gistfile1.txt

// This simple variant gets the job done without
// causing too much trouble with template depth:
//
// * Always allows an empty state I==0
// * emplace() and get() support 1-based indexes only
// * Basic exception guarantee
// * Max 255 types
//
template<class... TN>
class variant
{
    std::array<char, max_sizeof<TN...>()> buf_;
    unsigned char i_ = 0;

    template<std::size_t I>
    using type = typename std::tuple_element<
        I , std::tuple<TN...>>::type;

    template<std::size_t I>
    using C = std::integral_constant<std::size_t, I>;

public:
    variant() = default;

    ~variant()
    {
        if(n_)
            destroy(C<0>{});
    }

    template<std::size_t I, class... Args>
    void
    emplace(Args&&... args)
    {
        if(n_ != 0)
            destroy(C<0>{});
        n_ = 0;
        new(buf_.data()) type<I>(
            std::forward<Args>(args)...);
        n_ = I;
    }

    template<std::size_t I>
    type<I-1>&
    get()
    {
        BOOST_ASSERT(I == n_ - 1);
        return *reinterpret_cast<
            type<I>*>(buf_.data());
    }

    template<std::size_t I>
    type<I-1>&
    get() const
    {
        BOOST_ASSERT(I == n_ - 1);
        return *reinterpret_cast<
            type<I> const*>(buf_.data());
    }

private:
    void
    destroy(C<sizeof...(TN)>)
    {
        return;
    }

    template<std::size_t I>
    void
    destroy(C<I>)
    {
        if(I == n_ - 1)
        {
            type<I>.~type<I>();
            return;
        }
        destroy(C<I+1>{});
    }
};