C++ convert json to object

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 ;)

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