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Niels Lohmann 759dd2e1b1 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>

2026-10-07 16:42:22 +02:00

2.9 KiB

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nlohmann::basic_json::is_discarded

constexpr bool is_discarded() const noexcept;

This function returns #!cpp true for a JSON value if either:

the value was discarded during parsing with a callback function (see parser_callback_t), or
the value is the result of parsing invalid JSON with parameter allow_exceptions set to #!cpp false; see parse for more information.

Return value

#!cpp true if type is discarded, #!cpp false otherwise.

Exception safety

No-throw guarantee: this member function never throws exceptions.

Complexity

Constant.

Notes

!!! note "Comparisons"

Discarded values are never compared equal with [`operator==`](operator_eq.md). That is, checking whether a JSON
value `j` is discarded will only work via:

```cpp
j.is_discarded()
```

because

```cpp
j == json::value_t::discarded
```

will always be `#!cpp false`.

!!! note "Removal during parsing with callback functions"

When a value is discarded by a callback function (see [`parser_callback_t`](parser_callback_t.md)) during parsing,
then it is removed when it is part of a structured value. For instance, if the second value of an array is discarded,
instead of `#!json [null, discarded, false]`, the array `#!json [null, false]` is returned. If the top-level value
itself is discarded by the callback, the `parse` call returns a `#!json null` value.

After a successful parse, this function always returns #!cpp false: discarded values can only occur during parsing and are either removed when inside a structured value or replaced by #!json null at the top level. The exception is parsing with allow_exceptions set to #!cpp false: a parse error then yields a discarded value for which this function returns #!cpp true (see parse).

Examples

??? example "Example: is_discarded() for ordinary JSON values"

The following code exemplifies `is_discarded()` for all JSON types.

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

Output:

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

??? example "Example: discarded values from parsing"

The following code shows the two situations in which a discarded value can be observed: parsing invalid JSON with
`allow_exceptions` set to `#!cpp false`, and a parser callback that discards the top-level value (which is replaced
by `#!json null` and therefore does *not* remain discarded).

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

Output:

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

Version history

Added in version 1.0.0.

2.9 KiB Raw Blame History