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