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  1. #include <cstddef>
  2. #include <type_traits>
  3.  
  4. #define alignas(...) __attribute__((__aligned__(alignof(decltype(::wheels::detail::get_type_with_alignment<__VA_ARGS__>())))))
  5.  
  6. namespace wheels {
  7.     namespace detail {
  8.         constexpr std::size_t max(std::size_t n) { return n; }
  9.         constexpr std::size_t max(std::size_t a, std::size_t b) { return a > b? a : b; }
  10.         template <typename... T>
  11.         constexpr std::size_t max(std::size_t h, T... t) { return max(h, max(t...)); }
  12.  
  13.         template <std::size_t... N> struct __attribute__((__aligned__(max(N...)))) alignment_type {};
  14.         template <typename... T> alignment_type<alignof(T)...> get_type_with_alignment();
  15.         template <std::size_t... N> alignment_type<N...> get_type_with_alignment();
  16.     }
  17.  
  18.     template <std::size_t Len, typename... T>
  19.     struct aligned_union {
  20.         static constexpr std::size_t alignment_value = detail::max(alignof(T)...);
  21.         struct type { alignas(T...) char unnamed[detail::max(Len, sizeof(T)...)]; };
  22.     };
  23.     template <typename... T>
  24.     using AlignedUnion = typename aligned_union<1, T...>::type;
  25.  
  26. #define STATIC_ASSERT(...) static_assert((__VA_ARGS__), #__VA_ARGS__)
  27.     STATIC_ASSERT(std::is_pod<AlignedUnion<char, int, double>>::value);
  28.     STATIC_ASSERT(sizeof(AlignedUnion<char, int, double>) >= sizeof(char));
  29.     STATIC_ASSERT(sizeof(AlignedUnion<char, int, double>) >= sizeof(int));
  30.     STATIC_ASSERT(sizeof(AlignedUnion<char, int, double>) >= sizeof(double));
  31.     STATIC_ASSERT(alignof(AlignedUnion<char, int, double>) >= alignof(char));
  32.     STATIC_ASSERT(alignof(AlignedUnion<char, int, double>) >= alignof(int));
  33.     STATIC_ASSERT(alignof(AlignedUnion<char, int, double>) >= alignof(double));
  34. }
  35.  
  36. int main() {}
  37.  
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https://ideone.com/8g57x
language:
C++14 (gcc 8.3)
created:
14 years ago
visibility:
public

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