; see parse.

Parameters

input (in): Input to check.
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)

Return value

Whether the input is valid JSON.

Exception safety

Strong guarantee: this function itself never throws for an invalid input; it can only throw what allocating the input's own copy (for inputs that are always read into a buffer) throws.

Complexity

Linear in the length of the input.

Examples

??? example

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

Output:

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

Version history

Added in version 3.13.0.

1.8 KiB Raw Blame History