Passing vector items to function-object with unknown number of parameters - c++

Question

Here is a simplified version of what I am trying to achieve:
template <class Func, class Params>
void foo(Func f, Params p) {
f(p[0], p[1], ...) // <-- this is the problem. How to do this?
}
...
foo([](int a, int b){ cout<<(a+b); }, std::vector<int>{1,2});
foo([](char a){ cout<<a; }, std::vector<char>{'a'});
I hope the problem is clear.
EDIT:
The above example did not convey the problem well. I have a vector, populated at some earlier stage, of the parameters. I want a function that will accept a function-object and call it with the parameters from the vector. I can assume that the vector size is equal to the number of parameters.
Hopefully better example:
class C {
std::vector<int> v;
public:
void add_param(int);
... // other functions that manipulate the vector in various ways
template<class Func>
void run(Func f) {
f(v[0], etc...); // <-- problem
}
};

Answer 1

You can use variardic templates:
template <class Func, class... Params>
void foo(Func f, Params... p) {
f(p...);
}
foo([](int a, int b){ cout<<(a+b); }, 1, 2);
foo([](char a){ cout<<a; }, 'a');

Answer 2

You may use something like:
// Minimal traits to have information about function
template <typename Func> struct function_traits;
template <typename Ret, typename ... Ts>
struct function_traits<Ret (Ts...)>
{
constexpr static auto arity = sizeof...(Ts);
};
template <typename Ret, typename ... Ts>
struct function_traits<Ret (*)(Ts...)> : function_traits<Ret(Ts...)> {};
template <typename C, typename Ret, typename ... Ts>
struct function_traits<Ret (C::*)(Ts...) const> : function_traits<Ret(Ts...)> {};
template <typename C>
struct function_traits : function_traits<decltype(&C::operator())> {};
namespace detail
{
template <typename F, typename Vec, std::size_t ... Is>
void call(const F& f, Vec&& v, std::index_sequence<Is...>)
{
f(v[Is]...);
}
}
template <class Func, class Vec>
void foo(const Func& f, Vec&& v) {
detail::call(f,
std::forward<Vec>(v),
std::make_index_sequence<function_traits<Func>::arity>());
}
Demo