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Output of the Program | Pointer to a Constant or Constant Pointer?

  • Difficulty Level : Easy
  • Last Updated : 13 Sep, 2021

Predict the output of the below program.
 

c




int main()
{
    int x = 5;
    int * const ptr = &x;
    ++(*ptr);
    printf("%d", x);
        
    getchar();
    return 0;  
}

Output: 6
Explananation: 
See following declarations to know the difference between constant pointer and a pointer to a constant. 
int * const ptr —> ptr is constant pointer. You can change the value at the location pointed by pointer p, but you can not change p to point to other location.
int const * ptr —> ptr is a pointer to a constant. You can change ptr to point other variable. But you cannot change the value pointed by ptr.
Therefore above program works well because we have a constant pointer and we are not changing ptr to point to any other location. We are only incrementing value pointed by ptr.
Try below program, you will get compiler error. 
 

c




int main()
{
    int x = 5;
    int const * ptr = &x;
    ++(*ptr);
    printf("%d", x);
       
    getchar();
    return 0; 
}
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