Can anyone explain how reinterpret cast is working in this code

Question

read the following program which is using reinterpret_cast.

#include <iostream>

class A
{
   public:
      A() : m_i(0) { }
   protected:
      int m_i;
};

class B
{
   public:
      B() : m_d(0.0) { }

   protected:
      double m_d;
};

class C : public A , public B
{
   public:
      C() : m_c('a') { }
   private:
      char m_c;
};

int main()
{
   C c;
   A *pa = &c;
   B *pb = &c;

   bool z = (reinterpret_cast<char*>(pa) == reinterpret_cast<char*>(pb));
   std::cout << z;
   return 0;
}

After running this program it prints 0, can anyone explain why z is coming false in this program??

Answer 1

Can anyone explain how reinterpret cast is working in this code

reinterpret_cast<char*>(pa) evaluates to a pointer of type char* whose numerical value is the same as that of pa. Use of:

bool z = (pa == pb);

leads to a compiler error since A and B are not directly related. Use of

bool z = (reinterpret_cast<char*>(pa) == reinterpret_cast<char*>(pb));

allows you to compare the numerical values of pa and pb.

After running this program it prints 0, can anyone explain why z is coming false in this program??

The numerical values of pa and pb are not same. Hence, the result. You can use:

cout << "pa: " << std::hex << (void*)pa << std::endl;
cout << "pb: " << std::hex << (void*)pb << std::endl;

to print those values and convince yourself that they are not same.

If you look at the memory layout of a C, it looks something like:

+------+
|  m_i |     // The A sub-object
+------+
|  m_d |     // The B sub-object
+------+
|  m_c |     
+------+

When you use

C c;
A* pa = &c;
B* pb = &c;

pa points to the A sub-object of C and pb points to the B sub-object of C. As you can see from the picture, there is an offset between the A sub-object and the B sub-object. Hence, the numerical values of pa and pb are different. The most likely differ by sizeof(m_i).