How to deep copy json_object in C++ ?

[daiziyi]

hi, I use 'json-c' in C++ and define json_object* mJson as a member variable of class Response. (Defined the code below)

In member function bool Response::GetJson(json_object **result) , I get json_object * pointer with passing param json_object **result , and then operate json_object *.

But I want to deep copy of json_object * in member functin bool Response::GetJson(json_object *result). I did not found 'releated copy funcion' in json-c api.
How can i do it? Or Is it necessary to deep copy of json_object * ? Thank you!

Here is my code:

/* File: response.h */
/* how to deep copy of json_object* ?  */

#ifndef RESPONSE_H
#define RESPONSE_H

#include <string>
#include <json-c/json.h>

class Response
{
private:
    std::string mStr;
    json_object *mJson;

public:
    Response(std::string str);
    ~Response();
    bool Str2Json();
    bool GetJson(json_object **result);

};

#endif // RESPONSE_H

/* File:response.cpp */
#include "response.h"
#include <iostream>

Response::Response(std::string str)
{
    mStr = str;
    //mJson = json_object_new_object();  /* 'json_tokener_parse_verbose' does not need 'new_object' ?*/
    mJson = NULL;   

}

Response::~Response()
{
    json_object_put(mJson);
    std::cout << "Destruct function: Response Class" << std::endl;
}


bool Response::Str2Json()
{
    bool result = true;
    enum json_tokener_error jerr = json_tokener_success;
    mJson = json_tokener_parse_verbose( mStr.c_str(), &jerr );

    if(  jerr != json_tokener_success ){
        result = false;
    }

    //std::cout << mJson._ref_count << std::endl;  //ERROR
    return result;

}

bool Response::GetJson(json_object **result)
{
    *result = mJson;
}



/*****************TEST*******************/
int main(int argc, char *argv[])
{
    std::string str = "{\"Name\":\"Tom\", \"Age\":18, \"Address\":\"New York City\"}";
    json_object *json;
    //json = json_object_new_object();
    bool result = false;

    Response response(str);
    result = response.Str2Json();
    if( !result ){
        std::cout << "Parse string to json error!" << std::endl;
        return -1;
    }

    response.GetJson(&json);
    std::cout << json_object_get_string(json) << std::endl;
    json = NULL;
    //json_object_put(json);

    return 0;
}
/***************************************/

[hawicz]

https://groups.google.com/forum/#!topic/json-c/FJ8Z18ExbW4

[hawicz]

There is currently no copy function available, but it could be implemented with json_c_visit. A pull request to add this would be welcome.

As a quick, though rather inefficient, hack people have converted their object to a string then re-parsed it to get an independent object.
Note that if you don't actually need the object to be entirely independent (e.g. if you know your use-case is read-only) then you can simple increase the reference count with json_object_get() and return the original object, which will be appropriately freed once all references are released.

[daiziyi]

Thanks to hawicz's detailed answer! And i will implement the Copy Function with your advice and blog. In addition, let me know another web site about ‘json-c’ "https://groups.google.com/forum/#!forum/json-c". Thank you!

[hawicz]

@commodo has written a deep copy function that walks the object tree instead of converting to/from a string. It's not yet merged, but I expect that will happen relatively soon once a few issues with that code are addressed, so i'm closing this issue. See PR #328 for future updates.