nlohmann::adl_serializer::
from_json
// (1)
template<typename BasicJsonType, typename TargetType = ValueType>
static auto from_json(BasicJsonType && j, TargetType& val) noexcept(
noexcept(::nlohmann::from_json(std::forward<BasicJsonType>(j), val)))
-> decltype(::nlohmann::from_json(std::forward<BasicJsonType>(j), val), void())
// (2)
template<typename BasicJsonType, typename TargetType = ValueType>
static auto from_json(BasicJsonType && j) noexcept(
noexcept(::nlohmann::from_json(std::forward<BasicJsonType>(j), detail::identity_tag<TargetType> {})))
-> decltype(::nlohmann::from_json(std::forward<BasicJsonType>(j), detail::identity_tag<TargetType> {}))
This function is usually called by the
get()
function of the
basic_json
class (either explicitly or via the conversion operators).
-
This function is chosen for default-constructible value types.
-
This function is chosen for value types which are not default-constructible.
Parameters
-
j
(in)
-
JSON value to read from
-
val
(out)
-
value to write to
Return value
Copy of the JSON value, converted to
ValueType
Examples
Example: (1) Default-constructible type
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
template get<ns::person>()
is called.
#include <iostream>
#include <nlohmann/json.hpp>
using json = nlohmann::json;
namespace ns
// a simple struct to model a person
struct person
std::string name;
std::string address;
int age;
} // namespace ns
namespace ns
void from_json(const json& j, person& p)
j.at("name").get_to(p.name);
j.at("address").get_to(p.address);
j.at("age").get_to(p.age);
} // namespace ns
int main()
json j;
j["name"] = "Ned Flanders";
j["address"] = "744 Evergreen Terrace";
j["age"] = 60;
auto p = j.template get<ns::person>();
std::cout << p.name << " (" << p.age << ") lives in " << p.address << std::endl;
Output:
Ned Flanders (60) lives in 744 Evergreen Terrace
Example: (2) Non-default-constructible type
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.
#include <iostream>
#include <nlohmann/json.hpp>
using json = nlohmann::json;
namespace ns
// a simple struct to model a person (not default constructible)
struct person
person(std::string n, std::string a, int aa)
: name(std::move(n)), address(std::move(a)), age(aa)
std::string name;
std::string address;
int age;
} // namespace ns
namespace nlohmann
template <>
struct adl_serializer<ns::person>
static ns::person from_json(const json& j)
return {j.at("name"), j.at("address"), j.at("age")};
// Here's the catch! You must provide a to_json method! Otherwise, you
// will not be able to convert person to json, since you fully
// specialized adl_serializer on that type
static void to_json(json& j, ns::person p)
j["name"] = p.name;
j["address"] = p.address;
j["age"] = p.age;
} // namespace nlohmann
int main()
json j;
j["name"] = "Ned Flanders";
j["address"] = "744 Evergreen Terrace";
j["age"] = 60;
