Performant C++17 implementation of Python's join function

join.cpp

#include <string_view>
#include <string>
#include <tuple>

class Joint {
    std::string _j;

    auto& _concat(std::string& s, std::string_view sv) { return s += sv; }

    template<typename... Args>
    auto& _concat(std::string& s, std::string_view sv, Args... args) {
        return _concat(s.append(sv).append(_j), args...);
    }

public:
    Joint(std::string_view sv) : _j{sv} {}

    template<typename... Args>
    auto operator()(Args&&... args) {
        auto svs = std::make_tuple(std::string_view{std::forward<Args>(args)}...);
        std::string s;

        auto len = [](auto... args) { return ( args.length() + ... ); };
        s.reserve(std::apply(len, svs));

        auto concat = [&](auto... args) { return _concat(s, args...); };
        return std::apply(concat, svs);
    }
};

auto operator"" _join(const char* arr, size_t sz) { return Joint{{arr, sz}}; }


// Usage:
// ","_join("Hello", "world", "!")    // returns: std::string{"Hello,world,!"}