c++ generic compile-time for loop

Question

In some contexts, it could be useful/necessary to have a for loop evaluated/unrolled at compile time. For example, to iterate over the elements of a tuple

Answer 1

I'll answer on the question how to fix your last code sample.

The reason why it doesn't compile is here:

template  class F, typename... Args>
void compile_time_for(F f, Args... args)
                      /\

F is a template, you can't have an object of a template class without template parameters being substituted. E.g. you can't have on object of std::vector type, but can have object of std::vector. I suggest you to make F functor with a template operator() :

#include 
#include 
#include 
#include 

template 
void compile_time_for(F f, Args... args)
{
  if constexpr (start < end)
         {
           f.template operator()(std::forward(args)...);
           compile_time_for(f, std::forward(args)...);
         }    
}

struct myprint
{
    template 
    void operator()(const std::tuple& x) { std::cout << std::get(x) << " "; }
};

int main()
{
  std::tuple x{1, 2, "hello"};

  compile_time_for<0, 3>(myprint(), x);

  return 0;
}