extern const char * vs. extern const char []http://tinyurl.com/47e3k

From: Thomas Matthews (Thomas_MatthewsSpamBotsSuck_at_sbcglobal.net)
Date: 07/31/04 

Date: Sat, 31 Jul 2004 15:35:48 GMT

Hi,

I'm puzzled over the difference between:
     extern const char *
and
     extern const char [].
My understanding is that the two are equivalent,
but the program below shows elsewise.

Given the simple program below:
// File names.cpp
extern const char Hello_Text[] = "Hello";
extern const char World_Text[] = "World!";

// File main.cpp
#include <iostream>
#include <cstdlib>
using std::cout;
using std::endl;

template <const char * name>
void print_text(void) // personal pref. here
{
   cout << name << endl;
   return;
}

/* Refer to variables in names.cpp */
/* Note declaration of char * here */
/* versus char [] in names.cpp. */
extern const char * Hello_Text;
extern const char * World_Text;

int main(void)
{
   cout << "Start of Test" << endl;
   print_text<Hello_Text>();
   print_text<World_Text>();
   cout << "End of Test" << endl;
   return EXIT_SUCCESS;
}

When compiled with Gnu G++ 3.3.1, I get the following
error message (line numbers omitted):
$ g++ -o main.cpp extern_names.cpp
  In function `int main()':
error: `Hello_Text' is not a valid template argument
         it must be the address of an object with
         external linkage
         no matching function for call to
        `print_text()'
error: `World_Text' is not a valid template argument
         it must be the address of an object with
         external linkage
         no matching function for call to
         `print_text()'

I know that the following two statments are different:
(1) extern const char text[] = "Hello";
(2) extern const char * text = "Hello";
because (1) allocates an array of 6 locations and
(2) creates a pointer to a 'C' string literal.

But because the variable 'text' above in (1) is an
array and it has a location, a pointer can be created
and assigned to the location of the first character.
So, I would expect that
     extern const char * text;
would be a pointer that refers to the start of the
character array, much the same as this statment:
     extern const char text[];

So why are they different when used as template
parameters?

Does Chris Torek's "The Rule" apply here?
http://web.torek.net/torek/c/pa.html 

-- 
Thomas Matthews
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Other sites:
     http://www.josuttis.com  -- C++ STL Library book
     http://www.sgi.com/tech/stl -- Standard Template Library

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