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15.9 const Fields

A structure field declared const cannot be assigned to (see const). 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.