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Niels Lohmann
77acd4563c
Give basic_json_view the read-only access functions of basic_json: operator[] and at() with keys and indices, front()/back(), find(), contains(), count(), begin()/end() and cbegin()/cend(), items() with structured bindings from C++17 on, and type_name(). Exceptions have the ids and messages of the const functions of basic_json. Where basic_json has undefined behavior the view answers safely: operator[] with a missing key or an out-of-range index returns a discarded view, and front()/back() of an empty container throw invalid_iterator.214. Objects are iterated in document order, and all members are visited; duplicate-key lookups find the first member (as yyjson and simdjson do), while parse(), materialize(), and the map conversions keep the last value, as parse() does. Keys of up to 16 bytes are compared with two overlapping loads. Add value conversions: get<T>()/get_to() for arithmetic types, bool, nullptr_t, strings (std::basic_string copied, string_view_t without a copy), BasicJsonType, views, std::vector, and maps with string keys; get_string() for the string without a copy; number_token() for the number exactly as written in the source; value() with keys and JSON pointers; and operator[]/at()/ contains() with JSON pointers. Everything else, including types with from_json(), goes through materialize() of that subtree. get<T>() of arithmetic types is inlined down to the conversion, so reading an integer needs no call. detail::json_pointer_access exposes a pointer's reference tokens to code outside basic_json. Signed-off-by: Niels Lohmann <mail@nlohmann.me>
23 lines
932 B
C++
23 lines
932 B
C++
#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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// a settings object whose source text records every update to a key as
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// a duplicate member. items() visits all of them, in document order, so
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// the update history is visible; operator[] only ever sees the first
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// one, and materialize() -- like basic_json::parse() -- keeps the last
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json_document updates = json_document::parse(R"({"retries": 1, "timeout": 30, "retries": 5})");
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const auto settings = updates.root();
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for (const auto& item : settings.items())
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{
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std::cout << item.key() << '=' << item.value().materialize().dump() << '\n';
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}
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std::cout << "first \"retries\" seen by operator[]: " << settings["retries"].materialize().dump() << '\n';
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std::cout << "last \"retries\" kept by materialize(): " << settings.materialize()["retries"].dump() << '\n';
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}
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