Okay, this is a really difficult one.
I want to be able to get the subset of a parameter pack by choosing the parameter types at a given set of valid indices and then use that parameter pack as the argument list of a function. IE:
template <size_t... indices_t>
void func(pack_subset<indices_t..., args_t...>); //incorrect syntax,
//not sure how else to write this
//args_t is predetermined, this
//function is a static member of
//a variadic class template
//For a parameter pack <float, double> and index set 0
void func(float); // <- result of template
I can get the type of a single index, but a set of indices is a bit harder, especially when that set is of a variable size. The syntax for that is:
pack_index<size_t index_t, typename... args>::type
Maybe I can string a bunch of these together, but I'm not sure how to go about expanding indices_t so that I get a pack_index for each value in the list.
If you wrap the arguments into a tuple, it's pretty straight-forward:
#include <tuple>
template <typename T, std::size_t ...Is>
struct selector
{
using type = std::tuple<typename std::tuple_element<Is, T>::type...>;
};
Example input: <int, double, float, char, bool>, 1, 3
#include <iostream>
#include <demangle.hpp>
int main()
{
std::cout
<< demangle<selector<std::tuple<int, double, float, char, bool>, 1, 3>::type>()
<< std::endl;
}
Output:
std::tuple<double, char>
All you need to do is use std::tuple<args_t...> where you have just args_t... at the moment.
Here's an alternative idea for structuring that idea into something easier to handle:
template <typename ...Args> struct selector
{
using T = std::tuple<Args...>;
template <std::size_t ...Is>
static void call(typename std::tuple_element<Is, T>::type ...args)
{
// ...
}
};
Usage:
selector<int, char, bool, double>::call<0, 2>(1, true); // int, bool
来源：https://stackoverflow.com/questions/19758955/get-the-subset-of-a-parameter-pack-based-on-a-given-set-of-indices