How can a type be removed from a template parameter pack?
c++, c++11, template-meta-programming, templates, variadic-templates
Solution
The following provides a non-recursive and direct way to remove `T` from `Ts...` and, like Jarod42's solutions, yields a `std::tuple<Us...>` but without the need to use `typename ...::type`:
#include <tuple>
#include <type_traits>
template<typename...Ts>
using tuple_cat_t = decltype(std::tuple_cat(std::declval<Ts>()...));
template<typename T, typename...Ts>
using remove_t = tuple_cat_t<
typename std::conditional<
std::is_same<T, Ts>::value,
std::tuple<>,
std::tuple<Ts>
>::type...
>;
int main()
{
static_assert(std::is_same<
remove_t<int, int, char, int, float, int>,
std::tuple<char, float>
>::value, "Oops");
}
Live example
Problem
I'm searching for a way to remove (let's say for now all occurences of) a type from a template parameter pack. The end result would be a struct that looked like this : ``` template<typename T, typename...Ts> struct RemoveT { using type = /* a new type out of Ts that does not contain T */ } ``` Let's say that the marginal case `RemoveT<int, int>` would be handled by returning `void` (not handled in the code that follows). My initial design looks like this: ``` // -------------------------------------------------------------- // 1. A "way" of typedefing variadic number of types ------------ template<typename...Ts> struct pack { using type = Ts; }; // -------------------------------------------------------------- // -------------------------------------------------------------- template<typename T, typename...Ts> struct RemoveT; template<typename T, typename T1, typename...Ts> struct RemoveT { using type = typename pack<T1, typename RemoveT<T, Ts...>::type>::type; }; template<typename T, typename T1> struct RemoveT<T, T1> { using type = T1; }; template<typename T, typename...Ts> struct RemoveT<T, T, Ts...> { using type = typename RemoveT<Ts...>::type; }; // -------------------------------------------------------------- ``` Now I can't even begin to test this code because the `pack` structure is not valid C++ Reiteration Just in case this is helpfull for an answer, some other thoughs on solving it - One could argue that `pack` is not even useful at all. We could instead move around the `RemoveT` structure, creating a new `RemoveT` that only contains the types needed. The problem then transforms in extracting the types out of the struct - We could create type pairs that mimic the behaviour of typelists and take a more recursive approach on this. Bottom Line For variadic types `Ts` and a type `T`: Can I create `Us` out of `Ts` ommiting `T` ?