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StructMapping - Mapping JSON to and from a C++ Structure

bk192077

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1 May 2022

I would like to define a C++ structure, pass the person instance to the mapping method along with JSON data, then use the filled structure. Or vice versa, get Person as JSON. StructMapping is trying to solve these problems.

In this article, we look at the installation and usage of StructMapping. Consider examples of use when mapping JSON to a C++ structure and when reverse mapping structure C++ to JSON.

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StructMapping

документация на русском

English translation provided by translate.google.com

Source code: https://github.com/bk192077/struct_mapping

Introduction
Compatibility
Installation
Usage
Exceptions

Mapping json to and from a c++ structure

I would like to define a c++ structure

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

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

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map_json_to_struct(person, json_data)

then use the filled structure

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

Or vice versa

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

get Person as json

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{
 "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
Clang 12.0.0 | 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

std::optional (with any supported type)
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):

(if necessary) set the environment variables CC и CXX:

export CC=/usr/bin/gcc
export CXX=/usr/bin/g++

create directory called build in StructMapping source directory
change to build directory
run cmake .. command to configure your build
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
(not for windows) to run the tests run ctest (you can get detailed output using ctest -V)
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

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

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

create an instance

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Person person;

set json data

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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

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map_json_to_struct(person, json_data);

use

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

Completed code

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#include "struct_mapping/struct_mapping.h"

#include <iostream>
#include <sstream>
#include <string>

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::ostringstream out_json_data;
 struct_mapping::map_struct_to_json(person, out_json_data, "  ");

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

result

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{
 "name": "Jeebs",
 "age": 42,
 "student": 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

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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

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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.

Optional fields

example/optional

Any of the supported types can be used in combination with std::optional. For example:

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#include "struct_mapping/struct_mapping.h"

#include <iostream>
#include <optional>
#include <sstream>
#include <string>

struct Engine
{
 int age;
};

struct Car
{
 std::optional<Engine> engine;
 std::optional<std::string> name;
 std::optional<int> max_speed;
};

int main()
{
 struct_mapping::reg(&Engine::age, "age");

 struct_mapping::reg(&Car::engine, "engine");
 struct_mapping::reg(&Car::name, "name", struct_mapping::NotEmpty{});
 struct_mapping::reg(&Car::max_speed, "max_speed", struct_mapping::Bounds{0, 100});

 std::istringstream json_data(R"json(
 {
  "engine": 
  {
   "age": 42
  },
  "name": "Mercedes"
 }
 )json");

 Car car;

 struct_mapping::map_json_to_struct(car, json_data);

 std::ostringstream out_json_data;
 struct_mapping::map_struct_to_json(car, out_json_data, "  ", false);
 std::cout << out_json_data.str() << std::endl;
}

result

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{
 "engine": {
  "age": 42
 },
 "name": "Mercedes",
 "max_speed": null
}

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:

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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

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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";
  }
 });

or under enumeration example

Structures represented in json as strings

Structures in json can be represented as strings. To use this method, you need to set conversion methods from string to structure and vice versa using:

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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 struct
function_to_string_ - conversion function from struct to string

for example

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struct Color {
 int value;
};

struct_mapping::MemberString<Color>::set(
 [] (const std::string & value) {
  if (value == "red") return Color{1};
  if (value == "blue") return Color{2};

  throw struct_mapping::StructMappingException("bad convert '" + value + "' to Color");
 },
 [] (Color color) {
  switch (color.value) {
  case 1: return "red";
  case 2: return "blue";
  default: return "green";
  }
 }
 });

or under an example of using structures represented in json as strings

simple types example

example/simple

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#include "struct_mapping/struct_mapping.h"

#include <iostream>
#include <sstream>
#include <string>

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::ostringstream out_json_data;
 struct_mapping::map_struct_to_json(earth, out_json_data, "  ");

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

result

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{
 "giant": false,
 "surface_area": 510072000000000,
 "mass": 5.97237e+24,
 "satellite": "Moon"
}

structure example

example/struct

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#include "struct_mapping/struct_mapping.h"

#include <iostream>
#include <sstream>
#include <string>

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::ostringstream out_json_data;
 struct_mapping::map_struct_to_json(earth, out_json_data, "  ");

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

result

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{
 "president": {
  "name": "Agent K",
  "mass": 75.6
 }
}

an example of using structures represented in json as strings

example/struct_from_string

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#include "struct_mapping/struct_mapping.h"

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

namespace sm = struct_mapping;

struct Color
{
 int value;

 bool operator<(const Color& o) const
 {
  return value < o.value;
 }
};

static Color color_from_string(const std::string & value)
{
 if (value == "red")
 {
  return Color{1};
 }
 else if (value == "blue")
 {
  return Color{2};
 }

 return Color{0};
}

static std::string color_to_string(const Color& color)
{
 switch (color.value)
 {
 case 1: return "red";
 case 2: return "blue";
 default: return "green";
 }
}

struct Background
{
 Color color;
};

struct Palette
{
 Color main_color;
 Background background;
 std::list<Color> special_colors;
 std::set<Color> old_colors;
 std::map<std::string, Color> new_colors;
};

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

 sm::reg(&Palette::main_color, "main_color", sm::Default{"red"});
 sm::reg(&Palette::background, "background", sm::Required{});
 sm::reg(&Palette::special_colors, "special_colors");
 sm::reg(&Palette::old_colors, "old_colors");
 sm::reg(&Palette::new_colors, "new_colors");

 sm::reg(&Background::color, "color");

 Palette palette;

 std::istringstream json_data(R"json(
 {
  "background": {
   "color": "blue"
  },
  "special_colors": ["red", "green", "red", "blue"],
  "old_colors": ["red", "green", "blue"],
  "new_colors": {
   "dark": "green",
   "light": "red",
   "neutral": "blue"
  }
 }
 )json");

 sm::map_json_to_struct(palette, json_data);

 std::ostringstream out_json_data;
 struct_mapping::map_struct_to_json(palette, out_json_data, "  ");

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

result

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{
 "main_color": "red",
 "background": {
  "color": "blue"
 },
 "special_colors": [
  "red",
  "green",
  "red",
  "blue"
 ],
 "old_colors": [
  "green",
  "red",
  "blue"
 ],
 "new_colors": {
  "dark": "green",
  "light": "red",
  "neutral": "blue"
 }
}

Enumeration example

example/enumeration

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#include "struct_mapping/struct_mapping.h"

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

namespace sm = struct_mapping;

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

static Color ColorFromString(const std::string& value)
{
 if (value == "red")
 {
  return Color::red;
 }
 else if (value == "blue")
 {
  return Color::blue;
 }

 return Color::green;
}

static std::string ColorToString(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;
};

int main()
{
 sm::MemberString<Color>::set(ColorFromString, ColorToString);

 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::ostringstream out_json_data;
 struct_mapping::map_struct_to_json(palette, out_json_data, "  ");

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

result

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

sequence container example

example/array_like

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#include "struct_mapping/struct_mapping.h"

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

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

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

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

struct MiB
{
 std::unordered_set<Friend, FriendHash> 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::ostringstream out_json_data;
 struct_mapping::map_struct_to_json(mib, out_json_data, "  ");

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

result

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

associative container example

example/map_like

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#include "struct_mapping/struct_mapping.h"

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

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::ostringstream out_json_data;
 struct_mapping::map_struct_to_json(library, out_json_data, "  ");

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

result

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{
 "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. For std::optional, the option refers to the value that is contained in std::optional.

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Bounds{lower, upper}

Example of setting an option:

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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. For std::optional, the option refers to the value that is contained in std::optional.

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Default{default_value}

Example of setting an option:

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

When using an option for a struct for which conversion to / from string is used (structures represented in json as strings), the default value must also be a string

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sm::reg(&Palette::main_color, "main_color", sm::Default{"red"});

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. For std::optional, the option refers to the value that is contained in std::optional.

Example of setting an option:

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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. The option cannot be used for std::optional.

Example of setting an option:

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

Options example

example/options

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#include "struct_mapping/struct_mapping.h"

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

namespace sm = struct_mapping;

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

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;
};

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::ostringstream out_json_data;
 struct_mapping::map_struct_to_json(starship, out_json_data, "  ");

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

result

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{
 "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 Capacity"
 ]
}

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

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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

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template<typename T>
void map_struct_to_json(
  T& source_struct,
  std::basic_ostream<char>& json_data,
  std::string indent = "",
  bool hide_null = true);

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)
hide_null:
- true - std::optional fields for which no value is set will be hidden
- false - std::optional fields for which no value is set will have the value 'null' in json

example/struct_to_json

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#include "struct_mapping/struct_mapping.h"

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

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

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{
 "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

C++

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#include "struct_mapping/struct_mapping.h"

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

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::ostringstream out_json_data;
 struct_mapping::map_struct_to_json(earth, out_json_data, "  ");

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

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

BEGIN_STRUCT
MEMBER
MEMBER_OPTIONS
END_STRUCT

BEGIN_STRUCT

Defines the begin of the structure and its name

BEGIN_STRUCT(name)

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BEGIN_STRUCT(Planet)

MEMBER

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

MEMBER(type, name)

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

MEMBER_OPTIONS

Adds a data member and registers it. You can set field options. At least one option must be setted:

MEMBER_OPTIONS(type, name, option_1, ...)

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MEMBER_OPTIONS(
    int,
    count,
    struct_mapping::Bounds{23, 47},
    struct_mapping::Default{35})

END_STRUCT

Defines the end of the structure

END_STRUCT

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END_STRUCT

example/macro_reg

C++

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#include "struct_mapping/struct_mapping.h"

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

BEGIN_STRUCT(Planet)

MEMBER(bool, giant)
MEMBER(long long, surface_area)
MEMBER_OPTIONS(
 double,
 mass,
 struct_mapping::Bounds{1.0e23, 1.0e27},
 struct_mapping::Default{1.0e25})
MEMBER(std::vector<std::string>, satellites)

END_STRUCT

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

 Planet earth;

 struct_mapping::map_json_to_struct(earth, json_data);

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

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

Exceptions

StructMapping throws a StructMappingException exception during the mapping process

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

License

This article has no explicit license attached to it but may contain usage terms in the article text or the download files themselves. If in doubt please contact the author via the discussion board below.

A list of licenses authors might use can be found here

StructMapping - Mapping JSON to and from a C++ Structure

StructMapping

Table of contents

Mapping json to and from a c++ structure

Compatibility

Installation

Usage

Implementing a scenario with a Person structure

Mapping json to c++ structure

Optional fields

Enumeration

Structures represented in json as strings

simple types example

structure example

an example of using structures represented in json as strings

Enumeration example

sequence container example

associative container example

Mapping options

Bounds

Default

NotEmpty

Required

Options example

Reverse mapping of c++ structure to json

Registration of data members combined with initialization

BEGIN_STRUCT

MEMBER

MEMBER_OPTIONS

END_STRUCT

Exceptions

License