Converting C++ class to JSON

Question

I\'d like to create a JSON string containing the instance variables of my class.

For example,

class Example {  
    std::string string;  
    std::ma         


        

Answer 1

You could use Boost.PropertyTree.

#include <map>
#include <vector>
#include <string>
#include <iostream>
#include <boost/property_tree/ptree.hpp>
#include <boost/property_tree/json_parser.hpp>

namespace pt = boost::property_tree;

int main() {
    // Create an empty property tree object.
    pt::ptree tree;

    // Put a string value into the tree.
    tree.put("string", "the-string-value");

    // Put a map object into the tree.
    pt::ptree child1;
    std::map<std::string, std::string> map = {{"key1", "val1"},
                                              {"key2", "val2"}};
    for (auto &p : map) {
        child1.add(p.first, p.second);
    }
    tree.add_child("map", child1);

    // Put a vector of numbers into the tree
    pt::ptree child2;
    std::vector<int> vector = {1, 2, 3, 4};
    for (auto &v : vector) {
        pt::ptree item;
        item.put("", v);
        child2.push_back(std::make_pair("", item));
    }
    tree.add_child("vector", child2);

    // Write property tree to JSON file
    pt::write_json("output.json", tree);

    return 0;
}

Output:

{
    "string": "the-string-value",
    "map": {
        "key1": "val1",
        "key2": "val2"
    },
    "vector": [
        "1",
        "2",
        "3",
        "4"
    ]
}

Answer 2

JSON Spirit would allow you to do it like so:

Object addr_obj;

addr_obj.push_back( Pair( "house_number", 42 ) );
addr_obj.push_back( Pair( "road",         "East Street" ) );
addr_obj.push_back( Pair( "town",         "Newtown" ) );

ofstream os( "address.txt" );
os.write( addr_obj, os, pretty_print );
os.close();

Output:

{
    "house_number" : 42,
    "road" : "East Street",
    "town" : "Newtown"
}

The json_map_demo.cpp would be a nice place to start, I suppose.

Answer 3

Have you looked at cereal (http://uscilab.github.io/cereal/) ? It has JSON archives for serializing to/from JSON using C++.

An example with minimal overhead (from cereal) can be found here on SO: https://stackoverflow.com/a/22587527/255635

Answer 4

Do you want to JSON-ify a map or an object? (your example shows a class, yet you say a map). For a map, check out this library - JSON Spirit.

For objects: There is no reflection support in C++ (apart from the very limited RTTI), so there is no "one-click" solution for serialization either. Any solution will require you to write additional, possibly tightly coupled code to the class you want to serialize and de-serialize (that depends on if you want to serialize non-public data).

Answer 5

Any good C++ JSON library should do this and it is sad to see that they don't -- with the exception of ThorsSerializer and apparently Nosjob as mentioned in this question.

Of course, C++ does not have reflection like Java, so you have to explicitly annotate your types:
(copied from the ThorsSerializer documentation)

#include "ThorSerialize/JsonThor.h"
#include "ThorSerialize/SerUtil.h"
#include <map>
#include <vector>
#include <string>
#include <iostream>

class Example {
    std::string string;
    std::map<std::string, std::string> map;
    std::vector<int> vector;

    // Allow access to the class by the serialization library.
    friend class ThorsAnvil::Serialize::Traits<Example>;

    public:
        Example(std::string const& s, std::map<std::string, std::string> const& m, std::vector<int> const& v)
            : string(s), map(m), vector(v)
        {}
};

// Define what members need to be serilizable
ThorsAnvil_MakeTrait(Example, string, map, vector);

Example Usage:

int main()
{
    using ThorsAnvil::Serialize::jsonExport;
    using ThorsAnvil::Serialize::jsonImport;


    Example     e1 {"Some Text", {{"ace", "the best"}, {"king", "second best"}}, {1 ,2 ,3, 4}};

    // Simply serialize object to json using a stream.
    std::cout << jsonExport(e1) << "\n";

    // Deserialize json text from a stream into object.
    std::cin  >> jsonImport(e1);
}

Running:

{
    "string": "Some Text",
    "map":
    {
        "ace": "the best",
        "king": "second best"
    },
    "vector": [ 1, 2, 3, 4]
}

You cannot do better than this in C++.

Answer 6

If the question is still actual, then look at json_dto library, a small header-only helper for converting data between JSON representation and c++ structs.

For example having the following structs:

struct message_source_t
{
  // Worker thread.
  std::int32_t m_thread_id;

  // Sender.
  std::string m_subsystem;
};

struct message_t
{
  // Who sent a message.
  message_source_t m_from;

  // When the message was sent (unixtime).
  std::tm m_when;

  // Message text.
  std::string m_text;
};

with the help of json_dto you can create the following JSON:

{
  "from" : 
    {
      "thread_id" : 4242,
      "sybsystem" : "json_dto"
    },
  "when" : "2016.09.28 19:55:00",
  "text" : "Hello world!"
}  

And given such JSON string you can convert it to structs.