c++ - libcurl how to request HTTPS in cpp -

curl *curl; curlcode res;  curl_global_init(curl_global_default); curl = curl_easy_init();   if (curl) {     string html;     curl_easy_setopt(curl, curlopt_url, "https://www.google.com");     curl_easy_setopt(curl, curlopt_followlocation, 1l);     curl_easy_setopt(curl, curlopt_writedata, &html);     curl_easy_setopt(curl, curlopt_writefunction, writer);     curl_easy_setopt(curl, curlopt_cainfo, "d:\\certification\\digicerthighassuranceevrootca.crt");      res = curl_easy_perform(curl);      cout << "error : " << curl_easy_strerror(res) << endl;      cout << html; 

the error message unsupported protocol

the http site okay https site doesn't work

what should add in curl_easy_setopt()