https://gist.github.com/owickstrom/3218376

#include <iostream>
#include <curl/curl.h>
#include <math.h>
#include <vector>
#include <future>
#include <boost/format.hpp>

#define logbase(a, b) (log(a)/log(b))

using namespace std;
using boost::format;

const char *fileSizes[] = {"byte", "K", "MB", "GB", "TB"};
const int numFileSizes = 5;

// adds chunk size to sum
size_t addChunkSize(char *ptr, size_t size, size_t nmemb, void *data)
{
    auto acc = (long *)data;
    auto chunkSize = size * nmemb;
    *acc += chunkSize;
    return chunkSize;
}

// returns a formatted string (number of bytes, K, MB, GB or TB)
string getSizeString(long fsize)
{
    int fmtIndex;
    format fmter;
    
    if(fsize == 0 || (fmtIndex = (int)floor(logbase(fsize, 1024))) == 0)
    {
        fmter = format("%ld %s") % fsize % fileSizes[0];
    }
    else
    {
        fmtIndex = (fmtIndex >= numFileSizes) ? numFileSizes : fmtIndex;
        fmter = format("%.2f %s")
            % (fsize / pow(1024, fmtIndex))
            % fileSizes[fmtIndex];
    }
    return fmter.str();
}

void usage(string name)
{
    cout << "Usage: " << name << " url1 url2 url3 ...\n";
}

int main(int argc, const char * argv[])
{
    long acc = 0L;
    auto results = vector<future<long>>();
    
    // if no urls
    if(argc == 1)
    {
        usage(string(argv[0]));
        exit(1);
    }
    
    for(int i = 1; i < argc; i++)
    {
        const char *url = argv[i];
        
        // create future using a lambda and push it to the result vector
        results.push_back(async(launch::async,
        [url]() -> long
        {
            CURL *curl;
            CURLcode res;
            long siteSize = 0;
            
            curl = curl_easy_init();
            
            if(curl) {
                // setup
                curl_easy_setopt(curl, CURLOPT_URL, url);
                // pass the siteSize pointer along (received by addChunkSize)
                curl_easy_setopt(curl, CURLOPT_WRITEDATA, &siteSize);
                // callback for downloaded chunks
                curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, addChunkSize);
                
                // make the request
                res = curl_easy_perform(curl);
                
                // something failed
                if(res != CURLE_OK)
                {
                    cerr    << "\rWebsite size fetch failed for " << url << ": "
                            << curl_easy_strerror(res) << "\n";
                    siteSize = 0;
                }
                // successful
                else
                {
                    cerr    << "\rSize of " << url << ": "
                            << getSizeString(siteSize) << "\n";
                }
                
                // cleanup
                curl_easy_cleanup(curl);
            }
            
            return siteSize;
        }));
    }
    
    // gather results from futures
    for(auto &result : results)
    {
        acc += result.get();
    }
    
    cerr << "\nTotal size of websites: " << getSizeString(acc) << "\n";
    return 0;
}