{{announcement.body}}
{{announcement.title}}

Mapping JSON to and from a C++ Structure

DZone 's Guide to

Mapping JSON to and from a C++ Structure

Full tutorial on how to map JSON to and from a C++ structure—including installation, usage, and exceptions.

· IoT Zone ·
Free Resource

Table of contents

  • Introduction
  • Compatibility
  • Installation
  • Usage
    • Implementing a scenario with a Person structure
    • Mapping JSON to c++ structure
      • Enumeration
      • Simple types example
      • Structure example
      • Enumeration example
      • Sequence container example
      • Associative container example
      • Mapping options
        • Bounds
        • Default
        • NotEmpty
        • Required
        • Options example
    • Reverse mapping of CPP structure to JSON
    • Registration of data members combined with initialization
  • Exceptions

Introduction

I would like to define a c++ structure

struct Person {
 std::string name;
 int age;
 bool student;
} person;

pass the person instance to the mapping method along with JSON data

map_json_to_struct(person, json_data)

then use the filled structure

std::cout << person.name << " : " << person.age;

Or vice versa

map_struct_to_json(person, json_data, "  ");
std::cout << json_data.str() << std::endl;

get Person as JSON

{
 "name": "Agent K",
 "age": 42,
 "student": false
}

StructMapping is trying to solve these problems

Compatibility 

Compilation is required with -std=c++17 Mainly for:

  • if constexpr
  • static inline

Compiler | platform combinations on which StructMapping has been tested:

  • GNU C++ 10.1.0 | Linux
  • Visual C++ 2019 and Microsoft (R) C/C++ Optimizing Compiler Version 19.26.28806 for x64 | Windows 64-bit (except tests)

As types of member-data can be used

  • bool
  • char, unsigned char, short, unsigned short, int unsigned int, long, long long
  • float, double
  • std::string
  • std::list
  • std::vector
  • std::set (expected to be represented in JSON as an array)
  • std::unordered_set (expected to be represented in JSON as an array)
  • std::multiset (expected to be represented in JSON as an array)
  • std::unordered_multiset (expected to be represented in JSON as an array)
  • std::map (the key can only be std::string)
  • std::unordered_map (the key can only be std::string)
  • std::multimap (the key can only be std::string)
  • std::unordered_multimap (the key can only be std::string)
  • c++ structure
  • enumerations

Installation 

StructMapping is a header-only C++ library. All library files are in the include folder.

To build examples and run tests proceed with the steps below (cmake required):

  1. create a directory called build in StructMapping source directory
  2. change to build directory
  3. run cmake .. command to configure your build
  4. run cmake --build . command. On successful build you will find binary files for examples and tests (under windows tests are not build) in the bin directory
  5. (not for windows) to run the tests run ctest (you can get detailed output using ctest -V)
  6. it is possible to install library system-wide by running cmake --install . command from the build  tree with administrative privileges. This will install all files according to system preferences.

Usage 

Implementing a scenario with a Person structure 

example/person

define the structure

struct Person {
 std::string name;
 int age;
 bool student;
};

register members

struct_mapping::reg(&Person::name, "name");
struct_mapping::reg(&Person::age, "age");
struct_mapping::reg(&Person::student, "student");

create an instance

Person person;

set JSON data

std::istringstream json_data(R"json(
{
 "name": "Jeebs",
 "age": 42,
 "student": true
}
)json");

pass the person instance to the mapping method along with JSON data

mapper::map_json_to_struct(person, json_data);

use

std::cout <<
 person.name << " : " <<
 person.age << " : " <<
 std::boolalpha << person.student << std::endl;

Completed code

#include <iostream>
#include <sstream>

#include "struct_mapping/struct_mapping.h"

struct Person {
 std::string name;
 int age;
 bool student;
};

int main() {
 struct_mapping::reg(&Person::name, "name");
 struct_mapping::reg(&Person::age, "age");
 struct_mapping::reg(&Person::student, "student");

 Person person;

 std::istringstream json_data(R"json(
  {
   "name": "Jeebs",
   "age": 42,
   "student": true
  }
 )json");

 struct_mapping::map_json_to_struct(person, json_data);

 std::cout <<
  person.name << " : " <<
  person.age << " : " <<
  std::boolalpha << person.student << std::endl;
}

result

Jeebs : 42 : true

Mapping JSON to c++ structure 

To map JSON to a structure, it is necessary to register all data members of all structures that you want to mapped using for each field

template<typename T, typename V, typename ... U, template<typename> typename ... Options>
inline void reg(V T::* ptr, std::string const & name, Options<U>&& ... options);
  • ptr - pointer to data member
  • name - member name
  • options - mapping options

and call the function

template<typename T>
void map_json_to_struct(T & result_struct, std::basic_istream<char> & json_data);
  • result_struct - reference to the resulting structure
  • json_data - reference to JSON data input stream

During the mapping process, the correspondence between the types of data members and the types of set value is checked, and (for numbers) the set value is checked to get out of the range of values of the data member type. In case of type mismatch or out of range values exceptions are generated.

Enumeration 

Enums in JSON will be represented as strings. Therefore, to use enumerations, it is required to establish conversion methods from string to enumeration value and vice versa, using:

template<typename From, typename To>
static void MemberString::set(From function_from_string_, To function_to_string_);
  • function_from_string_ - conversion function from string to enum value
  • function_to_string_ - conversion function from enum value to string

for example

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";
  }
 });

simple types example 

example/simple

#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;
}

result

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

structure example 

example/struct

#include <iostream>
#include <sstream>

#include "struct_mapping/struct_mapping.h"

struct President {
 std::string name;
 double mass;
};

struct Earth {
 President president;
};

int main() {
 struct_mapping::reg(&President::name, "name");
 struct_mapping::reg(&President::mass, "mass");

 struct_mapping::reg(&Earth::president, "president");

 Earth earth;

 std::istringstream json_data(R"json(
  {
   "president": {
    "name": "Agent K",
    "mass": 75.6
   }
  }
 )json");

 struct_mapping::map_json_to_struct(earth, json_data);

 std::cout << "earth.president:" << std::endl;
 std::cout << " name : " << earth.president.name << std::endl;
 std::cout << " mass : " << earth.president.mass << std::endl;
}

result

earth.president:
 name : Agent K
 mass : 75.6

Enumeration example 

example/enumeration

#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;
}

result

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

sequence container example 

example/array_like

#include <iostream>
#include <list>
#include <set>
#include <sstream>
#include <string>
#include <unordered_set>
#include <vector>

#include "struct_mapping/struct_mapping.h"

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

 bool operator==(const Friend & o) const {
  return name == o.name;
 }
};

struct Friend_hash {
 size_t operator()(const Friend & o) const {
  return static_cast<size_t>(o.name.size());
 }
};

struct MiB {
 std::unordered_set<Friend, Friend_hash> friends;
 std::vector<std::list<std::string>> alien_groups;
 std::vector<std::list<std::vector<std::string>>> planet_groups;
};

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

 struct_mapping::reg(&MiB::friends, "friends");
 struct_mapping::reg(&MiB::alien_groups, "alien_groups");
 struct_mapping::reg(&MiB::planet_groups, "planet_groups");

 std::istringstream json_data(R"json(
  {
   "friends": [
    {
     "name": "Griffin",
     "counters": [1,3,4]
    },
    {
     "name": "Boris",
     "counters": []
    },
    {
     "name": "Agent K",
     "counters": [42, 128]
    }
   ],
   "alien_groups": [
    [
     "Edgar the Bug",
     "Boris the Animal",
     "Charlie",
     "Serleena"
    ],
    [
     "Agent J",
     "Agent K",
     "Zed",
     "Griffin",
     "Roman the Fabulist"
    ]
   ],
   "planet_groups": [
    [
     [
      "Mercury",
      "Venus",
      "Earth",
      "Mars"
     ],
     [
      "Jupiter",
      "Saturn",
      "Uranus",
      "Neptune"
     ]
    ],
    [
     [
      "Titan",
      "Ganymede"
     ],
     [
      "Eris",
      "Titania"
     ]
    ]
   ]
  }
 )json");

 MiB mib;

 struct_mapping::map_json_to_struct(mib, json_data);

 std::cout << "mib:" << std::endl;

 std::cout << " friends :" << std::endl;
 for (auto& f : mib.friends) {
  std::cout << "  name: [ " << f.name << " ], counters: [";
  for (auto& c : f.counters) {
   std::cout << c << ", ";
  }
  std::cout << "]" << std::endl;
 }

 std::cout << std::endl << " aliens_groups :" << std::endl;
 for (auto& alien : mib.alien_groups) {
  for (auto& name : alien) {
   std::cout << "  " << name << std::endl;
  }
  std::cout << std::endl;
 }

 std::cout << " planets_groups :" << std::endl;
 for (auto& group : mib.planet_groups) {
  std::cout << "  ---" << std::endl;
  for (auto& category : group) {
   for (auto& planet : category) {
    std::cout << "   " << planet << std::endl;
   }
   std::cout << std::endl;
  }
  std::cout << std::endl;
 }

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

result

mib:
 friends :
  name: [ Griffin ], counters: [1, 3, 4, ]
  name: [ Boris ], counters: []
  name: [ Agent K ], counters: [42, 128, ]

  Agent J
  Agent K
  Zed
  Griffin
  Roman the Fabulist

 planets_groups :
  ---
   Mercury
   Venus
   Earth
   Mars

   Jupiter
   Saturn
   Uranus
   Neptune


  ---
   Titan
   Ganymede

   Eris
   Titania

associative container example 

example/map_like

#include <iostream>
#include <map>
#include <sstream>
#include <unordered_map>

#include "struct_mapping/struct_mapping.h"

struct Library {
 std::unordered_map<std::string, std::multimap<std::string, int>> counters;
 std::multimap<std::string, std::unordered_multimap<std::string, std::string>> books;
};

int main() {
 struct_mapping::reg(&Library::counters, "counters");
 struct_mapping::reg(&Library::books, "books");

 Library library;

 std::istringstream json_data(R"json(
  {
   "counters": {
    "first": {
     "112": 13,
     "142": 560,
     "112": 0
    },
    "second": {
     "2": 28,
     "20": 411
    },
    "third": {
    }
   },
   "books": {
    "asd": {
     "Leo": "aaa",
     "Leo": "bbb",
     "Mark": "ccc"
    },
    "wwert": {
     "Gogol": "ddd",
     "Tom": "eee"
    }
   }
  }
 )json");

 struct_mapping::map_json_to_struct(library, json_data);

 std::cout << "library:" << std::endl;

 std::cout << " counters :" << std::endl;
 for (auto [n1, v] : library.counters) {
  std::cout << "  " << n1 << " : ";
  for (auto [n2, c] : v) {
   std::cout << "[" << n2 << ", " << c << "], ";
  }
  std::cout << std::endl;
 }

 std::cout << " books :" << std::endl;
 for (auto [n1, v] : library.books) {
  std::cout << "  " << n1 << " : ";
  for (auto [n2, b] : v) {
   std::cout << "[" << n2 << ", " << b << "], ";
  }
  std::cout << std::endl;
 }

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

result

library:
 counters :
  third : 
  second : [2, 28], [20, 411], 
  first : [112, 13], [112, 0], [142, 560], 
 books :
  asd : [Mark, ccc], [Leo, bbb], [Leo, aaa], 
  wwert : [Tom, eee], [Gogol, ddd],

Mapping options 

When registering a data member, you can specify one or more options that will customize the mapping.

Bounds 

Sets the range of values in which (including the limits of the range) the value to be set is located. Applicable for integer types and floating point types. The option accepts two parameters - the range limits. Throws an exception when the set during the mapping of the value goes out of bounds.

Bounds{lower, upper}

Example of setting an option:

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

Sets the default value for the data member. Applicable for bool, integer types, floating point types, strings, containers, CPP structures, and enumerations. The option accepts one parameter - the default value.

Default{default_value}

Example of setting an option:

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

Notes that a data member cannot be set to an empty value. Applicable for strings and containers. The option does not accept parameters. Throws an exception if, after completion of the mapping, the field value is an empty string or an empty container.

Example of setting an option:

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

Notes that a data member must be set to a value. Applicable for bool, integer types, floating point types, strings, containers, CPP structures, and enumerations. The option does not accept parameters. Throws an exception if, after completion of the mapping, value for the field has not been set.

Example of setting an option:

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

example/options

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

#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;
 std::set<int> ids;
 std::unordered_set<std::string> tools;

 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;
  os << "ids: " << std::endl;
  for (auto& i : o.ids) os << " " << i << std::endl;
  os << "tools: " << std::endl;
  for (auto& t : o.tools) os << " " << t << 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"}});
 sm::reg(&Spacecraft::ids, "ids", sm::Default{std::set<int>{14, 159}});
 sm::reg(&Spacecraft::tools, "tools", sm::NotEmpty{});

 Spacecraft starship;

 std::istringstream json_data(R"json(
 {
  "in_development": false,
  "name": "Vostok",
  "stages": {
   "first": {
    "engine_count": 31,
    "fuel": "compressed gas",
    "length": 70
   },
   "second": {}
  },
  "tools": [
   "Reverberating Carbonizer With Mutate Capacity",
   "Noisy Cricket",
   "The Series 4 De-Atomizer"
  ]
 }
 )json");

 sm::map_json_to_struct(starship, json_data);

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

result

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
ids: 
 14
 159
tools: 
 The Series 4 De-Atomizer
 Noisy Cricket
 Reverberating Carbonizer With Mutate Capacit

Reverse mapping of c++ structure to JSON 

For the structure to be mapped back to JSON, it is necessary to register all data members of all the structures that need to be mapped using for each field

template<typename T, typename V, typename ... U, template<typename> typename ... Options>
inline void reg(V T::* ptr, std::string const & name, Options<U>&& ... options);
  • ptr - pointer to data member
  • name - member name
  • options - mapping options

and call the function

template<typename T>
void map_struct_to_json(T & source_struct, std::basic_ostream<char> & json_data, std::string indent);
  • source_struct - reference to the source structure
  • json_data - reference to JSON data output stream
  • indent - indentation (if set, makes the output format better readable)

example/struct_to_json

#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;
}

result

{
  "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
        ]
      }
    ]
  }
}

Registration of data members combined with initialization 

In order not to take out the registration of data members from the structure, registration can be combined with initialization

example/in_struct_reg

#include <iostream>
#include <sstream>
#include <vector>

#include "struct_mapping/struct_mapping.h"

struct Planet {
 bool giant = [] {struct_mapping::reg(&Planet::giant, "giant"); return false;} ();
 long long surface_area = [] {struct_mapping::reg(&Planet::surface_area, "surface_area"); return 0;} ();
 double mass = [] {struct_mapping::reg(&Planet::mass, "mass"); return 0;} ();
 std::vector<std::string> satellites = [] {
  struct_mapping::reg(&Planet::satellites, "satellites"); return std::vector<std::string>{};} ();
};

int main() {
 std::istringstream json_data(R"json(
  {
   "giant": false,
   "surface_area": 510072000000000,
   "mass": 5.97237e24,
   "satellites": ["Moon", "R24"]
  }
 )json");

 Planet earth;

 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    : [ ";
 for (auto & s : earth.satellites) std::cout << s << ", ";
 std::cout << "]" << std::endl;
}

To simplify the use of this method, three macros are defined

  • BEGIN_STRUCT
  • MEMBER
  • END_STRUCT

BEGIN_STRUCT

Defines the begin of the structure and its name

BEGIN_STRUCT(name)

BEGIN_STRUCT(Planet)

MEMBER

Adds a data member, registers it, and initializes it with a default value:

MEMBER(type, name)

MEMBER(bool, giant)
MEMBER(long long, surface_area)
MEMBER(double, mass)
MEMBER(std::vector<std::string>, satellites)

END_STRUCT

Defines the end of the structure

END_STRUCT

END_STRUCT

example/macro_reg

#include <iostream>
#include <sstream>
#include <vector>

#include "struct_mapping/struct_mapping.h"

BEGIN_STRUCT(Planet)

MEMBER(bool, giant)
MEMBER(long long, surface_area)
MEMBER(double, mass)
MEMBER(std::vector<std::string>, satellites)

END_STRUCT

int main() {
 std::istringstream json_data(R"json(
  {
   "giant": false,
   "surface_area": 510072000000000,
   "mass": 5.97237e24,
   "satellites": ["Moon", "R24"]
  }
 )json");

 Planet earth;

 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    : [ ";
 for (auto & s : earth.satellites) std::cout << s << ", ";
 std::cout << "]" << std::endl;
}

Exceptions 

StructMapping throws a StructMappingException exception during the mapping process

  • when setting a value for an unregistered field (in JSON, the name of the value in the object does not correspond to any of the registered fields in the c++ structure)
  • when setting a value whose type does not match the type of field (for enumerations when setting values other than a string)
  • (for numerical fields) when setting a value that is outside the range of the field type
  • when using an enumeration, if functions for converting to/from a string were not specified for it
  • when setting a value that exceeds the limits set by the Bounds option
  • with an empty string field, if the NotEmpty option was set for it
  • if the Required option was set, but the value was not set
  • when setting the Bounds option to a value that is outside the range of values for the field type
  • when setting the Bounds option to values when the lower bound value is greater than the upper bound value
  • when setting the Default option to a value that is outside the range of values for the field type
  • when reaching the end of the JSON stream, when parsing is not completed (the process of parsing an object, value, etc.)
  • when a character is received in the JSON stream that is not valid at this position (for example, an array termination character, if the beginning character of the array was not received earlier)
  • when conversion of JSON string representing number to number fails

Project on GitHub

English translation provided by translate.google.com

Topics:
c++, json, json serialization

Opinions expressed by DZone contributors are their own.

{{ parent.title || parent.header.title}}

{{ parent.tldr }}

{{ parent.urlSource.name }}