C JSON Serialization

Question

I want a way to serialize and deserialize Objects to JSON  as automatic as possible   Serialize  For me  the ideal way is that if I call in an instance JSONSerialize   it returns an string with a JSON object that has all the public properties of the object as  name of property    value   For those values that are primitives  it is straightforward  for objects it should try to call on each JSONSerialize   or ToString   or something like that to recursively serialize all the public properties  For collections it should also behave correctly  just vectors arrays will be ok    Deserialize  Just make an instance of the given object  let s say a dog  and call JSONDeserialize json string   and that should fill all the public properties  creating the needed objects in case that the properties are not primitives  or the needed collections   An example should run like that   Dog  d1   new Dog    d1- gt name    myDog    string serialized   d1- gt JSONSerialize     Dog  d2   new Dog    d2- gt JSONDeserialize serialized   std  cout  lt  lt  d2- gt name     This will print  myDog    Or like that   Dog  d1   new Dog    d1- gt name    myDog    string serialized   JSONSerializer Serialize d1    Dog  d2   JSONSerializer Deserialize serialized  Dog   std  cout  lt  lt  d2- gt name     This will print  myDog    How can I pull this off easily

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Using ThorsSerializer  Dog  d1   new Dog    d1- gt name    myDog    std  stringstream  stream  lt  lt  ThorsAnvil  Serialize  jsonExport d1   string serialized   stream str     Dog  d2   nullptr  stream  gt  gt  ThorsAnvil  Serialize  jsonImport d2   std  cout  lt  lt  d2- gt name     This will print  myDog    I think that is pretty close to your original  There is a tiny bit of set up  You need to declare your class is Serializable    include  ThorSerialize Traits h   include  ThorSerialize JsonThor h   struct Dog       std  string  name         Declare the  Dog  class is Serializable  Serialize the member  name  ThorsAnvil MakeTrait Dog  name     No other coding is required   Complete Examples can be found    Simple Example Container Example Polymorphic Example

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This is my attempt using Qt  https   github com carlonluca lqobjectserializer  A JSON like this    quot menu quot          quot header quot    quot SVG Viewer quot        quot items quot               quot id quot    quot Open quot              quot id quot    quot OpenNew quot    quot label quot    quot Open New quot            null            quot id quot    quot ZoomIn quot    quot label quot    quot Zoom In quot              quot id quot    quot ZoomOut quot    quot label quot    quot Zoom Out quot              quot id quot    quot OriginalView quot    quot label quot    quot Original View quot            null            quot id quot    quot Quality quot              quot id quot    quot Pause quot              quot id quot    quot Mute quot            null            quot id quot    quot Find quot    quot label quot    quot Find    quot              quot id quot    quot FindAgain quot    quot label quot    quot Find Again quot              quot id quot    quot Copy quot              quot id quot    quot CopyAgain quot    quot label quot    quot Copy Again quot              quot id quot    quot CopySVG quot    quot label quot    quot Copy SVG quot              quot id quot    quot ViewSVG quot    quot label quot    quot View SVG quot              quot id quot    quot ViewSource quot    quot label quot    quot View Source quot              quot id quot    quot SaveAs quot    quot label quot    quot Save As quot            null            quot id quot    quot Help quot              quot id quot    quot About quot    quot label quot    quot About Adobe CVG Viewer    quot             can be deserialized by declaring classes like these  L BEGIN CLASS Item  L RW PROP QString  id  setId  QString    L RW PROP QString  label  setLabel  QString    L END CLASS  L BEGIN CLASS Menu  L RW PROP QString  header  setHeader  L RW PROP ARRAY WITH ADDER Item   items  setItems  L END CLASS  L BEGIN CLASS MenuRoot  L RW PROP Menu   menu  setMenu  nullptr  L END CLASS  and writing writing  LDeserializer lt MenuRoot gt  deserializer  QScopedPointer lt MenuRoot gt  g deserializer deserialize jsonString     You also need to inject mappings for meta objects once  QHash lt QString  QMetaObject gt  factory         QSL  quot Item  quot    Item  staticMetaObject          QSL  quot Menu  quot    Menu  staticMetaObject       I m looking for a way to avoid this

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Does anything  easy like that  exists   THANKS       C   does not store class member names in compiled code  and there s no way to discover  at runtime  which members  variables methods  class contains  In other words  you cannot iterate through members of a struct  Because there s no such mechanism  you won t be able to automatically create  JSONserialize  for every object   You can  however  use any json library to serialize objects  BUT you ll have to write serialization deserialization code yourself for every class  Either that  or you ll have to create serializeable class similar to QVariantMap that ll be used instead of structs for all serializeable objects    In other words  if you re okay with using specific type for all serializeable objects  or writing serialization routines yourself for every class   it can be done  However  if you want to automatically serialize every possible class  you should forget about it  If this feature is important to you  try another language

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Not yet mentioned  though it was the first in my search result  https   github com nlohmann json  Perks listed    Intuitive syntax  looks great   Single header file to include  nothing else Ridiculously tested   Also  it s under the MIT License   I ll be honest  I have yet to use it  but through some experience I have a knack for determining when I come across a really well-made c   library

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For that you need reflection in C C   language  that doesn t exists  You need to have some meta data describing the structure of your classes  members  inherited base classes   For the moment C C   compilers doesn t provide automatically that information in built binaries   I had the same idea in mind  and I used GCC XML project to get this information  It outputs XML data describing class structures   I have built a project and I m explaining some key points in this page    Serialization is easy  but we have to deal with complex data structure implementations  std  string  std  map for example  that play with allocated buffers   Deserialization is more complex and you need to rebuild your object with all its members  plus references to vtables     a painful implementation   For example you can serialize like that           Random class initialization     com  class1  aObject   new com  class1         for  int i 0  i lt 10  i                 aObject- gt setData i i                    aObject- gt pdata   new char 7       for  int i 0  i lt 7  i                 aObject- gt pdata i    7-i               dictionary initialization     cjson  dictionary aDict    data dictionary xml            json transformation     std  string aJson   aDict toJson lt com  class1 gt  aObject           print encoded class     cout  lt  lt  aJson  lt  lt  std  endl     To deserialize data it works like that          decode the object     com  class1  aDecodedObject   aDict fromJson lt com  class1 gt  aJson           modify data     aDecodedObject- gt setData 4 22           json transformation     aJson   aDict toJson lt com  class1 gt  aDecodedObject           print encoded class     cout  lt  lt  aJson  lt  lt  std  endl     Ouptuts    gt    cjson    main    index  54   inner       ident   test   pi  3 141593    name   first   com  class0   type   type   com  class0  data   0 1 2 3 4 5 6 7 8 9   com  classb   ref   ref   com  classm1   type   typem1   com  classm1  pdata   7 6 5 4 3 2 1      index  54   inner     ident   test   pi  3 141593    name   first   com  class0   type   type   com  class0  data   0 1 2 3 22 5 6 7 8 9   com  classb   ref   ref   com  classm1   type   typem1   com  classm1  pdata   7 6 5 4 3 2 1    gt    cjson     Usually these implementations are compiler dependent  ABI Specification for example   and requires external description to work  GCCXML output   such are not really easy to integrate to projects

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In case that anyone still has this need  I have   I have written a library myself to deal with this problem  See here  It isn t completely automatic in that you have to describe all the fields in your classes  but it is as close as what we can get as C   lacks reflection

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The jsoncons C   header-only library also supports conversion between JSON text and C   objects   Decode and encode are defined for all C   classes that have json type traits defined  The standard library containers are already supported  and json type traits can be specialized for user types in the jsoncons namespace   Below is an example    include  lt iostream gt   include  lt jsoncons json hpp gt   namespace ns       enum class hiking experience  beginner intermediate advanced        class hiking reputon               std  string rater           hiking experience assertion           std  string rated           double rating       public          hiking reputon const std  string amp  rater                         hiking experience assertion                         const std  string amp  rated                         double rating                rater  rater   assertion  assertion   rated  rated   rating  rating                               const std  string amp  rater   const  return rater            hiking experience assertion   const  return assertion            const std  string amp  rated   const  return rated            double rating   const  return rating                class hiking reputation               std  string application           std  vector lt hiking reputon gt  reputons       public          hiking reputation const std  string amp  application                             const std  vector lt hiking reputon gt  amp  reputons                application  application                  reputons  reputons                      const std  string amp  application   const   return application            const std  vector lt hiking reputon gt  amp  reputons   const   return reputons                 namespace ns     Declare the traits using convenience macros  Specify which data members need to be serialized   JSONCONS ENUM TRAITS DECL ns  hiking experience  beginner  intermediate  advanced  JSONCONS ALL CTOR GETTER TRAITS ns  hiking reputon  rater  assertion  rated  rating  JSONCONS ALL CTOR GETTER TRAITS ns  hiking reputation  application  reputons   using namespace jsoncons     for convenience  int main      std  string data   R                 application    hiking           reputons                          rater    HikingAsylum               assertion    advanced               rated    Marilyn C               rating   0 90                                       Decode the string of data into a c   structure     ns  hiking reputation v   decode json lt ns  hiking reputation gt  data           Iterate over reputons array value     std  cout  lt  lt    1  n       for  const auto amp  item   v reputons                  std  cout  lt  lt  item rated    lt  lt        lt  lt  item rating    lt  lt    n                 Encode the c   structure into a string     std  string s      encode json lt ns  hiking reputation gt  v  s  indenting  indent       std  cout  lt  lt    2  n       std  cout  lt  lt  s  lt  lt    n           Output    1  Marilyn C  0 9  2         application    hiking        reputons                            assertion    advanced                rated    Marilyn C                rater    HikingAsylum                rating   0 9

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Using quicktype  you can generate C   serializers and deserializers from JSON sample data   For example  given the sample JSON        breed    Boxer      age   5     tail length   6 5     quicktype generates    include  json hpp   namespace quicktype       using nlohmann  json       struct Dog           int64 t age          std  string breed          double tail length               inline json get untyped const json  amp j  const char  property            if  j find property     j end                  return j at property  get lt json gt                       return json             namespace nlohmann        inline void from json const json amp   j  struct quicktype  Dog amp   x             x age    j at  age   get lt int64 t gt              x breed    j at  breed   get lt std  string gt              x tail length    j at  tail length   get lt double gt                inline void to json json amp   j  const struct quicktype  Dog amp   x             j   json   age    x age     breed    x breed     tail length    x tail length              To parse the Dog JSON data  include the code above  install Boost and json hpp  then do   Dog dog   nlohmann  json  parse jsonString

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There is no reflection in C    True  But if the compiler can t provide you the metadata you need  you can provide it yourself   Let s start by making a property struct   template lt typename Class  typename T gt  struct PropertyImpl       constexpr PropertyImpl T Class   aMember  const char  aName    member aMember   name aName          using Type   T       T Class   member      const char  name      template lt typename Class  typename T gt  constexpr auto property T Class   member  const char  name        return PropertyImpl lt Class  T gt  member  name       Of course  you also can have a property that takes a setter and getter instead of a pointer to member  and maybe read only properties for calculated value you d like to serialize  If you use C  17  you can extend it further to make a property that works with lambdas   Ok  now we have the building block of our compile-time introspection system   Now in your class Dog  add your metadata   struct Dog       std  string barkType      std  string color      int weight   0       bool operator   const Dog amp  rhs  const           return std  tie barkType  color  weight     std  tie rhs barkType  rhs color  rhs weight              constexpr static auto properties   std  make tuple          property  amp Dog  barkType   barkType            property  amp Dog  color   color            property  amp Dog  weight   weight               We will need to iterate on that list  To iterate on a tuple  there are many ways  but my preferred one is this   template  lt typename T  T    S  typename F gt  constexpr void for sequence std  integer sequence lt T  S    gt   F amp  amp  f        using unpack t   int         void unpack t  static cast lt void gt  f std  integral constant lt T  S gt       0      0       If C  17 fold expressions are available in your compiler  then for sequence can be simplified to   template  lt typename T  T    S  typename F gt  constexpr void for sequence std  integer sequence lt T  S    gt   F amp  amp  f         static cast lt void gt  f std  integral constant lt T  S gt                 This will call a function for each constant in the integer sequence   If this method don t work or gives trouble to your compiler  you can always use the array expansion trick   Now that you have the desired metadata and tools  you can iterate through the properties to unserialize      unserialize function template lt typename T gt  T fromJson const Json  Value amp  data        T object          We first get the number of properties     constexpr auto nbProperties   std  tuple size lt decltype T  properties  gt   value          We iterate on the index sequence of size  nbProperties      for sequence std  make index sequence lt nbProperties gt       amp   auto i               get the property         constexpr auto property   std  get lt i gt  T  properties               get the type of the property         using Type   typename decltype property   Type              set the value to the member            you can also replace  asAny  by  fromJson  to recursively serialize         object   property member    Json  asAny lt Type gt  data property name                 return object      And for serialize   template lt typename T gt  Json  Value toJson const T amp  object        Json  Value data          We first get the number of properties     constexpr auto nbProperties   std  tuple size lt decltype T  properties  gt   value          We iterate on the index sequence of size  nbProperties      for sequence std  make index sequence lt nbProperties gt       amp   auto i               get the property         constexpr auto property   std  get lt i gt  T  properties               set the value to the member         data property name    object   property member                return data      If you want recursive serialization and unserialization  you can replace asAny by fromJson   Now you can use your functions like this   Dog dog   dog color    green   dog barkType    whaf   dog weight   30   Json  Value jsonDog   toJson dog      produces   color   green    barkType   whaf    weight   30  auto dog2   fromJson lt Dog gt  jsonDog    std  cout  lt  lt  std  boolalpha  lt  lt   dog    dog2   lt  lt  std  endl     pass the test  both dog are equal    Done  No need for run-time reflection  just some C  14 goodness   This code could benefit from some improvement  and could of course work with C  11 with some ajustements   Note that one would 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   Here s a complete  working example made from the various code snippet of this answer  Feel free to use it   As mentionned in the comments  this code won t work with msvc  Please refer to this question if you want a compatible code  Pointer to member  works in GCC but not in VS2015

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Try json dto  It is header-only and easy to use    Simple example   struct message t     std  string m from    std  string m text        Entry point for json dto    template  lt  typename JSON IO  gt    void   json io  JSON IO  amp  io           io        amp  json dto  mandatory   from   m from          amp  json dto  mandatory   text   m text             This will be convertable to and from JSON      from     json dto    text     Hello world

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