Json-cpp - how to initialize from string and get string value?

Question

My code below crashes(Debug Error! R6010 abort() has been called). Can you help me? I'd also would like to know how to initialize the json object from a string value.

Json::Value obj;
obj["test"] = 5;
obj["testsd"] = 655;
string c = obj.asString();

Answer 1

Hello it is pretty simple:

1 - You need a CPP JSON value object (Json::Value) to store your data

2 - Use a Json Reader (Json::Reader) to read a JSON String and parse into a JSON Object

3 - Do your Stuff :)

Here is a simple code to make those steps:

#include <stdio.h>
#include <jsoncpp/json/json.h>
#include <jsoncpp/json/reader.h>
#include <jsoncpp/json/writer.h>
#include <jsoncpp/json/value.h>
#include <string>

int main( int argc, const char* argv[] )
{

    std::string strJson = "{\"mykey\" : \"myvalue\"}"; // need escape the quotes

    Json::Value root;   
    Json::Reader reader;
    bool parsingSuccessful = reader.parse( strJson.c_str(), root );     //parse process
    if ( !parsingSuccessful )
    {
        std::cout  << "Failed to parse"
               << reader.getFormattedErrorMessages();
        return 0;
    }
    std::cout << root.get("mykey", "A Default Value if not exists" ).asString() << std::endl;
    return 0;
}

To compile: g++ YourMainFile.cpp -o main -l jsoncpp

I hope it helps ;)

Answer 2

Json::Reader is deprecated. Use Json::CharReader and Json::CharReaderBuilder instead:

std::string strJson = R"({"foo": "bar"})";

Json::CharReaderBuilder builder;
Json::CharReader* reader = builder.newCharReader();

Json::Value json;
std::string errors;

bool parsingSuccessful = reader->parse(
    strJson.c_str(),
    strJson.c_str() + strJson.size(),
    &json,
    &errors
);
delete reader;

if (!parsingSuccessful) {
    std::cout << "Failed to parse the JSON, errors:" << std::endl;
    std::cout << errors << std::endl;
    return;
}

std::cout << json.get("foo", "default value").asString() << std::endl;

Kudos to p-a-o-l-o for their answer here: Parsing JSON string with jsoncpp

Answer 3

You can avoid using Json::CharReader and Json::CharReaderBuilder by using stringstream instead.

#include <string>
#include <sstream>
#include "jsoncpp/json/json.h"
    
int main() {
        std::string strJson = "{\"mykey\" : \"myvalue\"}"; 
        Json::Value obj;
    
        // read a JSON String
        stringstream(strJson) >> obj;
    
        // get string value
        std::string value1 = obj["mykey"].asString();

        // or to get a default value if it isn't set
        std::string value2 = obj.get("mykey", "...").asString();
        return 0;
}