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759dd2e1b1
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>
32 lines
832 B
C++
32 lines
832 B
C++
#include <iostream>
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#include <string>
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#include <vector>
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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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std::vector<std::string> messages =
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{
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R"({"id": 1})", R"({"id": 2, "tag": "x"})", R"({"id": 3})"
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};
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// parse into the same document over and over: its node index and decode
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// buffer are reused instead of being freed and reallocated for each message
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json_document doc;
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std::size_t total = 0;
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for (const auto& msg : messages)
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{
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doc.read(msg);
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total += doc.root().size();
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}
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std::cout << total << '\n';
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// read() can also change what kind of input is owned/borrowed between calls
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doc.read(std::string(R"({"owned": true})"));
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std::cout << doc.owns_source() << '\n';
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}
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