overload.hpp

// @copyright Louis Dionne 2013-2017
// Distributed under the Boost Software License, Version 1.0.
// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)

template <typename F, typename ...G>
struct overload_t
    : overload_t<F>::type
    , overload_t<G...>::type
{
    using type = overload_t;
    using overload_t<F>::type::operator();
    using overload_t<G...>::type::operator();

    template <typename F_, typename ...G_>
    constexpr explicit overload_t(F_&& f, G_&& ...g)
        : overload_t<F>::type(static_cast<F_&&>(f))
        , overload_t<G...>::type(static_cast<G_&&>(g)...)
    { }
};

template <typename F>
struct overload_t<F> { using type = F; };

template <typename R, typename ...Args>
struct overload_t<R(*)(Args...)> {
    using type = overload_t;
    R (*fptr_)(Args...);

    explicit constexpr overload_t(R (*fp)(Args...))
        : fptr_(fp)
    { }

    constexpr R operator()(Args ...args) const
    { return fptr_(static_cast<Args&&>(args)...); }
};

struct make_overload_t {
    template <typename ...F,
        typename Overload = typename overload_t<
            typename std::decay<F>::type...
        >::type
    >
    constexpr Overload operator()(F&& ...f) const {
        return Overload(static_cast<F&&>(f)...);
    }
};

constexpr make_overload_t overload{};