Empty value when converting CURL to string

I would like to get source code of a page, I have this function to populate a string.

My header file :

class Explorateur {
public:
    Explorateur();
    ~Explorateur();
    std::string RecupererCodeSource(std::string pAdresse);

    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;
    }

};

My class file :

std::string Explorateur::RecupererCodeSource(std::string pAdresse)
{
      CURL *curl;
      CURLcode res;
      std::string readBuffer;

      curl = curl_easy_init();
      if(curl)
      {
        curl_easy_setopt(curl, CURLOPT_URL, pAdresse);
        curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, WriteCallback);
        curl_easy_setopt(curl, CURLOPT_WRITEDATA, &readBuffer);
        std::cout << "curl  : " << curl << std::endl;
        res = curl_easy_perform(curl);
        std::cout << "res  : " << res << std::endl;
        curl_easy_cleanup(curl);

      }
      std::cout << "Read : " << readBuffer << std::endl;
      return readBuffer;
}

My main :

int main()
{
    Explorateur explorateur;
    std::string valeur = explorateur.RecupererCodeSource("https://www.google.com/");
    return 0;
}

The result is not excepted, it's empty, do you have an idea why ?

curl  : 0x1450d60
res  : 6
Read :