From:	kagel AT quasar DOT bloomberg DOT com
Date:	Fri, 7 Feb 1997 10:22:59 -0500
Message-Id:	<9702071522.AA27953@quasar.bloomberg.com >
To:	tudor AT cam DOT org
Cc:	djgpp AT delorie DOT com
In-Reply-To:	<32FA7146.3883@cam.org> (message from Tudor on Thu, 06 Feb 1997 16:03:18 -0800)
Subject:	Re: Question on pointers and arrays
Reply-To:	kagel AT dg1 DOT bloomberg DOT com

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   From: Tudor <tudor AT cam DOT org>
   Newsgroups: comp.os.msdos.djgpp
   Date: Thu, 06 Feb 1997 16:03:18 -0800
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   Demandred wrote:
   > 
   > Probably a silly question, but...
   > 
   > If I declare an array of objects of type Foo
   > 
   > Foo FooArray[5];
   > 
   > And pass a pointer to a Foo into a function (or class constructor)
   > 
   > Bar(Foo *array) {...
   > 
   > Can I access elements in the array in the function, like so?
   > 
   > ...array[3]...}
   I guess you can.
   When you say Foo array[5] then 'array' is actually a pointer to the
   first element.
   char string[5]="abcde" and
   char *string="abcde" are equivalent.
No they are not!
   char string[5]="abcde"     -- This line allocates an array of 5 chars and places
				the 6 chars "abcde\0" beginning at that location 
				trashing the next byte.  (I'll give you the typo.)
   char *string="abcde"       -- This line allocates a pointer variable and assigns
				the address of the static string "abcde\0" to that
				pointer.
That's the same thing you say?  Try this:

  char *string1 = "abcde", *string2 = "abcde";
  char string3[6] = "abcde", string4[6] = "abcde";


  printf( "&string1=%p\n", string1 );/* String1 & string2 hold the same address */
  printf( "&string2=%p\n", string2 );/*   unless -fwritable-strings is included */
  printf( "&string3=%p\n", string3 );/*   in the compile. String3 and string4 */
  printf( "&string4=%p\n", string4 );/*   are at different addresses and in a */
				     /*   whole other segment than string1 & 2 */
  fflush( stdout );
  strcpy( string1, "fghij" );    /* First problem, most compilers, including 
				   GCC/DJGPP, will SEGV here because static strings
				   are usually put into an unwritable string space.
				   With all references to the same string pointing 
				   to the same single constant value.  Some compilers
				   will not fault but string2 will also
				   have been modified.  GCC solves both problems 
				   with the -fwritable-strings compile argument. */
  strcpy( string3, "fghij" );

  printf( "string1: %s\n", string1 );
  printf( "string2: %s\n", string2 );
  printf( "string3: %s\n", string3 );
  printf( "string4: %s\n", string4 );

-- 
Art S. Kagel, kagel AT quasar DOT bloomberg DOT com

A proverb is no proverb to you 'till life has illustrated it.  -- John Keats

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