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From: Ehud Lamm <mslamm@mscc.huji.ac.il>
Subject: Re: Representation question (void *)
Date: 1999/11/17
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|Lipscomb, Kevin <KLipscomb@C2CEN.USCG.mil> wrote in message
|news:7E47A2C771ADD211AD3C00A0C960172D01269B57@ISCPORTSEX...
|> How would one represent the following C declaration in Ada 95?
|>
|>    void *data;
|>
|> Is this just a placeholder for a pointer that should never be referenced?
|>

In discussing the creating of a general sort routine for any type, I
recently wrote this:

"Let's examine the solution used in the C standard library. The C
standard library offers two interesting functions: \textbf{qsort} which is
a
sorting function and \textbf{bsearch} which is used for
searching. They are defined as follows:
\begin{verbatim}
void *bsearch(const void *key, const void *base, 
              size_t n, size_t size,
              int (*cmp)(const void *keyval, const void *datum))
  bsearch searches base[0]..base[n-1] for an item that matches
  *key. The function cmp must return negaitve if its first
  argument (the search key) is less than its second (a
  table entry), zero if equal, and positive if
  greater. Items in the array base must be in ascending
  order. bsearch returns a pointer to a matching item, or
  NULL if none exists.

void qsort(void *base, size_t n, size_t size,
           int (*cmp)(const void *, const void *))
  qsort sorts into ascending order an array base[0]..base[n-1] or
  objects of size size. The comparison function cmp is as in bsearch.
\end{verbatim}

\par
The C routines make use of a common programing trick in C. They
receive ``void *'' arguments, which are taken to mean any memory
address, and do the low-level pointer manipulation using the sizes
supplied as parameters. 
\par
Notice that this approach ignores type safety, since ``void *'' is
simply an idiom of ``any type.''  This has disadvantages for the user
of the routine, but also for the routines programmer, who must do low
level pointer arithmetic, instead of using the higher level of
abstraction provided by types.
\par
Ada provides a type safe solution, by providing \textit{generic
  programming} facilites,...."[1]

Of course you can mimic the C coid* trick by using the unchecked
programming facilies in Ada, with all their charm :-)

[1] Again from "The Little Abstractionsist"

Ehud Lamm mslamm@mscc.huji.ac.il
http://purl.oclc.org/NET/ehudlamm <== My home on the web 
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