MichHash.cc

#pragma once

#include <city.h>
#include <sstream>

void michHashInternal(std::stringstream& ss){}

template<typename first, typename... args>
void michHashInternal(std::stringstream& ss, first&& f, args&&... a){
    ss << std::forward<first>(f);
    michHashInternal(ss, std::forward<args>(a)...);
}

template<typename... args>
uint64 michHash(args&&... a){
    std::stringstream ss;
    michHashInternal(ss, std::forward<args>(a)...);
    std::string str = ss.str();
    return CityHash64(str.c_str(), str.length());
}

get rid of first on michHash.
argument passing should be either be std::forwarded (so add a && to the arguments and std::forward(f) on access or do it by const&. For both functions.
Get rid of strlen. You already have the size in ss.str() so avoid recomputing it.