C++ convert json to object
Solution 1
There is two solutions.
Do it manually
you can provide a function that takes a json::value
and return the object of your type:
User fromJson(json::value data) {
return User{data[U("username")].as_string(), data[U("info")].as_string()};
}
Do it automatically
There is no reflection in C++
. True. But if the compiler can't provide you with metadata, you can provide it yourself.
Let's start by making a propery struct:
template<typename Class, typename T>
struct Property {
constexpr Property(T Class::*aMember, const char* aName) : member{aMember}, name{aName} {}
using Type = T;
T Class::*member;
const char* name;
};
Ok, now we have the building block of our compile-time introspection system.
Now in your class user, add your metadata:
struct User {
constexpr static auto properties = std::make_tuple(
Property<User, std::string>{&User::username, "username"},
Property<User, std::string>{&User::info, "info"}
);
private:
std::string username;
std::string info;
};
Now that you have the desired metadata, you can iterate through it by recursion:
template<std::size_t iteration, typename T>
void doSetData(T&& object, const json::value& data) {
// get the property
constexpr auto property = std::get<iteration>(std::decay_t<T>::properties);
// get the type of the property
using Type = typename decltype(property)::Type;
// set the value to the member
object.*(property.member) = asAny<Type>(data[U(property.name)]);
}
template<std::size_t iteration, typename T, typename = std::enable_if_t<(iteration > 0)>>
void setData(T&& object, const json::value& data) {
doSetData<iteration>(object, data);
// next iteration
setData<iteration - 1>(object, data);
}
template<std::size_t iteration, typename T, typename = std::enable_if_t<(iteration == 0)>>
void setData(T&& object, const json::value& data) {
doSetData<iteration>(object, data);
}
template<typename T>
T fromJson(Json::Value data) {
T object;
setData<std::tuple_size<decltype(T::properties)>::value - 1>(object, data);
return object;
}
That will do the trick.
I did not test this code, so if you have trouble, tell me in the comments.
Note that you will need to write the asAny
function. It's just a function that takes a Json::Value and call the right as_...
function, or another fromJson
;)
Solution 2
I have reworked the Guillaume solution to support c++11. A full working solution with some "polyfill" of c++14's decay_t and enable_if_t features to work with c++11 is below:
// main.cpp
#include <iostream>
#include <type_traits>
#include <tuple>
#include <jsoncpp/json/json.h>
template<typename Class, typename T>
struct Property
{
constexpr Property(T Class::*aMember, const char *aName) :
member{aMember}, name{aName}
{}
using Type = T;
T Class::*member;
const char *name;
};
class User
{
std::string username;
std::string info;
public:
constexpr static auto properties = std::make_tuple(Property<User, std::string>{&User::username, "username"},
Property<User, std::string>{&User::info, "info"});
const std::string &getUsername() const
{
return username;
}
void setUsername(const std::string &username)
{
User::username = username;
}
const std::string &getInfo() const
{
return info;
}
void setInfo(const std::string &info)
{
User::info = info;
}
};
template< class T >
using decay_t = typename std::decay<T>::type;
template< bool B, class T = void >
using enable_if_t = typename std::enable_if<B,T>::type;
template<std::size_t iteration, typename T>
void doSetData(T &&object, const Json::Value &data)
{
constexpr auto property = std::get<iteration>(decay_t<T>::properties);
using Type = typename decltype(property)::Type;
object.*(property.member) = data[property.name].asString();
}
template<std::size_t iteration, typename T, enable_if_t<(iteration > 0)>* = nullptr>
void setData(T &&object, const Json::Value &data)
{
doSetData<iteration>(object, data);
setData<iteration - 1>(object, data);
}
template<std::size_t iteration, typename T, enable_if_t<(iteration == 0)>* = nullptr>
void setData(T &&object, const Json::Value &data)
{
doSetData<iteration>(object, data);
}
template<typename T>
T fromJson(Json::Value data)
{
T object;
setData<std::tuple_size<decltype(T::properties)>::value - 1>(object, data);
return object;
}
int main()
{
Json::Value value;
value["username"] = "fiorentinoing";
value["info"] = "https://www.linkedin.com/in/fiorentinoing/";
User u = fromJson<User>(value);
std::cout << "Hello, "<< u.getUsername() <<"!" << std::endl;
std::cout << "Please, visit "<< u.getInfo() <<"." << std::endl;
return 0;
}
With libjsoncpp-dev as dependency, in order to build under Ubuntu 18.04 you can issue:
g++ --std=c++11 -o static_reflection main.cpp -ljsoncpp
Piotrek
Updated on June 07, 2022Comments
-
Piotrek almost 2 years
I'm downloading json from my server. Object that I'm sending from server is C# object and looks like this:
public class User { public string UserName { get; set; } public string Info { get; set; } }
Now, I have to get this data in my C++ app. I use this library for it.
Object that I've got from server is of the type:
web::json::value
How can I get UserName from this
web::json::value
?