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cconstantsdeclarationfunction-signature
Learath2
Learath2

Reputation: 21343

What's the meaning of multiple const qualifiers?

Code:

const char* const* const* foo(int bar);

I've seen double consts before which prevent the modification of the pointer too. First time i've seen triple const in my life. Wondering what its use is.

Upvotes: 19

Views: 1254

Answers (4)

HelloWorld123456789
HelloWorld123456789

Reputation: 5359

foo returns a pointer which points to a constant pointer which points to another constant pointer which points to a constant char. Have a look at the spiral rule.

int bar;
const char* const* const* ptr = foo(bar);

So basically you cannot change any of *ptr or **ptr or ***ptr.

Upvotes: 4

pmg
pmg

Reputation: 108978

In your example all but the top level of indirection all const qualified.

const char            /* const qualified base level */
    *const            /* const qualified level 1 */
    *const            /* const qualified level 2 */
    *                 /* regular level 3 */
    foo(int bar);

foo is a function that accepts an int argument and returns a regular pointer.
The pointer it returns points to a const qualified pointer
which, in turn, points to another const qualified pointer
which points to const qualified char

Upvotes: 19

Filipe Gonçalves
Filipe Gonçalves

Reputation: 21213

Hah - the confusion around const.

Basically, if it qualifies a pointer, then it applies to the pointer immediately to its left. For example:

char *const c;

makes c a read-only pointer to char, whereas

const char *c;

Makes c a pointer to a read-only char, though you can change where c points to.

Of course, both the pointer and what it points to can be made read-only:

const char *const c;

This makes c a read-only pointer to a read-only char. With the former declaration, you can modify *c, but you can't modify c (i.e. you can't make it point to somewhere else). With the second, you can assign to c but not to *c, and with the third, you can't modify where it points to, nor can you modify the pointed-to object.

This extends to multiple levels of indirection. For your case, you might want to regroup the const keywords with the matching pointer that they are qualifying. This is mainly a whitespaces issue, but it would be like this:

const char *const *const *foo(int bar);

Which makes foo return a pointer to a read-only pointer to a read-only pointer to a read-only char. Read that carefully: note that given this piece of code:

const char *const *const *ptr = foo(0);

Then you know that assigning to ptr is legal, but any other assignment is illegal, namely, you can't assign to *ptr, **ptr, and ***ptr.

Upvotes: 8

cmaster - reinstate monica
cmaster - reinstate monica

Reputation: 40604

If you have a multilevel pointer, you have several pointer variables. Example:

char*** foo;

is accessed like this:

| foo | pointer1 | pointer2 | string |
   |    ^     |    ^     |    ^
    \___/      \___/      \___/

You can qualify each of the four locations in memory as being const, as in the declaration

const char *const *const *const foo;

However, best avoid becoming a three star programmer.

Upvotes: 14

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