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  1.     #include <iostream>
  2.     struct A 
  3.     {
  4.       virtual void print (void) { std::cout << "A called." << std::endl; }
  5.       size_t x;
  6.     };
  7.  
  8.     struct B : A 
  9.     {
  10.       void print (void) { std::cout << "B called." << std::endl; }
  11.     };
  12.     // "Real" memory layout of A
  13.     struct Real_A
  14.     {
  15.       void * vtable;
  16.       size_t x_value;
  17.     };
  18.     // "Real" memory layout of B
  19.     struct Real_B : Real_A
  20.     {
  21.       size_t y_value;
  22.     };
  23.     // "Pseudo virtual table structure for classes with 1 virtual function"
  24.     struct VT
  25.     {
  26.       void * func_addr;
  27.     };
  28.  
  29.     int main (void) 
  30.     {
  31.       A * pa = new A;
  32.       pa->x = 15;
  33.       B * pb = new B;
  34.       pb->x = 20;
  35.       A * pa_b = new B;
  36.       pa_b->x = 25;
  37.       // reinterpret addrress of A and B objects as Real_A and Real_B
  38.       Real_A& ra(*(Real_A*)pa);
  39.       Real_B& rb(*(Real_B*)pb);
  40.       // reinterpret addrress of B object through pointer to A as Real_B
  41.       Real_B& rb_a(*(Real_B*)pa_b);
  42.       // Print x_values to know whether we meet the class layout
  43.       std::cout << "Value of ra.x_value = " << ra.x_value << std::endl;
  44.       std::cout << "Value of rb.x_value = " << rb.x_value << std::endl;
  45.       std::cout << "Value of rb.x_value = " << rb_a.x_value << std::endl;
  46.       // Print vtable addresses
  47.       std::cout << "VT of A through A*: " << ra.vtable << std::endl;
  48.       std::cout << "VT of B through B*: " << rb.vtable << std::endl;
  49.       std::cout << "VT of B through A*: " << rb_a.vtable << std::endl;
  50.       // Reinterpret memory pointed to by the vtable address as VT objects
  51.       VT& va(*(VT*)ra.vtable);
  52.       VT& vb(*(VT*)rb.vtable);
  53.       VT& vb_a(*(VT*)rb_a.vtable);
  54.       // Print addresses of functions in the vtable
  55.       std::cout << "FA of A through A*: " << va.func_addr << std::endl;
  56.       std::cout << "FA of B through B*: " << vb.func_addr << std::endl;
  57.       std::cout << "FA of B through A*: " << vb_a.func_addr << std::endl;
  58.  
  59.       delete pa;
  60.       delete pb;
  61.       delete pa_b;
  62.  
  63.       return 0;
  64.     }
Success #stdin #stdout 0s 3432KB
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Value of ra.x_value = 15
Value of rb.x_value = 20
Value of rb.x_value = 25
VT of A through A*: 0x8048f38
VT of B through B*: 0x8048f48
VT of B through A*: 0x8048f48
FA of A through A*: 0x8048d40
FA of B through B*: 0x8048cc0
FA of B through A*: 0x8048cc0
https://ideone.com/iKyBB3
language:
C++14 (gcc 8.3)
created:
11 years ago
visibility:
public

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