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https://github.com/nlohmann/json.git
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Niels Lohmann
89f52818da
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>
56 lines
1.6 KiB
C++
56 lines
1.6 KiB
C++
#include <iostream>
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#include <nlohmann/json_view.hpp>
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#include <string>
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#include <vector>
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using json_document = nlohmann::json_document;
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using json_view = nlohmann::json_view;
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// address has no direct conversion in get<T>(), so get<address>() falls back
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// to materialize().get<address>() -- a real nlohmann::json value is built
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// for just this one member, and its own from_json() runs on that
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struct address
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{
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std::string city;
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int zip = 0;
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};
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void from_json(const nlohmann::json& j, address& a)
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{
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j.at("city").get_to(a.city);
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j.at("zip").get_to(a.zip);
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}
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int main()
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{
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json_document doc = json_document::parse(R"(
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{
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"name": "Alice",
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"active": true,
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"orders": [1, 2, 3],
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"address": {"city": "Berlin", "zip": 10115}
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}
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)");
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const json_view customer = doc.root();
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// read typed fields straight into C++ variables -- none of these build
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// a nlohmann::json value
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const std::string name = customer["name"].get<std::string>();
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const bool active = customer["active"].get<bool>();
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std::cout << name << (active ? " (active)" : " (inactive)") << '\n';
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// std::vector<json_view> keeps views of the array elements instead of
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// copies of their values
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bool first = true;
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for (const json_view order : customer["orders"].get<std::vector<json_view>>())
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{
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std::cout << (first ? "" : " ") << order.get<int>();
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first = false;
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}
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std::cout << '\n';
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// everything else goes through materialize()
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const address a = customer["address"].get<address>();
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std::cout << a.city << ' ' << a.zip << '\n';
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}
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