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  1. #include <limits>
  2. #include <type_traits>
  3. #include <iostream>
  4.  
  5. template <typename T> struct unsigned_integer_traits;
  6. template <> struct unsigned_integer_traits<unsigned char> {
  7.   typedef signed char signed_type;
  8. };
  9. template <> struct unsigned_integer_traits<unsigned short int> {
  10.   typedef signed short int signed_type;
  11. };
  12. template <> struct unsigned_integer_traits<unsigned int> {
  13.   typedef signed int signed_type;
  14. };
  15. template <> struct unsigned_integer_traits<unsigned long int> {
  16.   typedef signed long int signed_type;
  17. };
  18. template <> struct unsigned_integer_traits<unsigned long long int> {
  19.   typedef signed long long int signed_type;
  20. };
  21.  
  22.  
  23. template <typename T>
  24. struct is_2complement_system {
  25.   typedef typename unsigned_integer_traits<T>::signed_type signed_type;
  26.   static const bool value = (static_cast<signed_type>(~(T())) 
  27.                              == static_cast<signed_type>(-1));
  28. };
  29. template <typename T>
  30. const bool is_2complement_system<T>::value; 
  31.  
  32. template <typename T, bool is_2complement_system_value>
  33. struct unsigned_to_signed_impl;
  34.  
  35. template <typename T>
  36. struct unsigned_to_signed_impl<T,true> {
  37.   typedef typename unsigned_integer_traits<T>::signed_type signed_type;
  38.   static signed_type convert(T value) 
  39.   { 
  40.      return static_cast<signed_type>(value); 
  41.   }
  42. };
  43.  
  44. template <typename T>
  45. struct unsigned_to_signed_impl<T,false> {
  46.   typedef typename unsigned_integer_traits<T>::signed_type signed_type;
  47.   static signed_type convert(T value) 
  48.   { 
  49.      // well do something complicated you don't care what
  50.      return T(); // just for tests 
  51.   }
  52. };
  53.  
  54. template <typename T>
  55. typename unsigned_integer_traits<T>::signed_type  unsigned_to_signed(T n)
  56. {
  57.     return unsigned_to_signed_impl<T,is_2complement_system<T>::value>::convert(n);
  58. }
  59.  
  60. int main() {
  61.   std::cout 
  62.     << unsigned_to_signed(100u) << '\n'
  63.     << unsigned_to_signed(0xFFFFffff) << '\n'
  64. ;
  65.  
  66. }
Success #stdin #stdout 0s 2928KB
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stdin
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Standard input is empty
stdout
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100
-1
https://ideone.com/bqDgS2
language:
C++14 (gcc 8.3)
created:
11 years ago
visibility:
public

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