15.9 const Fields

A structure field declared const cannot be assigned to (see const Variables and Fields). For instance, let’s define this modified version of struct intlistlink:

struct intlistlink_ro /* “ro” for read-only. */
 {
 const int datum;
 struct intlistlink *next;
 };

This structure can be used to prevent part of the code from modifying the datum field:

/* p has type struct intlistlink *.
 Convert it to struct intlistlink_ro *. */
struct intlistlink_ro *q
 = (struct intlistlink_ro *) p;
q->datum = 5; /* Error! */
p->datum = 5; /* Valid since *p is
 not a struct intlistlink_ro. */

A const field can get a value in two ways: by initialization of the whole structure, and by making a pointer-to-structure point to an object in which that field already has a value.

Any const field in a structure type makes assignment impossible for structures of that type (see Structure Assignment). That is because structure assignment works by assigning the structure’s fields, one by one.