How to initialize const members of structs on the heap

Question

I would like to allocate a structure on the heap, initialize it and return a pointer to it from a function. I'm wondering if there's a way for me to initialize const members of a struct in this scenario:

#include <stdlib.h>

typedef struct {
  const int x;
  const int y;
} ImmutablePoint;

ImmutablePoint * make_immutable_point(int x, int y)
{
  ImmutablePoint *p = (ImmutablePoint *)malloc(sizeof(ImmutablePoint));
  if (p == NULL) abort();
  // How to initialize members x and y?
  return p;
}

Should I conclude from this that it is impossible to allocate and initialize a struct on the heap which contains const members?

Answer 1

Like so:

ImmutablePoint *make_immutable_point(int x, int y)
{
  ImmutablePoint init = { .x = x, .y = y };
  ImmutablePoint *p = malloc(sizeof *p);

  if (p == NULL) abort();
  memcpy(p, &init, sizeof *p);

  return p;
}

(Note that unlike C++, there is no need to cast the return value of malloc in C, and it is often considered bad form because it can hide other errors).

Answer 2

If this is C and not C++, I see no solution other than to subvert the type system.

ImmutablePoint * make_immutable_point(int x, int y)
{
  ImmutablePoint *p = malloc(sizeof(ImmutablePoint));
  if (p == NULL) abort();

  // this 
  ImmutablePoint temp = {x, y};
  memcpy(p, &temp, sizeof(temp));

  // or this
  *(int*)&p->x = x;
  *(int*)&p->y = y;

  return p;
}

Answer 3

If you insist on keeping the const in the structure, you are going to have to do some casting to get around that:

int *cheat_x = (int *)&p->x;
*cheat_x = 3;

Answer 4

I like caf's approach, but this occured to me too

ImmutablePoint* newImmutablePoint(int x, int y){ 
   struct unconstpoint {
      int x;
      int y;
   } *p = malloc(sizeof(struct unconstpoint));
   if (p) { // guard against malloc failure
      *p.x = x;
      *p.y = y;
   }
   return (ImmutablePoint*)p;
}