Question

I have a struct which contains some pointers. I want the value of these to be unmodifiable. But simply writing const infront doesn’t make the structs members unmutable

typedef struct{
  int *x;
  int *y;
}point;

void get(const  point *p,int x, int y){
  p->x[0]=x;//<- this should not be allowed
  p->y[0]=y;//<- this should not be allowed
}

Can someone point me in the right direction.

EDIT:

So it would seem that there is no simple way of using the function prototype to tell that everything belonging to the struct should be unmodifiable

Answer 1

To explain what you need to establish, when you write

point str;

point *p=&str;

Here p is a pointer to str which is of type point

When you declare it as const, it means that p is a constant pointer. This does not restrict the pointers that the structure may contain.

If you want the const ness to apply inside the structure, you have to define the pointers inside the structure also as const

typedef struct{
   const int *  x;
   const int *  y;
}point;

Again to push home my point declare the parameter as

    void get(point * const  p,int x, int y) 
   //Constant Pointer ( *to prevent p from pointing to anything else*)

    //    AND

   //Make the pointers inside point structure constant
   //( *to prevent the int pointers x & y from pointing to anything else*)

If the structure it is pointing to is also const use

      void get(const point * const p, int x, int y)
     //Constant Pointer to constant structure