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  1. #include <iostream>
  2. #include <stdio.h>
  3. #include <string>
  4.  
  5. struct A
  6. {
  7.     A(const std::string& pa) : a(pa) {printf("CTR: A address: %p\n", this) ;}
  8.     std::string a;
  9. };
  10.  
  11. struct B
  12. {
  13.     B(const std::string& pb) : b(pb) {printf("CTR: B address: %p\n", this) ;}
  14.     std::string b;
  15. };
  16.  
  17. // assumed data layout:
  18. struct C {
  19.  
  20.     C() : a("astring"), b("bstring") {}
  21.   // ...
  22.   A a;
  23.   // ...
  24.   B b;
  25. };
  26.  
  27. void approach1( A *pa, B *pb )
  28. {
  29.  
  30.     printf("approach1: A address: %p B address: %p\n", pa, pb); 
  31.     // compute offset:
  32.     std::ptrdiff_t offset = static_cast<char*>((void*)pb) - static_cast<char*>((void*)pa);
  33.     // then in some other function...
  34.     // given offset and ptr to b, compute ptr to a:
  35.     A *a = static_cast<A*>( (void*)(static_cast<char*>((void*)pb) + offset) );
  36.     printf("approach1: a address: %p \n", a); 
  37.  
  38.     std::cout << "approach1: A->a=" << a->a << std::endl;
  39. }
  40.  
  41.  
  42. void approach2( A *pa, B *pb )
  43. {
  44.     printf("approach2: A address: %p B address: %p\n", pa, pb); 
  45.  
  46.     std::ptrdiff_t offset = reinterpret_cast<char*>(pb) - reinterpret_cast<char*>(pa);
  47.  
  48.     A *a = reinterpret_cast<A*>( reinterpret_cast<char*>(pb) + offset );
  49.     printf("approach2: a address: %p \n", a); 
  50.     std::cout << "approach2: A->a=" << a->a << std::endl;
  51. }
  52.  
  53. main()
  54. {
  55.   C c;
  56.   std::cout << c.a.a << std::endl;
  57.  
  58.   approach1(&c.a, &c.b);
  59.   approach2(&c.a, &c.b);
  60. }
Success #stdin #stdout 0s 3432KB
comments (?)
stdin
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Standard input is empty
stdout
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CTR: A address: 0xbfb572b8
CTR: B address: 0xbfb572bc
astring
approach1: A address: 0xbfb572b8 B address: 0xbfb572bc
approach1: a address: 0xbfb572c0 
approach1: A->a=
approach2: A address: 0xbfb572b8 B address: 0xbfb572bc
approach2: a address: 0xbfb572c0 
approach2: A->a=
https://ideone.com/U8ahAL
language:
C++ (gcc 8.3)
created:
11 years ago
visibility:
public

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