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  1. #include <iostream>
  2. #include <vector>
  3.  
  4. class A;
  5. class B;
  6.  
  7. // a buffer contains array of A followed by B
  8. // | A[N - 1]...A[0] | B |
  9.  
  10. class A
  11. {
  12. 	unsigned char index;
  13. 	unsigned char a;
  14. public:
  15.     A(size_t index) : index(index) {}
  16.     const B* getB() const { return reinterpret_cast<const B*>(this + index + 1); }
  17.     void print() const { std::cout << "A[" << index << "]\n"; }
  18. };
  19.  
  20. class B
  21. {
  22. 	size_t a_count;
  23. 	int b[16];
  24. public:
  25. 	virtual ~B() {}
  26. 	B(size_t a_count) : a_count(a_count) {}
  27. 	const A* getA(size_t i) const { return reinterpret_cast<const A*>(this) - i - 1; }
  28. 	void print() const
  29. 	{
  30. 		std::cout << "B has " << a_count << " As\n";
  31. 		for(auto i = 0; i < a_count; ++i)
  32. 		{
  33. 			getA(i)->print();
  34. 		}
  35. 	}
  36. };
  37.  
  38. B* make_record(size_t a_count)
  39. {
  40. 	char* buf = new char[a_count*sizeof(A) + sizeof(B)];
  41. 	for(auto i = 0; i < a_count; ++i)
  42. 	{
  43. 		new(buf) A(a_count - i - 1);
  44. 		buf += sizeof(A);
  45. 	}
  46. 	return new(buf) B(a_count);
  47. }
  48.  
  49. int main() 
  50. {
  51. 	std::vector<B*> vec;
  52. 	vec.push_back(make_record(0));
  53. 	vec.push_back(make_record(1));
  54. 	vec.push_back(make_record(2));
  55.  
  56. 	for(auto b : vec) {
  57.       std::cout << "B starts at " << b << ", needs alignment " << alignof(B)
  58.    	    << ", misaligned by " << (reinterpret_cast<uintptr_t>(b) % alignof(B)) << std::endl;
  59.    	  b->print();
  60. 	}
  61. }
Success #stdin #stdout 0s 3432KB
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stdin
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Standard input is empty
stdout
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B starts at 0x9003008, needs alignment 4, misaligned by 0
B has 0 As
B starts at 0x900306a, needs alignment 4, misaligned by 2
B has 1 As
A[]
B starts at 0x90030cc, needs alignment 4, misaligned by 0
B has 2 As
A[]
A[]
https://ideone.com/eC7l17
Demonstration of object misalignment.
language:
C++14 (gcc 8.3)
created:
12 years ago
visibility:
public

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