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https://github.com/nlohmann/json.git
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Niels Lohmann
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>
34 lines
1.0 KiB
C++
34 lines
1.0 KiB
C++
#include <iostream>
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#include <string>
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#include <nlohmann/json.hpp>
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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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// an lvalue std::string is BORROWED: the document only stores a pointer
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// into `text`, so `text` must outlive `borrowed`
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std::string text = R"({"count": 3})";
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json_document borrowed = json_document::parse(text);
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std::cout << borrowed.owns_source() << '\n'; // false
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// an rvalue std::string is MOVED into the document -- no copy of the text
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json_document owned = json_document::parse(std::string(R"({"count": 3})"));
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std::cout << owned.owns_source() << '\n'; // true
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// errors are identical to basic_json::parse: same exception id, message,
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// and position, because the library parser runs on the same bytes on a
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// failing input
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try
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{
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static_cast<void>(json_document::parse(R"({"count": )"));
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}
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catch (const nlohmann::json::parse_error& e)
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{
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std::cout << e.id << '\n';
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}
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}
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