Changes promised in the replies to the review of #5620:
- read() reuses the owned copy: a copied input (an rvalue byte container,
a stream, FILE*, wide characters, an iterator range) is appended to the
cleared owned buffer instead of a fresh std::string being move-assigned
over it. A moved-in std::string still replaces it.
- The error replay runs the library parser with a SAX handler that stores
nothing and throws the reported exception, instead of
BasicJsonType::parse(), which built a DOM up to the error.
- Pointers to and arrays of wide characters, and FILE*, take the input
adapter path (converted to UTF-8 into an owned buffer) instead of
failing a static_assert. Wide pointers are NUL-terminated.
- Streams are read in blocks through rdbuf() into the buffer, instead of
character by character through the input adapter; eofbit is set at the
end, as by the library's stream adapter.
- tape/tape_size/tape_cap are now nodes/node_count/node_capacity.
- The builder's members get descriptive, m_-prefixed names.
- The shallow frame stack is a std::array (in a wrapper with a
user-provided constructor: -Weffc++ wants the member initialized, and
value-initializing the 64 frames costs up to 30 % on small documents).
- The value dispatch macro has no NEXT parameter; the switch is left with
break, and the macro is undefined after its last use.
- literal() and digits_exceed() deduce the length from the string literal;
each literal has one helper; the two `if (!is_float)` blocks are merged.
- arena_ensure() sits right before the writes; the decode arena is a
std::vector<char>.
- The builder records the deepest nesting, and materialize() sizes its
stack from it once (growing it only if a subtree is deeper).
- Comment fixes (hex4, slow_string, the NUL rule, put_utf8, 0 - m, the
arena resize, grow()) and formatting (macro backslashes, digit cases,
one power of ten per line, value_t assertion next to is_container()).
- build_with's template parameters follow the builder's order.
Greg's idea of keeping the source offset in `off` and the arena offset in
`next` for escaped strings was measured and not adopted: string access got
7-18 % slower with a select, and 3.5-18 % slower with a branch-free field
pick (Apple M-series, clang -O3), parsing was unchanged.
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
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>
Niels Lohmann