Niels Lohmann
4303ba674d
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>
2.6 KiB
nlohmann::basic_json_view::materialize
BasicJsonType materialize() const;
Builds the BasicJsonType value of this subtree: the value BasicJsonType::parse() would
have produced for the same source text, allocated for the first time by this call.
Return value
The BasicJsonType value of this subtree, or a discarded BasicJsonType value (#!cpp BasicJsonType(value_t::discarded))
if the view is discarded.
Exception safety
Strong guarantee: if an exception is thrown, there are no changes to the view or the document it refers to (nothing about either is mutated by this function).
Exceptions
May throw #!cpp std::bad_alloc (via BasicJsonType's allocator) if constructing the result fails.
Complexity
Linear in the size of the subtree.
Notes
materialize() replays the subtree through the same SAX builder BasicJsonType::parse()
uses internally, so the result matches it exactly -- including, for an object, that a repeated key keeps only its
last value. The replay is iterative, so it is not limited by the call stack the way a naive recursive conversion
would be; the JSON nesting depth is limited only by available memory, as for BasicJsonType::parse() itself.
Unlike parsing with JSON_DIAGNOSTIC_POSITIONS enabled, the values
produced by materialize() do not carry source positions: there is no lexer run during the replay to record them.
Calling materialize() on the same view repeatedly builds a new, independent BasicJsonType value each time; it
never caches the result.
Examples
??? example
The example below skips messages that are not useful -- a discarded value, or an empty array -- using only
[`is_array()`](is_array.md) and [`empty()`](empty.md), and calls `materialize()` only for the messages that are
actually used, so no `BasicJsonType` value (and none of its per-element allocations) is ever built for the
skipped ones.
```cpp
--8<-- "examples/basic_json_view__materialize.cpp"
```
Output:
```json
--8<-- "examples/basic_json_view__materialize.output"
```
See also
- root - the view of a document's root value
BasicJsonType::parse- build aBasicJsonTypevalue directly from a JSON textJSON_DIAGNOSTIC_POSITIONS- source positions on parsed values (not produced bymaterialize())
Version history
- Added in version 3.13.0.