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  1. #include <iostream>
  2. #include <stdint.h>
  3. #include <stddef.h>
  4. #include <assert.h>
  5.  
  6. #ifndef UINT64_C
  7. #define UINT64_C(n) n##ULL
  8. #endif
  9.  
  10. static const uint64_t stchkValue = UINT64_C(0xbbbbfeedfacebbbb);
  11. static const uint64_t enchkValue = UINT64_C(0xeeeefeedfaceeeee);
  12.  
  13. template<typename T>
  14. struct Item
  15. {
  16.     uint64_t stchk;
  17.     union {
  18.         char mem[sizeof(T)];
  19.         //__m128i align; ideone compiler options disallow
  20.     } data;
  21.     uint64_t enchk;
  22.  
  23.     Item()
  24.      : stchk(stchkValue)
  25.      , enchk(enchkValue)
  26.     {
  27.     }
  28.  
  29.     void construct()
  30.     {
  31.         new (&data) T();
  32.     }
  33. };
  34.  
  35. template<typename T>
  36. static void destruct(T *p)
  37. {
  38.     // Try to reverse calculate the Item pointer from the mem pointer
  39.     // tried auto ofs = offsetof(Item<T>, data);
  40.     // tried auto ofs = offsetof(Item, data);
  41.     typedef Item<T> NiceItem;
  42.     auto ofs = offsetof(NiceItem, data);
  43.     //auto ofs = (size_t)(uintptr_t)(&((T*)0)->data);
  44.  
  45.     std::cout << "offset is " << ofs << std::endl;
  46.  
  47.     auto self = reinterpret_cast<Item<T>*>((uintptr_t)p - ofs);
  48.     assert(self->stchk == stchkValue);
  49.     assert(self->enchk == enchkValue);
  50.     assert(reinterpret_cast<T*>(self->data.mem) == p);
  51.  
  52.     p->~T();
  53. }
  54.  
  55. int main()
  56. {
  57.     Item<int> x;
  58.     x.construct();
  59.     auto p = &x.data;
  60.     destruct(p);
  61. }
  62.  
Success #stdin #stdout 0s 2884KB
comments (?)
stdin 
Standard input is empty
stdout 
offset is 8
https://ideone.com/98LONC
language:
C++14 (gcc 8.3)
created:
12 years ago
visibility:
public

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