void vk::account::login()
{
    if (this->username != "" && this->password != "")
    {
        string fields = "grant_type=password&client_id=" + to_string(this->client_id) + "&client_secret=" + this->client_secret + "&username=" + this->username + "&password=" + this->password + "&scope=notify,friends,photos,audio,video,docs,notes,pages,status,offers,questions,wall,groups,messages,notifications,stats,ads,offline";
        string nwresp = vk::net::GetNWPOSTResponse("https://oauth.vk.com/token", fields);
        if (nwresp[0] == '{')
        {
            size_t pos = nwresp.find("access_token");
            if (pos != string::npos)
            {
                json::value val;
                parse(nwresp, val);
                this->a_t = val["access_token"].as<string>();
            }
            else throw string("vk::account::login error: Bad login.");
        }
        else throw string("vk::account::login error: No { symbol found; response not JSON object.");
    }
    else throw string("vk::account::login error: No login/password found.");
}

void vk::account::login(string username, string password)
{
    if (username != "" && password != "")
    {
        string fields = "grant_type=password&client_id=" + to_string(this->client_id) + "&client_secret=" + this->client_secret + "&username=" + username + "&password=" + password + "&scope=notify,friends,photos,audio,video,docs,notes,pages,status,offers,questions,wall,groups,messages,notifications,stats,ads,offline";
        string nwresp = vk::net::GetNWPOSTResponse("https://oauth.vk.com/token", fields);
        if (nwresp[0] == '{')
        {
            size_t pos = nwresp.find("access_token");
            if (pos != string::npos)
            {
                json::value val;
                parse(nwresp, val);
                this->a_t = val["access_token"].as<string>();
            }
            else throw string("vk::account::login error: Bad login.");
        }
        else throw string("vk::account::login error: No { symbol found; response not JSON object.");
    }
    else throw string("vk::account::login error: No login/password found.");
}