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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
1.1 KiB
C++
34 lines
1.1 KiB
C++
#include <iostream>
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#include <string>
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#include <utility>
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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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// BORROWED: doc only points into `text`; `text` must outlive `doc`
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std::string text = R"({"a": 1})";
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json_document doc = json_document::parse(text);
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std::cout << doc.owns_source() << '\n'; // false
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// a view is valid as long as the document is alive, has not been
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// re-parsed (read()) or shrunk (shrink_to_fit()), and -- if borrowed --
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// the source text is alive
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nlohmann::json_view v = doc.root();
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std::cout << v.is_object() << '\n';
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// re-parsing the SAME document invalidates views taken before the call;
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// `v` above must not be used after this line
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doc.read(R"([1, 2, 3])");
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v = doc.root(); // take a fresh view instead
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std::cout << v.is_array() << '\n';
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// moving the document does not invalidate views: the node index is
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// heap-allocated and does not move with the document object
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json_document moved = std::move(doc);
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std::cout << v.is_array() << '\n';
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}
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