Why isn’t a for-loop a compile-time expression?

Here’s a way to do it that does not need too much boilerplate, inspired from http://stackoverflow.com/a/26902803/1495627 :

template<std::size_t N>
struct num { static const constexpr auto value = N; };

template <class F, std::size_t... Is>
void for_(F func, std::index_sequence<Is...>)
{
  using expander = int[];
  (void)expander{0, ((void)func(num<Is>{}), 0)...};
}

template <std::size_t N, typename F>
void for_(F func)
{
  for_(func, std::make_index_sequence<N>());
}

Then you can do :

for_<N>([&] (auto i) {      
  std::get<i.value>(t); // do stuff
});

If you have a C++17 compiler accessible, it can be simplified to

template <class F, std::size_t... Is>
void for_(F func, std::index_sequence<Is...>)
{
  (func(num<Is>{}), ...);
}