C++ std::get fails

Question

How do I use a variable to index into a tuple using std::get<>? I have the following code:

#include 
#include 
using n         


        

Answer 1

#include 

template
auto index_over( std::index_sequence ) {
  return [](auto&& f)->decltype(auto){
    return decltype(f)(f)( std::integral_constant{}... );
  };
}
template
auto index_upto( std::integral_constant ={} ) {
  return index_over( std::make_index_sequence{} );
}
template
auto foreacher( F&& f ) {
  return [f=std::forward(f)](auto&&...args)mutable {
    (void(), ..., void(f(decltype(args)(args))));
  };
}
template
auto count_upto( std::integral_constant ={} ) {
  return [](auto&& f){
    index_upto()(foreacher(decltype(f)(f)));
  };
}

you can just do:

#include 
#include 

int main() {
  std::tuple data(5, 10);
  count_upto<2>()([&](auto I){
    std::cout << "#" << (I+1) << ":" << std::get(data) << "\n";
  });
}

Live example.

There is no unbounded recursion in this solution. It does require C++1z -- you can replace the body of foreacher with the using unused=int[]; trick in C++14.