Confusing pointer declaration

Question

I'm reading appendix A of Accelerated C++. There the authors show an example of a declaration which looks like this:

const char * const * const * cp;

They say const char is the specifier and * const * const * cp is the declarator. I'm confused about the purpose of the extra const and *s. Is this a declaration of a const pointer to a const char?

It's a declaration of a "pointer to const pointer to const pointer to const char", I believe. And this is why C++. — aardvarkk
– aardvarkk, Commented May 29, 2013 at 19:09

Daniel Fischer · Accepted Answer · 2013-05-29 19:10:01Z

7

It's the declaration of

a pointer
to a const pointer
to a const pointer
to const char

Thus you may change cp, but you may not change any of

*cp
**cp
***cp

answered May 29, 2013 at 19:10

Daniel Fischer

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6 Comments

crush Over a year ago

And why would anyone ever want to do this?

Daniel Fischer Over a year ago

@crush Sadism? I have never used triple pointers yet, I don't know when you would need them.

crush Over a year ago

It is exceedingly rare that I find an instance where a double pointer is useful.

Daniel Fischer Over a year ago

@crush Rare, but that happens, consider for example long int strtol(const char *nptr, char **endptr, int base); and friends.

crush Over a year ago

Yeah, so I guess a triple pointer might be used in conjunction with an array of strings where you are pointing to a specific character of a specific string in that array? There are certainly better ways to handle this that take advantage of C++

|

Bill · Accepted Answer · 2013-05-29 19:23:18Z

4

As we can see from cdecl, cp is a pointer to const pointer to const pointer to const char.

You can see this by breaking it down right-to-left:

const char * const * const * cp;
                             cp is
                           a pointer
                   to const pointer
           to const pointer
to const char

Also, the standard (§ 8) says that:

The speciﬁers indicate the type, storage class or other properties of the entities being declared. The declarators specify the names of these entities and (optionally) modify the type of the speciﬁers with operators such as * (pointer to) and () (function returning).

edited May 29, 2013 at 19:23

answered May 29, 2013 at 19:09

Bill

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Comments

Nik Bougalis · Accepted Answer · 2013-05-29 19:09:49Z

1

From cdecl.org:

declare cp as pointer to const pointer to const pointer to const char

answered May 29, 2013 at 19:09

Nik Bougalis

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Comments

aaronman · Accepted Answer · 2013-05-29 21:39:13Z

1

This is the same as char * * * cp except that all the pointers are const, except one. Since char***cp is a pointer to a pointer to a pointer your example is the same but it is a pointer to a const pointer to a const pointer

edited May 29, 2013 at 21:39

answered May 29, 2013 at 19:10

aaronman

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4 Comments

Nik Bougalis Over a year ago

Formatting-cat got your tongue? ;)

aaronman Over a year ago

Not anymore, the stars made some of the text bold

kotlomoy Over a year ago

"all the pointers are const" - no, two pointers are const, one pointers is non-const

aaronman Over a year ago

@kotlomoy fixed it but notice that I had it correct in the second half previously

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