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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>
34 lines
1.2 KiB
C++
34 lines
1.2 KiB
C++
#include <array>
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#include <iostream>
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#include <nlohmann/json_view.hpp>
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using json_document = nlohmann::json_document;
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int main()
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{
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// three incoming messages; skip the ones that are not useful without ever
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// building a nlohmann::json value for them
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json_document heartbeat = json_document::parse("null");
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json_document empty_batch = json_document::parse("[]");
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json_document batch = json_document::parse(R"([{"id": 1}, {"id": 2}, {"id": 3}])");
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std::array<const json_document*, 3> messages = {{&heartbeat, &empty_batch, &batch}};
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for (const json_document* d : messages)
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{
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// is_array()/empty() only look at the flat index: a discarded
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// heartbeat or an empty batch is never turned into a nlohmann::json
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// value, so no per-element allocation happens for them
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if (!d->root().is_array() || d->root().empty())
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{
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std::cout << "skipped\n";
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continue;
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}
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// materialize() replays the subtree through the same SAX builder
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// basic_json::parse() uses, so the result is exactly what
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// basic_json::parse() would have produced for the same text
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nlohmann::json value = d->root().materialize();
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std::cout << value.dump() << '\n';
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}
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}
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