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5bc36a6947
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
1.4 KiB
C++
34 lines
1.4 KiB
C++
#include <iostream>
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#include <string>
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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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std::cout << std::boolalpha;
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// source_offset() points into source(): useful to report *where* in the
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// original text a value came from (error messages, syntax highlighting,
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// forwarding a sub-range verbatim, ...) without materializing it
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json_document doc = json_document::parse(R"( 123)");
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auto v = doc.root();
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std::cout << v.source_offset() << ' '
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<< std::string(doc.source().data() + v.source_offset(), 3) << '\n';
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// a string without escapes also stays in the source text
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json_document plain = json_document::parse(R"("ab")");
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std::cout << plain.source()[plain.root().source_offset()] << '\n';
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// a string with escapes is decoded once into the document's own buffer, so
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// there is no single byte range in source() to point at: source_offset()
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// returns the "not applicable" sentinel
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json_document escaped = json_document::parse(R"("a\nb")");
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std::cout << (escaped.root().source_offset() == static_cast<std::size_t>(-1)) << '\n';
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// a discarded view -- default-constructed, or the root of a failed parse
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// with allow_exceptions == false -- has no offset either
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json_document failed = json_document::parse("not json", false);
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std::cout << (failed.root().source_offset() == static_cast<std::size_t>(-1)) << '\n';
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}
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