C: declare a constant pointer to an array of constant characters

Question

Hi,

I am trying to understand array declarations, constness, and their resulting variable types.

The following is allowed (by my compiler):

      char s01[] = "abc" ;  // typeof(s01) = char*
const char s02[] = "abc" ;  // typeof(s02) = const char* (== char const*)
char const s03[] = "abc" ;  // typeof(s03) = char const* (== const char*)

Alternatively, we can declare the array size manually:

      char s04[4] = "abc" ;  // typeof(s04) = char*
const char s05[4] = "abc" ;  // typeof(s05) = const char* (== char const*)
char const s06[4] = "abc" ;  // typeof(s06) = char const* (== const char*)

How do I get a resulting variable of type const char* const? The following are not allowed (by my compiler):

const char s07 const[] = "abc" ;
char const s08 const[] = "abc" ;
const char s09[] const = "abc" ;
char const s10[] const = "abc" ;
const char s11 const[4] = "abc" ;
char const s12 const[4] = "abc" ;
const char s13[4] const = "abc" ;
char const s14[4] const = "abc" ;

Thanks

Answer 1

const char *const s15 = "abc";

Answer 2

s01 et al are not really pointer types, they're array types. In that sense, they already act a bit like const pointers (you cannot re-assign s01 to point somewhere else, for instance).

Answer 3

Use cdecl:

cdecl> declare foo as constant pointer to array of constant char
Warning: Unsupported in C -- 'Pointer to array of unspecified dimension'
        (maybe you mean "pointer to object")
const char (* const foo)[]
cdecl> declare foo as constant pointer to array 4 of constant char
const char (* const foo)[3]
cdecl> declare foo as constant pointer to constant char
const char * const foo

Pointers to arrays are rarely used in C; usually API functions expect a pointer to the first element.

Answer 4

Your first typeof comments aren't really correct. The type of s01 is char [4], and the types of s02 and s03 are const char [4]. When used in an expression and not the subject of either the & or sizeof operators, they will evaluate to rvalues of type char * and const char * respectively, pointing at the first element of the array.

You can't declare them in such a way that they decay to an rvalue that itself is const-qualified; it doesn't really make any sense to have a const-qualified rvalue, since rvalues can't be assigned to. It's like saying you want a 5 constant that is of type const int rather than int.