02bd821f2e
This allows templates such as `NLOHMANN_DEFINE_TYPE_*` templates and other generators with things like `std::vector<std::optional<T>>`. Co-authored-by: John Ericson <John.Ericson@Obsidian.Systems>
110 lines
2.9 KiB
C++
110 lines
2.9 KiB
C++
#pragma once
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///@file
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#include <nlohmann/json.hpp>
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#include <list>
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namespace nix {
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const nlohmann::json * get(const nlohmann::json & map, const std::string & key);
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nlohmann::json * get(nlohmann::json & map, const std::string & key);
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/**
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* Get the value of a json object at a key safely, failing
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* with a Nix Error if the key does not exist.
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*
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* Use instead of nlohmann::json::at() to avoid ugly exceptions.
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*
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* _Does not check whether `map` is an object_, use `ensureType` for that.
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*/
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const nlohmann::json & valueAt(
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const nlohmann::json & map,
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const std::string & key);
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/**
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* Ensure the type of a json object is what you expect, failing
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* with a Nix Error if it isn't.
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*
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* Use before type conversions and element access to avoid ugly exceptions.
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*/
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const nlohmann::json & ensureType(
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const nlohmann::json & value,
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nlohmann::json::value_type expectedType);
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/**
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* For `adl_serializer<std::optional<T>>` below, we need to track what
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* types are not already using `null`. Only for them can we use `null`
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* to represent `std::nullopt`.
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*/
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template<typename T>
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struct json_avoids_null;
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/**
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* Handle numbers in default impl
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*/
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template<typename T>
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struct json_avoids_null : std::bool_constant<std::is_integral<T>::value> {};
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template<>
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struct json_avoids_null<std::nullptr_t> : std::false_type {};
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template<>
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struct json_avoids_null<bool> : std::true_type {};
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template<>
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struct json_avoids_null<std::string> : std::true_type {};
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template<typename T>
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struct json_avoids_null<std::vector<T>> : std::true_type {};
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template<typename T>
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struct json_avoids_null<std::list<T>> : std::true_type {};
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template<typename K, typename V>
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struct json_avoids_null<std::map<K, V>> : std::true_type {};
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}
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namespace nlohmann {
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/**
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* This "instance" is widely requested, see
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* https://github.com/nlohmann/json/issues/1749, but momentum has stalled
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* out. Writing there here in Nix as a stop-gap.
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*
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* We need to make sure the underlying type does not use `null` for this to
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* round trip. We do that with a static assert.
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*/
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template<typename T>
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struct adl_serializer<std::optional<T>> {
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/**
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* @brief Convert a JSON type to an `optional<T>` treating
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* `null` as `std::nullopt`.
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*/
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static void from_json(const json & json, std::optional<T> & t) {
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static_assert(
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nix::json_avoids_null<T>::value,
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"null is already in use for underlying type's JSON");
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t = json.is_null()
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? std::nullopt
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: std::make_optional(json.template get<T>());
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}
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/**
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* @brief Convert an optional type to a JSON type treating `std::nullopt`
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* as `null`.
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*/
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static void to_json(json & json, const std::optional<T> & t) {
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static_assert(
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nix::json_avoids_null<T>::value,
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"null is already in use for underlying type's JSON");
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if (t)
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json = *t;
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else
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json = nullptr;
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}
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};
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}
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