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  1. #include <iostream>
  2.  
  3. struct B1 {
  4.     char x[1];
  5. };
  6.  
  7. struct B2 {
  8.     char x[1];
  9. };
  10.  
  11. struct D :
  12.     private B1,
  13.     private B2 {
  14.     char x[1];
  15. };
  16.  
  17. #define RAW_PTR static_cast<const void*>
  18.  
  19. int main()
  20. {
  21.     using namespace std;
  22.  
  23.     D d;
  24.  
  25.     D* pd = &d;
  26.  
  27.     cout << "-------" << endl;
  28.     cout << "upcasts" << endl;
  29.     cout << "-------" << endl;
  30.     cout << endl;
  31.     {
  32.         //B2* p = pd; // error: 'B2' is an inaccessible base of 'D'
  33.     }
  34.     {
  35.         //B2* p = static_cast<B2*>(pd); // error: 'B2' is an inaccessible base of 'D'
  36.     }
  37.     {
  38.         B2* p = reinterpret_cast<B2*>(pd); // Compiles, but is wrong!
  39.                                            //  simply copies bits, doesn't adjust
  40.         cout << "reinterpret_cast:" << endl;
  41.         cout << pd << endl;
  42.         cout << p << endl;
  43.         cout << ((RAW_PTR(p) == RAW_PTR(pd)) ? "Wrong!" : "Right.") << endl;
  44.         cout << endl;
  45.     }
  46.     {
  47.         B2* p = (B2*)pd; // Correct conversion, here equivalent to
  48.                          //  implicit conversion ignoring 'private' access specifier;
  49.                          //  makes the needed adjustment
  50.         cout << "C-style cast:" << endl;
  51.         cout << pd << endl;
  52.         cout << p << endl;
  53.         cout << ((RAW_PTR(p) == RAW_PTR(pd)) ? "Wrong!" : "Right.") << endl;
  54.         cout << endl;
  55.     }
  56.  
  57.     cout << endl;
  58.  
  59.     B2* pb2 = (B2*)&d; // (correct conversion)
  60.  
  61.     cout << "---------" << endl;
  62.     cout << "downcasts" << endl;
  63.     cout << "---------" << endl;
  64.     cout << endl;
  65.     {
  66.         //D* p = pb2; // error: invalid conversion from 'B2*' to 'D*' [-fpermissive]
  67.                       // (then also error: 'B2' is an inaccessible base of 'D')
  68.     }
  69.     {
  70.         //D* p = static_cast<D*>(pb2); // error: 'B2' is an inaccessible base of 'D'
  71.     }
  72.     {
  73.         D* p = reinterpret_cast<D*>(pb2); // Compiles, but is wrong!
  74.                                           //  simply copies bits, doesn't adjust
  75.         cout << "reinterpret_cast:" << endl;
  76.         cout << pb2 << endl;
  77.         cout << p << endl;
  78.         cout << ((RAW_PTR(p) == RAW_PTR(pb2)) ? "Wrong!" : "Right.") << endl;
  79.         cout << endl;
  80.     }
  81.     {
  82.         D* p = (D*)pb2; // Correct conversion, here equivalent to
  83.                         //  static_cast ignoring 'private' access specifier;
  84.                         //  makes the needed adjustment
  85.         cout << "C-style cast:" << endl;
  86.         cout << pb2 << endl;
  87.         cout << p << endl;
  88.         cout << ((RAW_PTR(p) == RAW_PTR(pb2)) ? "Wrong!" : "Right.") << endl;
  89.         cout << endl;
  90.     }
  91. }
Success #stdin #stdout 0s 2852KB
comments (?)
stdin
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Standard input is empty
stdout
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-------
upcasts
-------

reinterpret_cast:
0xbfa8f93d
0xbfa8f93d
Wrong!

C-style cast:
0xbfa8f93d
0xbfa8f93e
Right.


---------
downcasts
---------

reinterpret_cast:
0xbfa8f93e
0xbfa8f93e
Wrong!

C-style cast:
0xbfa8f93e
0xbfa8f93d
Right.
https://ideone.com/rvqdL5
language:
C++ (gcc 8.3)
created:
10 years ago
visibility:
public

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