Clever Geek Handbook

Mapping data in json format to a c ++ structure and back (work on errors)

The previous version of solving the problem of mapping between the C ++ structure and json turned out like the first pancake - a lump. Fortunately, development is an iterative process and there will always be a second version behind the first version. The comments (thanks everyone) and the hole analysis in the first pancake made some improvements.



What was bad



  • it is impossible to use ordinary C ++ structures (including those already existing). All structures must be defined from scratch in a special way
  • Json object can only be mapped to a specially defined structure
  • Json array can only be mapped to a special class
  • it is impossible to use stl containers
  • macros are simply necessary (it is possible without them, but the registration of methods for setting fields is rigidly combined with the initialization of these fields, therefore, without macros, the definition of the structure is unreadable)
  • the display is not configurable in any way, i.e. cannot be set, for example, default values ​​or value limits


How it became now



The registration of the fields that will participate in the display is no longer tied to the structure. To register, use the function



reg(V T::* ptr, std::string const & name, Options<U>&& ... options);


  • ptr - pointer to the field
  • name - field name
  • options - display options


The following can be used as field types:



  • bool
  • char, unsigned char, short, unsigned short, int unsigned int, long, long long
  • float, double
  • std :: string
  • std :: list
  • std :: vector
  • std::map ( std::string)
  • std::unordered_map ( std::string)
  • std::multimap ( std::string)
  • std::unordered_multimap ( std::string)
  • ++




struct Friend {
 std::string name;
 std::list<int> counters;
};

struct MiB {
 std::list<Friend> friends;
 std::vector<std::list<std::string>> groups;
 std::map<std::string, std::vector<std::string>> books;
};

struct_mapping::reg(&Friend::name, "name");
struct_mapping::reg(&Friend::counters, "counters");

struct_mapping::reg(&MiB::friends, "friends");
struct_mapping::reg(&MiB::groups, "groups");
struct_mapping::reg(&MiB::books, "books");


, , 



map_json_to_struct(T & result_struct, std::basic_istream<char> & json_data);


  • result_struct —
  • json_data — json


. .





#include <iostream>
#include <sstream>

#include "struct_mapping/struct_mapping.h"

struct Planet {
 bool giant;
 long long surface_area;
 double mass;
 std::string satellite;
};

int main() {
 struct_mapping::reg(&Planet::giant, "giant");
 struct_mapping::reg(&Planet::surface_area, "surface_area");
 struct_mapping::reg(&Planet::mass, "mass");
 struct_mapping::reg(&Planet::satellite, "satellite");

 Planet earth;

 std::istringstream json_data(R"json(
  {
   "giant": false,
   "surface_area": 510072000000000,
   "mass": 5.97237e24,
   "satellite": "Moon"
  }
 )json");

 struct_mapping::map_json_to_struct(earth, json_data);

 std::cout << "earth" << std::endl;
 std::cout << " giant        : " << std::boolalpha << earth.giant << std::endl;
 std::cout << " surface_area : " << earth.surface_area << std::endl;
 std::cout << " mass         : " << earth.mass << std::endl;
 std::cout << " satellite    : " << earth.satellite << std::endl;
}




earth
 giant        : false
 surface_area : 510072000000000
 mass         : 5.97237e+24
 satellite    : Moon




, json . , :



MemberString::set(From function_from_string_, To function_to_string_);


  • function_from_string_ —
  • function_to_string_ — 




enum class Color {
 red,
 blue,
 green,
};

struct_mapping::MemberString<Color>::set(
 [] (const std::string & value) {
  if (value == "red") return Color::red;
  if (value == "green") return Color::green;
  if (value == "blue") return Color::blue;

  throw struct_mapping::StructMappingException("bad convert '"+value+"' to Color");
 },
 [] (Color value) {
  switch (value) {
  case Color::red: return "red";
  case Color::green: return "green";
  default: return "blue";
  }
 });




#include <iostream>
#include <list>
#include <map>
#include <sstream>
#include <string>

#include "struct_mapping/struct_mapping.h"

namespace sm = struct_mapping;

enum class Color {
 red,
 blue,
 green,
};

Color color_from_string(const std::string & value) {
 if (value == "red") return Color::red;
 if (value == "blue") return Color::blue;

 return Color::green;
}

std::string color_to_string(Color color) {
 switch (color) {
 case Color::red: return "red";
 case Color::green: return "green";
 default: return "blue";
 }
}

struct Palette {
 Color main_color;
 Color background_color;
 std::list<Color> special_colors;
 std::map<std::string, Color> colors;

 friend std::ostream & operator<<(std::ostream & os, const Palette & o) {
  os << "main_color       : " << color_to_string(o.main_color) << std::endl;
  os << "background_color : " << color_to_string(o.background_color) << std::endl;
  os << "special_colors   : ";
  for (auto color : o.special_colors)
   os << color_to_string(color) << ", ";
  os << std::endl << "colors           : ";
  for (auto [name, color] : o.colors)
   os << "[" << name << ", " << color_to_string(color) << "], ";
  os << std::endl;

  return os;
 }
};

int main() {
 sm::MemberString<Color>::set(color_from_string, color_to_string);

 sm::reg(&Palette::main_color, "main_color", sm::Required{});
 sm::reg(&Palette::background_color, "background_color", sm::Default{Color::blue});
 sm::reg(&Palette::special_colors, "special_colors");
 sm::reg(&Palette::colors, "colors");

 Palette palette;

 std::istringstream json_data(R"json(
 {
  "main_color": "green",
  "special_colors": ["green", "green", "red"],
  "colors": {
   "dark": "green",
   "light": "red",
   "neutral": "blue"
  }
 }
 )json");

 sm::map_json_to_struct(palette, json_data);

 std::cout << palette << std::endl;
}




main_color       : green
background_color : blue
special_colors   : green, green, red, 
colors           : [dark, green], [light, red], [neutral, blue],




,



  • Bounds
  • Default
  • NotEmpty
  • Required


Bounds



, ( ) . . — . .



Bounds{ ,  }


:



reg(&Stage::engine_count, "engine_count", Bounds{1, 31});


Default



. bool, , , , , ++ . — .



Default{  }


:



reg(&Stage::engine_count, "engine_count", Default{3});


NotEmpty



, . . . , .



:



reg(&Spacecraft::name, "name", NotEmpty{}));


Required



, . bool, , , , , ++ . . , .



:



reg(&Spacecraft::name, "name", Required{}));




#include <iostream>
#include <list>
#include <map>
#include <sstream>
#include <string>

#include "struct_mapping/struct_mapping.h"

namespace sm = struct_mapping;

struct Stage {
 unsigned short engine_count;
 std::string fuel;
 long length;

 friend std::ostream & operator<<(std::ostream & os, const Stage & o) {
  os << "  engine_count : " << o.engine_count << std::endl;
  os << "  fuel         : " << o.fuel << std::endl;
  os << "  length       : " << o.length << std::endl;

  return os;
 }
};

struct Spacecraft {
 bool in_development;
 std::string name;
 int mass;
 std::map<std::string, Stage> stages;
 std::list<std::string> crew;

 friend std::ostream & operator<<(std::ostream & os, const Spacecraft & o) {
  os << "in_development : " << std::boolalpha << o.in_development << std::endl;
  os << "name           : " << o.name << std::endl;
  os << "mass           : " << o.mass << std::endl;
  os << "stages: " << std::endl;
  for (auto& s : o.stages) os << " " << s.first << std::endl << s.second;
  os << "crew: " << std::endl;
  for (auto& p : o.crew) os << " " << p << std::endl;

  return os;
 }
};

int main() {
 sm::reg(&Stage::engine_count, "engine_count", sm::Default{6}, sm::Bounds{1, 31});
 sm::reg(&Stage::fuel, "fuel", sm::Default{"subcooled"});
 sm::reg(&Stage::length, "length", sm::Default{50});

 sm::reg(&Spacecraft::in_development, "in_development", sm::Required{});
 sm::reg(&Spacecraft::name, "name", sm::NotEmpty{});
 sm::reg(&Spacecraft::mass, "mass",
  sm::Default{5000000}, sm::Bounds{100000, 10000000});
 sm::reg(&Spacecraft::stages, "stages", sm::NotEmpty{});
 sm::reg(&Spacecraft::crew, "crew",
  sm::Default{std::list<std::string>{"Arthur", "Ford", "Marvin"}});

 Spacecraft starship;

 std::istringstream json_data(R"json(
 {
  "in_development": false,
  "name": "Vostok",
  "stages": {
   "first": {
    "engine_count": 31,
    "fuel": "compressed gas",
    "length": 70
   },
   "second": {}
  }
 }
 )json");

 sm::map_json_to_struct(starship, json_data);

 std::cout << starship << std::endl;
}




in_development : false
name           : Vostok
mass           : 5000000
stages: 
 first
  engine_count : 31
  fuel         : compressed gas
  length       : 70
 second
  engine_count : 6
  fuel         : subcooled
  length       : 50
crew: 
 Arthur
 Ford
 Marvin


c++ json



json , , 



reg(V T::* ptr, std::string const & name, Options<U>&& ... options);




map_struct_to_json(T & source_struct, std::basic_ostream<char> & json_data, std::string indent);


  • source_struct —
  • json_data — json
  • indent — ( , )


#include <iostream>
#include <sstream>

#include "struct_mapping/struct_mapping.h"

struct OceanPart {
 std::string name;
 double average_depth;
 std::vector<int> temperature;
};

struct OceanColor {
 std::string name;
};

struct Ocean {
 double water_volume;
 long long surface_area;
 bool liquid;
 std::string name;

 OceanColor color;

 std::vector<OceanPart> parts;
};

struct Planet {
 bool giant;
 long long surface_area;
 double mass;
 double volume;
 long long orbital_period;
 std::string name;
 bool terrestrial;
 std::string shape;

 Ocean ocean;
};

int main() {
 struct_mapping::reg(&OceanPart::name, "name");
 struct_mapping::reg(&OceanPart::average_depth, "average_depth");
 struct_mapping::reg(&OceanPart::temperature, "temperature");

 struct_mapping::reg(&OceanColor::name, "name");

 struct_mapping::reg(&Ocean::water_volume, "water_volume");
 struct_mapping::reg(&Ocean::surface_area, "surface_area");
 struct_mapping::reg(&Ocean::liquid, "liquid");
 struct_mapping::reg(&Ocean::name, "name");
 struct_mapping::reg(&Ocean::color, "color");
 struct_mapping::reg(&Ocean::parts, "parts");

 struct_mapping::reg(&Planet::giant, "giant");
 struct_mapping::reg(&Planet::surface_area, "surface_area");
 struct_mapping::reg(&Planet::mass, "mass");
 struct_mapping::reg(&Planet::volume, "volume");
 struct_mapping::reg(&Planet::orbital_period, "orbital_period");
 struct_mapping::reg(&Planet::name, "name");
 struct_mapping::reg(&Planet::terrestrial, "terrestrial");
 struct_mapping::reg(&Planet::shape, "shape");
 struct_mapping::reg(&Planet::ocean, "ocean");

 Planet earth;

 earth.giant = false;
 earth.terrestrial = true;
 earth.surface_area = 510072000;
 earth.orbital_period = 365 * 24 * 3600;
 earth.mass = 5.97237e24;
 earth.name = "Terra";
 earth.volume = 1.08321e12;
 earth.shape = "nearly spherical";

 earth.ocean.water_volume = 1332000000;
 earth.ocean.surface_area = 361132000;
 earth.ocean.liquid = true;
 earth.ocean.name = "World Ocean";
 earth.ocean.color.name = "blue";

 OceanPart pacific;
 pacific.name = "Pacific Ocean";
 pacific.average_depth = 4.280111;
 pacific.temperature = std::vector<int>{-3, 5, 12};

 OceanPart atlantic;
 atlantic.name = "Atlantic Ocean";
 atlantic.average_depth = 3.646;
 atlantic.temperature = std::vector<int>{-3, 0};

 earth.ocean.parts.push_back(pacific);
 earth.ocean.parts.push_back(atlantic);

 std::ostringstream json_data;
 struct_mapping::map_struct_to_json(earth, json_data, "  ");

 std::cout << json_data.str() << std::endl;
}




{
  "giant": false,
  "surface_area": 510072000,
  "mass": 5.97237e+24,
  "volume": 1.08321e+12,
  "orbital_period": 31536000,
  "name": "Terra",
  "terrestrial": true,
  "shape": "nearly spherical",
  "ocean": {
    "water_volume": 1.332e+09,
    "surface_area": 361132000,
    "liquid": true,
    "name": "World Ocean",
    "color": {
      "name": "blue"
    },
    "parts": [
      {
        "name": "Pacific Ocean",
        "average_depth": 4.28011,
        "temperature": [
          -3,
          5,
          12
        ]
      },
      {
        "name": "Atlantic Ocean",
        "average_depth": 3.646,
        "temperature": [
          -3,
          0
        ]
      }
    ]
  }
}




  • ++ ( )
  • Json objects can be mapped both to a specially defined structure (this possibility remains) and to ordinary structures
  • Json arrays can be mapped to std :: vector and std :: list. The general requirements for containers on which arrays can be mapped are not fully formed yet.
  • json objects can be mapped to associative containers, with the restriction that the key must be a string. The general requirements for containers, as well as with arrays, are not fully formed yet.
  • macros are unnecessary and certainly not necessary. The possibility of using them remained (as an option) during registration, combined with the initialization of fields. But most likely it will be cut.
  • display can be customized using options


The library is available on GitHub



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