Add const qualifier to array reference typedef

Question

Consider the following type definition:

typedef int (&foo_t)[3];

or

using foo_t = int(&)[3];

When adding a const qualifier to the type, it is ignored:

int foo[3] = {1, 2, 3};
const foo_t fooref = foo;
fooref[0] = 4;  // No error here

Or when I try to assign a const array to it:

const int foo[3] = {1, 2, 3};
const foo_t fooref = foo;
/* error: binding reference of type ‘foo_t’ {aka ‘int (&)[3]’} to ‘const int [3]’ discards qualifiers
 const foo_t fooref = foo;
                      ^~~ */

How can I add const to a typedef'ed array reference?

BeyelerStudios · Accepted Answer · 2018-12-09 17:41:45Z

1

You cannot simply add a const to the type referenced by a typedefed type. Think of typedefing a pointer type:

typedef int* pint_t;

The type const pint_t names an unmodifiable pointer to a modifiable int.

If you can, simply add it to the definition (or define a const variant of your type):

typedef const int (&foo_t)[3];

or

using foo_t = const int(&)[3];

If that's out of the question, a general unpacking scheme to make the inner-most type const may be implementable, but probably not advisable - i.e. check your design.

answered Dec 9, 2018 at 17:41

BeyelerStudios

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