PRODUCT  :  C++                                    NUMBER  :  802
  VERSION  :  ANY
       OS  :  PC DOS
     DATE  :  October 19, 1993                         PAGE  :  1/1

    TITLE  :  char ** Vs. char[][]




  One of the pitfalls of the C language is based on the assumption
  that char **apple, char *orange[], and char cherry[][] are
  equivalent declarations. In practice, the types are virtually
  identical, but some subtle differences can lead to unintended
  results.

  'apple' is a pointer to a pointer, 'orange' is an array of
  pointers, and 'cherry' is simply a two dimensional array. If
  'apple' is declared as above, then expressions like apple[1], and
  *apple[3] are valid because the compiler knows what the latter
  expressions mean because it knows what type 'apple' really is.

  But, suppose the compiler had been told the incorrect type?  What
  happens then?  Well, this can only happen if the variable is
  defined in one module, say as 'char apple[][]' and declared
  external in another, as 'extern char **apple'. In the second
  module, when we use expressions for apple, the compiler will
  interpret them as if apple were a char ** type, since that is how
  it's declared (the compiler knows nothing of the definition in
  module one). What this means for the implementation is this:
  Let's compare char **apple with char cherry[][] 'apple' is the
  label for a pointer, so to get at the pointer, the compiler
  dereferences the label to get the first pointer.  Two more
  dereferences will be required to get at the data now.  But
  'cherry' is the label of an array, not a pointer.  So to access
  an array member, the compiler needs to calculate the offset into
  the array and use that as an offset of the label 'cherry'. There
  is considerable difference between looking at the data offset
  from a certain value, and dereferencing a sequence of pointers.
  These differences will cause the data to appear corrupted, when
  in reality, the data is fine and it is the measuring tool that is
  incorrect.  The moral of this is, of course, to remember the
  distinction between a char ** type and a char [][] type and not
  to mix declarations of such types.
  DISCLAIMER: You have the right to use this technical information
  subject to the terms of the No-Nonsense License Statement that
  you received with the Borland product to which this information
  pertains.