C++ Metaprogramming Iterating with std::integer_sequence
Example
Since C++14, the standard provides the class template
template <class T, T... Ints>
class integer_sequence;
template <std::size_t... Ints>
using index_sequence = std::integer_sequence<std::size_t, Ints...>;
and a generating metafunction for it:
template <class T, T N>
using make_integer_sequence = std::integer_sequence<T, /* a sequence 0, 1, 2, ..., N-1 */ >;
template<std::size_t N>
using make_index_sequence = make_integer_sequence<std::size_t, N>;
While this comes standard in C++14, this can be implemented using C++11 tools.
We can use this tool to call a function with a std::tuple of arguments (standardized in C++17 as std::apply):
namespace detail {
template <class F, class Tuple, std::size_t... Is>
decltype(auto) apply_impl(F&& f, Tuple&& tpl, std::index_sequence<Is...> ) {
return std::forward<F>(f)(std::get<Is>(std::forward<Tuple>(tpl))...);
}
}
template <class F, class Tuple>
decltype(auto) apply(F&& f, Tuple&& tpl) {
return detail::apply_impl(std::forward<F>(f),
std::forward<Tuple>(tpl),
std::make_index_sequence<std::tuple_size<std::decay_t<Tuple>>::value>{});
}
// this will print 3
int f(int, char, double);
auto some_args = std::make_tuple(42, 'x', 3.14);
int r = apply(f, some_args); // calls f(42, 'x', 3.14)
