Re: Uninitialized "out" parameters

[fjh@mundook.cs.mu.OZ.AU (Fergus Henderson)]

bobduff@world.std.com (Robert A Duff) writes:

>Fergus Henderson <fjh@mundook.cs.mu.OZ.AU> wrote:
>>I haven't seen your example.  I tried searching for it using dejanews,
>>but I did not find any article in this thread posted by you that
>>gave an example of a 2 gig array.
>
>I haven't seen it, either, but I can guess what Robert meant.  You
>declare an array (1..One_Zillion), and you trust the operating system to
>*not* create storage for most of that array.  Normally, you just set the
>first few elements, and the others are memory-mapped to non-existent
>storage.  If you were required to say ":= (others => something);",

I would hope that a good static checking system would not require
initialization at the point of declaration as the only method of
initializing arrays; I would hope that it would allow common patterns
of initialization such as initializing all the elements one by one
in a loop.

>the compiler might actually cause writes to all that memory, causing the
>OS to actually allocate it, causing Storage_Error, or perhaps just a
>huge waste of memory.

I suppose that could be a serious problem (even if it only occurred
for uncommon patterns of initialization).  But you give a solution:

>On the other hand, if "something" happens to be, say, "0", then some
>operating systems can deal with it by initializing memory pages to zero
>as they are needed.  The compiler has to know about that fact, and act
>accordingly.  The DEC Ada-83 compiler on VAX/VMS does this.

--
Fergus Henderson <fjh@cs.mu.oz.au>   |  "I have always known that the pursuit
WWW: <http://www.cs.mu.oz.au/~fjh>   |  of excellence is a lethal habit"
PGP: finger fjh@128.250.37.3         |     -- the last words of T. S. Garp.