Remove the last type of a template parameter pack

Question

I want to use the types of a template parameter pack as parameters to a different template, but cut off the last parameter.

For example:

template <class... Ts> struct some_template;

template <class... Ts> struct foo
{
    using bar = some_template<magically_get_all_but_last(Ts)...>;
};

// I might be missing a few "typename"s, but you get the idea.
static_assert(std::is_same<foo<int, bool, std::string>::bar, some_template<int,bool> >::value); 

Note that this is the opposite of getting only the last parameter.

Answer 1

Here is a simple approach which use std::tuple_element<I, Tuple> together with std::index_sequence<sizeof...(Ts) - 1 to get all but the last type in a list of variadic arguments. Since the parameter pack for the indices is needed, there is an extra indirection which is put into a base but could be anywhere.

template <class T, class... Ts> struct foobase;
template <std::size_t... I, class... Ts>
struct foobase<std::index_sequence<I...>, Ts...> {
    using bar = some_template<typename std::tuple_element<I, std::tuple<Ts...>>::type...>;
};

template <class... Ts> struct foo
    : foobase<std::make_index_sequence<sizeof...(Ts) - 1>, Ts...>
{
};

Answer 2

Here is my solution which uses C++11:

template <typename ...P>
struct dummy {};

template <template <typename ...> class Obj, typename T, typename ...P>
struct internal;

template <template <typename ...> class Obj, typename ...P1, typename T, typename ...P2>
struct internal<Obj, dummy<P1...>, T, P2...>
{
    using type = typename internal<Obj, dummy<P1..., T>, P2...>::type;
};

template <template <typename ...> class Obj, typename ...P1, typename T, typename L>
struct internal<Obj, dummy<P1...>, T, L>
{
    using type = Obj<P1..., T>;
};

template <template <typename ...> class T, typename ...P>
struct subst_all_but_last
{
    using type = typename internal<T, dummy<>, P...>::type;
};

Use like this:

using bar = typename subst_all_but_last<some_template, Ts...>::type;

instead of

using bar = some_template<magically_get_all_but_last(Ts)...>;