Re: Array of char pointers.

From:

"Ben Voigt [C++ MVP]" <rbv@nospam.nospam>

Newsgroups:

microsoft.public.vc.language

Date:

Mon, 21 Apr 2008 11:11:38 -0500

Message-ID:

<eZyzOo8oIHA.552@TK2MSFTNGP06.phx.gbl>

Barry Schwarz wrote:

On Thu, 17 Apr 2008 09:54:42 -0500, "Ben Voigt [C++ MVP]"
<rbv@nospam.nospam> wrote:

New code should always use const whenever you make a pointer to a
string literal.

Not if his code he contains lines like
char x[] = "abc";
arr[0] = x;

Like I said in the beginning, const is appropriate only if all the
elements of arr always point to non-modifiable data.

That snippet doesn't make a pointer to a string literal. It
initializes an array in writable memory. There's a big difference.

I stand by the correctness and universal applicability of my earlier
rule.

So how does your rule deal with

int main(void)
{
   char *arr[] = {"a", "bc", "def", "ghij"};
   char var[] = "variable text";
   int i;
   if (i = function_that_decides_predefined_text_is_inappropriate())
       {
       if (i > 0)
           {
           var[7] = 'q';
           var[8] = '\0';
           }
       else
           {
           var[3] = 'y';
           var[4] = '\0';
           }
       arr[0] = var;
       }
   for (i = 0; i < 4; i++)
       puts(arr[i]);
   return 0;
}

By changing the type of arr to:

const char *arr[] = { ... };

Which is as it should be.

My only point is that just because a pointer starts out pointing to a
string literal does not mean it will always point to one.

Then it's a polymorphic pointer. char* is a subtype of const char*, if your
collection can contain either then it should be defined using the supertype
as only the operations allowed on the supertype are safe.

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