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  1. #include <tuple>
  2. #include <cassert>
  3. #include <iostream>
  4.  
  5. template <size_t I>
  6. struct visit_impl
  7. {
  8.     template <typename T, typename F>
  9.     static void visit(T& tup, size_t idx, F fun)
  10.     {
  11.         if (idx == I - 1) fun(std::get<I - 1>(tup));
  12.         else visit_impl<I - 1>::visit(tup, idx, fun);
  13.     }
  14. };
  15.  
  16. template <>
  17. struct visit_impl<0>
  18. {
  19.     template <typename T, typename F>
  20.     static void visit(T& tup, size_t idx, F fun) { assert(false); }
  21. };
  22.  
  23. template <typename F, typename... Ts>
  24. void visit_at(std::tuple<Ts...> const& tup, size_t idx, F fun)
  25. {
  26.     visit_impl<sizeof...(Ts)>::visit(tup, idx, fun);
  27. }
  28.  
  29. template <typename F, typename... Ts>
  30. void visit_at(std::tuple<Ts...>& tup, size_t idx, F fun)
  31. {
  32.     visit_impl<sizeof...(Ts)>::visit(tup, idx, fun);
  33. }
  34.  
  35. int main()
  36. {
  37. 	auto tup = std::make_tuple('a', 42, 13.37);
  38. 	auto doSomething = [](auto v) { std::cout << v << "\n"; };
  39. 	int n(0);
  40. 	std::cin >> n;
  41. 	visit_at(tup, n, doSomething);
  42. }
Success #stdin #stdout 0s 5284KB
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13.37
https://ideone.com/fdcRSX
language:
C++14 (gcc 8.3)
created:
11 days ago
visibility:
public

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