Getting cURL JSON response C++ - c++

Question

I would like to know how to get a JSON response with cURL. My current code doesn't work, it just returns an empty string. Any help would be greatly appreciated! I've included my current code below.
static size_t WriteCallback(void* contents, size_t size, size_t nmemb, void* userp)
{
((std::string*)userp)->append((char*)contents, size * nmemb);
return size * nmemb;
}
int ipCheck(std::string ip)
{
nlohmann::json j;
CURL* curl;
CURLcode res;
std::string response;
struct curl_slist* headers = NULL;
headers = curl_slist_append(headers, "Accept: application/json");
headers = curl_slist_append(headers, "Content-Type: application/json");
curl = curl_easy_init();
if (curl) {
curl_easy_setopt(curl, CURLOPT_URL, "http://api.ipstack.com/" + ip + "?access_key=SECRET_KEY");
curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, WriteCallback);
curl_easy_setopt(curl, CURLOPT_WRITEDATA, response);
res = curl_easy_perform(curl);
curl_easy_cleanup(curl);
}
if (response.empty()) {
std::cout << "\n\nInvalid request!";
_getch();
}
std::cout << response;
}

Answer 1

you have an error here :
curl_easy_setopt(curl, CURLOPT_WRITEDATA, response);
you are passing a temporary copy of std::string here instead of a pointer to the variable you expect to be passed to the callback so the original variable isn't touched ! worse : the callback is passed a dangling pointer which is the casted to std::string and operated on (undefined behavior) so expect to see strange things (usually crash)