How do I terminate variadic template recursion based on the number of elements?

debugcn Published at Dev

Michael Gazonda

I've written a compile time search and find through template parameters, and it's working fine. I'm stumped about how to go about providing a default value for when there is no item found.

I've tried using sizeof...(args_t) to create a template specialization to terminate on. That isn't allowed. And so, I'm not sure how to go about doing this.

Here's what I've got right now:

template <typename... args_t> class c
{
    template <size_t pos, typename _t, typename... a_t> struct at : at<pos - 1, a_t...> { };
    template <typename _t, typename... a_t> struct at<0, _t, a_t...>
    {
        using t = _t;
    };
};

What I need is something like:

template <typename... args_t> class c
{
    template <> struct at<sizeof...(args_t)>
    {
        using t = default_value;
    };
};

So, how do I go about creating a template specialization based on the number of variadic elements?

Jarod42

You may use the following to manage case where index is outside bound:

template <typename... args_t> class c
{
    template <std::size_t pos, typename... Ts> struct at;

    template <std::size_t pos, typename T, typename... Ts>
    struct at<pos, T, Ts...> : at<pos - 1, Ts...>
    {};

    template <typename T, typename... Ts>
    struct at<0, T, Ts...>
    {
        using type = T;
    };

    template <std::size_t N>
    struct at<N>
    {
        using type = void; // default type
    };

};

Live example

Or using std::tuple and std::conditional (with lazy evaluation):

template <typename... args_t> class c
{
public:
    struct default_type { using type = void; };

    template <std::size_t pos, typename... Ts>
    struct at
    {
        using type =
            typename std::conditional<
                (pos < sizeof...(Ts)),
                std::tuple_element<pos, std::tuple<Ts...>>, // don't use ::type here
                default_type
            >::type::type; // expand type 'twice'.
    };

};

Live example

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