janus
5/18/2011 10:32:00 AM
On Tuesday, May 17, 2011 6:43:58 PM UTC+1, Shao Miller wrote:
> On 5/17/2011 04:21, janus wrote:
> > On Monday, May 16, 2011 3:25:35 PM UTC+1, Shao Miller wrote:
> >> On 5/16/2011 07:05, janus wrote:
> >>>> Shao Miller
> >>>> /* Join non-empty strings with a specified, non-empty delimiter string */
> >>>> int join_strings_with_delim(
> >>>> struct string * output_str,
> >>>> const struct string * const * input_strings,
> >>>> const unsigned int input_string_count,
> >>>> const struct string * delim,
> >>>> int allocation
> >>>> ) {
> >>>> int i;
> >>>> size_t output_size;
> >>>> char * buf;
> >>>>
> >>>> /* Check output string */
> >>>> ...
> >>>
> >>> I am confuse... Could explain the following;
> >>>
> >>> const struct string * const * input_strings, # Having two consts and pointer symbol staggered
> >>
> >> Absolutely, I'm happy to explain. :)
> >>
> >> 'input_strings' is a pointer ('*') to a 'const'-qualified pointer ('*')
> >> to a 'const'-qualified 'struct string'.
> >>
> >> ( [ (const struct string) * const] * input_strings)
> >>
> >> What does this madness mean? It means:
> >>
> >> - 'input_strings' is not 'const'-qualified, so it can be changed. For
> >> example, you could do:
> >>
> >> input_strings++;
> >>
> >> - 'input_strings' points to a pointer. _That_ pointer is
> >> 'const'-qualified, so it mustn't be changed. For example, you could not do:
> >>
> >> input_strings[0] = ...
> >> *input_strings = ...
> >>
> >> - 'input_strings' points to a pointer, and _that_ pointer points to a
> >> 'struct string'. That 'struct string' is 'const'-qualified, so it
> >> mustn't be changed. For example, you could not do:
> >>
> >> input_strings[0][0].len = ...
> >> (*input_strings)[0].len = ...
> >> (*input_strings)->len = ...
> >> (*(*input_strings)).len = ...
> >
> > Let me explain what I have picked all this while.
> > "const struct string * const * input_strings" is a two dimensional array.Which I could re-write as
> > const struct string const input_strings[n][m]. input_strings[i] may be updated, however input_strings[i][k] will not update. And input_strings[i][k].len will not update.
>
> Well, not exactly. As my understanding goes, an array, no matter how
> many dimensions it has, implies a contiguous range of elements of the
> base-level element type. For example:
>
> int main(void) {
> static const char ca[3][5] = {
> {'0', '1', '2', '3', '4'},
> {'5', '6', '7', '8', '9'},
> {'A', 'B', 'C', 'D', 'E'},
> };
> return 0;
> }
>
> The above multi-dimensional array's characters, in memory, would
> resemble the characters in:
>
> char ca_bytes[] = {'0', '1', '2', '3', '4', '5', '6', '7', '8', '9',
> 'A', 'B', 'C', 'D', 'E'};
>
> However in the code sample you are posting in response to, there is a
> one-dimensional array of 'struct string' for "New York", "New Jersey",
> "New London". Then there is a _separate_ array of _pointers_ where each
> element (a pointer to 'struct string') _points_ to an element of the
> first array. The two arrays are separate, and even have different base
> element types: 'struct string' for the first, but 'struct string *' for
> the second.
>
> The 'join_strings_with_delim()' function takes an array of 'struct
> string *' rather than an array of 'struct string'. The 'const'
> qualifiers mean "I shall not modify the array of pointers, nor shall I
> modify the pointed-to structures." Since there are two subjects that
> are not modified, that is why there are two 'const's in there.
Miller, Thanks so much. It makes sense now.
Janus