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nlohmann
158 lines
3.8 KiB
Markdown
158 lines
3.8 KiB
Markdown
# nlohmann::adl_serializer::from_json
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```
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// (1)
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template<typename BasicJsonType, typename TargetType = ValueType>
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static auto from_json(BasicJsonType && j, TargetType& val) noexcept(
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noexcept(::nlohmann::from_json(std::forward<BasicJsonType>(j), val)))
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-> decltype(::nlohmann::from_json(std::forward<BasicJsonType>(j), val), void())
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// (2)
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template<typename BasicJsonType, typename TargetType = ValueType>
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static auto from_json(BasicJsonType && j) noexcept(
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noexcept(::nlohmann::from_json(std::forward<BasicJsonType>(j), detail::identity_tag<TargetType> {})))
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-> decltype(::nlohmann::from_json(std::forward<BasicJsonType>(j), detail::identity_tag<TargetType> {}))
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```
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This function is usually called by the [`get()`](https://json.nlohmann.me/api/basic_json/get/index.md) function of the [basic_json](https://json.nlohmann.me/api/basic_json/index.md) class (either explicitly or via the conversion operators).
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1. This function is chosen for default-constructible value types.
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1. This function is chosen for value types which are not default-constructible.
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## Parameters
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`j` (in) : JSON value to read from
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`val` (out) : value to write to
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## Return value
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1. (none) -- the converted value is written to the output parameter `val`.
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1. the JSON value `j` converted to `TargetType`
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## Examples
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Example: (1) Default-constructible type
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The example below shows how a `from_json` function can be implemented for a user-defined type. This function is called by the `adl_serializer` when `get<ns::person>()` is called.
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```
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#include <iostream>
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#include <nlohmann/json.hpp>
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using json = nlohmann::json;
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namespace ns
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{
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// a simple struct to model a person
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struct person
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{
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std::string name;
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std::string address;
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int age;
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};
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} // namespace ns
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namespace ns
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{
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void from_json(const json& j, person& p)
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{
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j.at("name").get_to(p.name);
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j.at("address").get_to(p.address);
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j.at("age").get_to(p.age);
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}
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} // namespace ns
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int main()
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{
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json j;
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j["name"] = "Ned Flanders";
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j["address"] = "744 Evergreen Terrace";
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j["age"] = 60;
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auto p = j.get<ns::person>();
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std::cout << p.name << " (" << p.age << ") lives in " << p.address << std::endl;
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}
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```
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Output:
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```
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Ned Flanders (60) lives in 744 Evergreen Terrace
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```
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Example: (2) Non-default-constructible type
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The example below shows how a `from_json` is implemented as part of a specialization of the `adl_serializer` to realize the conversion of a non-default-constructible type.
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```
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#include <iostream>
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#include <nlohmann/json.hpp>
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using json = nlohmann::json;
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namespace ns
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{
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// a simple struct to model a person (not default constructible)
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struct person
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{
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person(std::string n, std::string a, int aa)
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: name(std::move(n)), address(std::move(a)), age(aa)
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{}
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std::string name;
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std::string address;
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int age;
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};
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} // namespace ns
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namespace nlohmann
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{
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template <>
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struct adl_serializer<ns::person>
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{
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static ns::person from_json(const json& j)
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{
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return {j.at("name"), j.at("address"), j.at("age")};
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}
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// Here's the catch! You must provide a to_json method! Otherwise, you
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// will not be able to convert person to json, since you fully
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// specialized adl_serializer on that type
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static void to_json(json& j, ns::person p)
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{
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j["name"] = p.name;
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j["address"] = p.address;
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j["age"] = p.age;
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}
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};
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} // namespace nlohmann
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int main()
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{
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json j;
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j["name"] = "Ned Flanders";
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j["address"] = "744 Evergreen Terrace";
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j["age"] = 60;
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auto p = j.get<ns::person>();
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std::cout << p.name << " (" << p.age << ") lives in " << p.address << std::endl;
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}
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```
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Output:
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```
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Ned Flanders (60) lives in 744 Evergreen Terrace
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```
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## See also
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- [to_json](https://json.nlohmann.me/api/adl_serializer/to_json/index.md)
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## Version history
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- Added in version 2.1.0.
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