same address shows different values for const variable with g++ compiler [duplicate]

Question

The following code shows different output with gcc and g++ on using const variable i. The addresses of i and value of ptr is same, but on accessing that address by printing value of i and derefrencing value of ptr I got value of i as 5 with g++ and 10 with gcc.

How g++ holds const variable in memory?

   #include <stdio.h>
   int main()
   {
     const  int i =5; 
     int *ptr =(int*)&i;
     *ptr = 10;
     printf("\n %u and %u   and %d  and %d  \n",&i,ptr,i,*ptr);

     return 0;
   }

Answer 1

You are modifying a const qualified object. This is not allowed in C ("undefined behavior"). Anything can happen.

Examples:

1. The compiler could put i into read-only memory. Writing to *ptr would crash your program.
2. It could put it into writable memory and you would just see the 10.
3. It could put it into writable memory but replace all read accesses to i by the number 5 (You promised it is const, didn't you?).

I guess the C compiler chose 2 while the C++ compiler went for 3.