| HN Mirror

Suppose I have the following struct and functions.

  struct my_struct{int a; char private[10];}
  void * get_struct_field_nonconst(struct my_struct *ptr)
  { return &(ptr->private[0]); }

With these elements you are only allowed to give non-const pointers to read or write inside private field. In order to truly apply this to const pointers (no casting etc.) you will need another get_struct_field() function but defined as this.

  const void * get_struct_field_const(const struct my_struct *ptr)
  { return &(ptr->private[0]); }

This will work but it is ugly. Here _Generic keyword allows you to write these functions but use them with a common name. Such as get_struct_field(). And that would be something like this:

  #define get_struct_field(ptr) \
  _Generic((ptr), \
  const struct my_struct * : get_struct_field_const, \
   struct my_struct * : get_struct_field_nonconst) \
  (ptr)

Hope this would help!