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  1. #include <iostream>
  2. #include <new>
  3. #include <cstring>
  4.  
  5. using namespace std;
  6.  
  7. template <typename STRUCT, typename TYPE> class flex_struct {
  8. 	flex_struct(){}
  9. public:
  10. 	//should be safe to access and check what length the array is
  11. 	static STRUCT* head(char*buff) {
  12. 		//is this next line wrong?
  13. 		//if((alignof(STRUCT)%reinterpret_cast<size_t>(buff))!=0) { throw std::exception(); }
  14. 		return reinterpret_cast<STRUCT*>(buff);
  15. 	}
  16.  
  17. 	struct struct_with_array : public STRUCT { TYPE buf[1]; };
  18.  
  19. 	TYPE* buff() {
  20. 		//if(length==0) { throw std::exception(); }
  21. 		auto p = reinterpret_cast<struct_with_array*>(this);
  22. 		return p->buf;
  23. 	}
  24. };
  25.  
  26. typedef short testtype;
  27.  
  28. struct MyVariableLengthStruct : public flex_struct<MyVariableLengthStruct, testtype> {
  29. 	int a, b;
  30. 	char c;
  31. };
  32.  
  33. struct MyVariableLengthStruct2 {
  34. 	int a, b;
  35. 	char c;
  36. 	testtype buf[1];
  37. };
  38. struct MyVariableLengthStruct3a {
  39. 	int a, b;
  40. 	char c;
  41. };
  42. struct MyVariableLengthStruct3 : MyVariableLengthStruct3a {
  43. 	testtype buf[1];
  44. };
  45. int main() {
  46.  
  47. 	auto srcarray=new char[1024];
  48. 	memset(srcarray, 0, sizeof(srcarray)); //whats a C++ way to do this w/o writing a loop or function?
  49. 	auto v = MyVariableLengthStruct::head(srcarray);
  50. 	auto buff = v->buff();
  51. 	auto dif1 = (int)buff-(int)v;
  52. 	printf("%X %X %d\n", v, buff, dif1);
  53. 	MyVariableLengthStruct2 v2;
  54. 	auto dif2 = (int)v2.buf-(int)&v2;
  55. 	printf("%X %X %d\n", &v2, v2.buf, dif2);
  56. 	MyVariableLengthStruct3 v3;
  57. 	auto dif3 = (int)v3.buf-(int)&v3;
  58. 	printf("%X %X %d\n", &v3, v3.buf, dif3);
  59. }
  60.  
Success #stdin #stdout 0s 2960KB
comments (?)
stdin
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Standard input is empty
stdout
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8853008 8853012 10
BF9DE184 BF9DE18E 10
BF9DE174 BF9DE180 12
https://ideone.com/6JrVo
language:
C++14 (gcc 8.3)
created:
12 years ago
visibility:
public

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