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  1. #include <iostream>
  2.  
  3. using value_t = unsigned long long int;
  4.  
  5. namespace peano
  6. {
  7.  
  8. struct zero {
  9.   static constexpr value_t value = 0;
  10.  
  11.   using type = zero;
  12. };
  13.  
  14. template <typename T>
  15. struct succ {
  16.     static constexpr value_t value = 1 + T::value;
  17.  
  18.     using pred = typename T::type;
  19.  
  20.     using type = succ<typename T::type>;
  21. };
  22.  
  23. template <typename S, typename T>
  24. struct add: succ<typename add<S, typename T::pred>::type>::type {};
  25.  
  26. template <typename S>
  27. struct add<S, zero>: S::type {};
  28.  
  29. template <typename S, typename T>
  30. struct mul: add<typename mul<S, typename T::pred>::type, S>::type {};
  31.  
  32. template <typename S>
  33. struct mul<S, zero>: zero {};
  34.  
  35. template <typename S>
  36. struct factorial: mul<S, typename factorial<typename S::pred>::type>::type {};
  37.  
  38. template <>
  39. struct factorial<zero>: succ<zero>::type {};
  40.  
  41. // constants
  42. using _0 = zero;
  43. using _1 = succ<_0>::type;
  44. using _2 = succ<_1>::type;
  45. using _3 = succ<_2>::type;
  46. using _4 = succ<_3>::type;
  47. using _5 = succ<_4>::type;
  48. using _6 = succ<_5>::type;
  49. using _7 = succ<_6>::type;
  50. using _8 = succ<_7>::type;
  51. using _9 = succ<_8>::type;
  52.  
  53. } // namespace peano
  54.  
  55. int main() {
  56.   // 9 + 9 == 18
  57.   std::cout
  58.     << peano::add<peano::_9, peano::_9>::value
  59.     << std::endl;
  60.  
  61.   // 5 * 7 == 35
  62.   std::cout
  63.     << peano::mul<peano::_5, peano::_7>::value
  64.     << std::endl;
  65.  
  66.   // let's try a more complex expression
  67.   // 2 * 7 + 3 * (4 + 5) == 41
  68.   std::cout <<
  69.     peano::add<
  70.       peano::mul<peano::_2, peano::_7>,
  71.       peano::mul<
  72.         peano::_3,
  73.         peano::add<peano::_4, peano::_5>
  74.       >
  75.     >::value
  76.     << std::endl;
  77.  
  78.   // and now factorial: 5! == 120
  79.   std::cout
  80.     << peano::factorial<peano::_5>::value
  81.     << std::endl;
  82.  
  83.   return 0;
  84. }
  85.  
Success #stdin #stdout 0.01s 5304KB
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120
https://ideone.com/9FW5H
Compile-time template computations
language:
C++ (gcc 8.3)
created:
1 year ago
visibility:
public

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