// Usage:  std::cout << " "_join("Hello", "World") << std::endl;
#include <string_view>
#include <string>
#include <tuple>
#include <iostream>
class Joinit
{
    public:
        Joinit(std::string_view sv): join(sv)
        {}

        template<typename... Args>
        auto operator()(Args&&... args)
        {
            auto svs = std::make_tuple(std::string_view{std::forward<Args>(args)}...);
            auto len = [](auto... args){ return (args.length() + ...);};
            std::string buffer;
            buffer.reserve(std::apply(len, svs));
            auto joined_string = [&](auto... args){ return concat(buffer, args...); };
            return std::apply(joined_string, svs);
        }
    private:
        auto& concat(std::string& str, std::string_view sv)
        {
            return str += sv;
        }
        template<typename... Args>
        auto& concat(std::string& str, std::string_view sv, Args... args)
        {
            return concat(str.append(sv).append(this->join), args...);
        }
        std::string join;
};

auto operator"" _join(const char* buffer, size_t size)
{
    return Joinit{{buffer, size}};
}