Dynamic JSON message in c++;

Question

I successfully send to a REST API a static message following this answer, however when I try to build my dynamic JSON message I have a problem of an extra character that is automatically adding the parsing function in c++, so at the server side, I got an error.

This is the minimal working code of the C++ client that sends the dynamic message:

#include <curl/curl.h>
#include <iostream> //cout
#include <sstream>      // std::ostringstream


using namespace std;

int main (int argc, char *argv[]) {
    CURL *curl;
    CURLcode res;

    curl_global_init(CURL_GLOBAL_ALL);
    curl = curl_easy_init();
    if (curl == NULL) {
        return 128;
    }
    int x,y,z=0;

    for (int var = 0; var < 10; ++var) {

        std::ostringstream ssright;
                   ssright<<"{ \"camID\" : \"";
                   ssright<<22;
                   ssright<<"\" , \"x\" : \"";
                   ssright<<x;
                   ssright<<"\" , \"y\" : \"";
                   ssright<<y;
                   ssright<<"\" , \"z\" : \"";
                   ssright<<z;
                   ssright<<"\" }";


       const char* jsonObj = ssright.str().c_str();

x+=100;    y+=15;    z+=500;
std::cout<<ssright.str().c_str()<<std::endl;
std::cout<<jsonObj<<std::endl;

        struct curl_slist *headers = NULL;
        headers = curl_slist_append(headers, "Accept: application/json");
        headers = curl_slist_append(headers, "Content-Type: application/json");
        headers = curl_slist_append(headers, "charsets: utf-8");

        curl_easy_setopt(curl, CURLOPT_URL, "http://192.168.4.7:3000/createEmp");

        curl_easy_setopt(curl, CURLOPT_CUSTOMREQUEST, "POST");
        curl_easy_setopt(curl, CURLOPT_HTTPHEADER, headers);
        curl_easy_setopt(curl, CURLOPT_POSTFIELDS,jsonObj);
        //curl_easy_setopt(curl, CURLOPT_POSTFIELDS, ssright.str().c_str());
        //curl_easy_setopt(curl, CURLOPT_USERAGENT, "camera");

        res = curl_easy_perform(curl);


    }

    curl_easy_cleanup(curl);
    curl_global_cleanup();
    return res;
}

When the server receives this message I got an Unexpected token error, which I think is the end of line token.

SyntaxError: Unexpected token �

If I send the message like this:

const char* jsonObj = "{ \"camID\" : \"22\" , \"x\" : \"0\" , \"y\" : \"0\" , \"z\" : \"0\" }";

The JSON is successfully received at the server.

I also tried to send it like this, but is not working either:

const char* jsonObj = ssright.str().substr(0,ssright.str().size()-1).c_str();

I tried to eliminate the last character of my string, but it keeps adding the end of the line. ssright.str().substr(0,ssright.str().size()-1).c_str() but is eliminating the '}' and still sending the end of the line.

My question is there is a way to create a dynamic String that does not add the end of the line character? (if this Unexpected token is the end of line character).

You print something to std::cout. Showing it certainly would help. — el.pescado - нет войне
– el.pescado - нет войне, Commented Jul 13, 2017 at 10:02
the problem was as @VTT said the creation of a dangling pointer to a temporary string buffer. and it was not an end of the line. — aburbanol
– aburbanol, Commented Jul 13, 2017 at 10:27

user7860670 · Accepted Answer · 2017-07-13 09:53:39Z

2

You are truncating last character } from json string and create a dangling pointer to a temporary string buffer.

const char* jsonObj = ssright.str().substr(0,ssright.str().size()-1).c_str();

This should be:

::std::string jsonObjStr(ssright.str());
const char * jsonObj(jsonObjStr.c_str());

answered Jul 13, 2017 at 9:53

user7860670

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2 Comments

aburbanol Over a year ago

why your answer is different to const char* jsonObj = ssright.str().c_str(); Now is working!

user7860670 Over a year ago

@aburbanol This also creates a dangling pointer to a temporary string buffer... The pointer returned by c_str() remains valid only until an object returned by str() is not destroyed or otherwise altered.

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