Der code:

#include <curl/curl.h>
        #include <cstdio>
        #include <iostream>
        #include <stdio.h>
        #include <stdlib.h>
        #include <string>
        #include <string.h>
        #include <fstream>

        using namespace std;    

        size_t write_data(void *ptr, size_t size, size_t nmemb, FILE *stream) {
         size_t written;
         written = fwrite(ptr, size, nmemb, stream);
         return written;
        }

        char* version = "0.0.0";
        void downloadFile(const char* url, const char* fname) {
         CURL *curl;
         FILE *fp;
         CURLcode res;
         curl = curl_easy_init();
         if (curl){
                fp = fopen(fname, "wb");
                curl_easy_setopt(curl, CURLOPT_URL, url);
                curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, write_data);
                curl_easy_setopt(curl, CURLOPT_WRITEDATA, fp);
                res = curl_easy_perform(curl);
                curl_easy_cleanup(curl);
                fclose(fp);
            }
        }

        string get_online_version()
        {
                char* appdata = getenv("APPDATA");
                char* newVersionPath = appdata;

                strcat( newVersionPath,"\\test\\onlineversion.txt" );

                downloadFile( "bla.tdl/update/version.txt",
                newVersionPath) ;

                string buffer;

                ifstream file(newVersionPath);
                getline(file,buffer);

                file.close();
                return buffer;

        }

        bool check_version()
        {

            string online_version = get_online_version();
            cout << "This: " << version << endl;
            cout << "New : " << online_version << endl;

            if (version < online_version){

                return true;

            }

            else {

                return false;

            }

        }

        bool update() {

                char* appdata = getenv("APPDATA");
                char* newVersionPath = appdata;

                strcat( newVersionPath,"\\test\\setup.exe" );            

                downloadFile( "bla.tdl/update/setup.exe",
                newVersionPath);

                //system(newVersionPath);

                return true;
        }

        int main()
        {
            curl_global_init(CURL_GLOBAL_ALL);

            if(!check_version()){

                cout << "Everything up to date" << endl;

                return 0;
            }
            else{
                cout << "Update verfügbar" << endl;
                cout << "Do you want to update (yes)?:";
                string line;
                getline(cin, line);
                if ( line.empty() )
                {
                    update();
                    return 0;
                }

                else if (line == "yes"){
                    update();
                    return 0;
                }

                else if (line == "no"){
                    cout << "no";
                }
            }
        }

Danke für eure Hilfe
--Crosant--