convert char* to int* C++

Question

I have this piece of code that I saw it somewhere and I tried to figure out how it works, but I couldn't.

This is it :

#include <iostream>
using namespace std;

int main() {
   int a = 2;
   char * p = (char *) &a;
   *(p + 1) = 1;
   cout << (int *) p << endl;
   return 0;
}

I thought that in p it stores the binary of variable a like 00000010 . Than in the next immediate address it stores 00000001 . When I try to print (int *) p it takes 4 bytes from that address and converts it into int.

When I ran the program the result wasn't that expected. It shows only the address of variable a . No change observed.

Could you please explain me how this works and why ?

PS : If I want to show the value of p it shows only 2 not 258 how I expected.

Answer 1

cout << (int *) p << endl; would be the same as cout << &a << endl; (just the address of a ).

int a = 2;
cout << sizeof(a) << endl;       // 4 (int is 4 bytes)
cout << sizeof(&a) << endl;      // 8 (64b machine, so pointer is 8 bytes)

char *p = (char *) &a;           // p points to first byte of a (normally 4 bytes)
cout << (int) *p << endl;        // 2    // Little Endian, the first byte is actually the last
cout << (int) *(p + 1) << endl;  // 0

*(p + 1) = 1;                    // second byte of a is now set to 1
cout << a << endl;               // a now changes to 258 (0000 0001 0000 0010)

Answer 2

PS : If I want to show the value of p it shows only 2 not 258 how I expected.

The value of p is the address of the object to which it points. It seems your confusion is here. If you dereference p you will get value of object to which it points, that is different.

Hence, in your case p is initialized to address of a . Afterwards, nothing is assigned to it (eg nowhere you do p=&SomeOtherObject ).

   cout << (int *) p << endl; // Print value of pointer -which is address

So above you are printing value of p which is address of a .

As noted keep in mind

*(p+1) = 1

might be undefined behaviour if sizeof (int) is same as sizeof (char)