Hi,

    Why multiplication of pointers is not allowed?
Till now I only know this, but not the reason why!

PS:  As a rule, I searched the FAQ, but could not
find an answer.

--
Vijay Kumar R Zanvar
My Home Page - http://www.oocities.org/vijoeyz/


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On Tue, 6 Jan 2004 12:37:08 +0530, "Vijay Kumar R Zanvar"
<vijoeyz@hotpop.com> wrote:

>Hi,
>
>    Why multiplication of pointers is not allowed?
>Till now I only know this, but not the reason why!
>
>PS:  As a rule, I searched the FAQ, but could not
>find an answer.

Adding an int to a pointer results in pointing to something a
specified distance further to the "right" in memory.

Subtracting an int from a pointer results in pointing to something a
specified distance further to the "left" in memory.

Subtracting one pointer from another results in how far apart the two
memory locations are.

If your program and data were to be magically relocated as a unit in
memory, each of the above expressions would still produce the same
result.

Until you can define a concept of either adding two pointers or
multiplying two pointers that meets the constraint in the previous
paragraph, the two operations make no sense.  (Hint: others have
thought this through and decided such a definition is either not
possible or of no programming value.)


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Vijay Kumar R Zanvar wrote:
>
> Hi,
>
>     Why multiplication of pointers is not allowed?
> Till now I only know this, but not the reason why!
>
> PS:  As a rule, I searched the FAQ, but could not
> find an answer.

    The result would be meaningless.  An easy (but
informal) way to understand this is to use an analogy:
think of pointer values as street addresses.  Then
the following operations make sense:

    - Find the house at "123 Main Street."

    - To find the neigboring house, compute "123
Main Street plus one."  (I'm ignoring such
real-world intrusions as odd-even numbering
schemes, discontinuities between city blocks,
and so on: we're just exploring an imperfect
analogy, after all.)

    - To find the distance between two Main Street
addresses, compute "123 Main Street minus 189
Main Street," yielding "minus 66 lots."

    However, some other arithmetical operations make
no sense at all:

    - Computing "123 Main Street plus 207 Main Street"
produces no useful answer.

    - Computing "123 Main Street minus 123 Elm Street"
produces no useful answer.

    - Similarly, computing "123 Main Street times
89 El Camino Real" makes no sense.  Perhaps the
result might be considered a kind of "area,"
but there seems to be no useful analogous
concept to "area" in the addressing of memory-
resident objects.

    - Finally, computing "123 Main Street times three"
might possibly make sense, arriving at "369 Main
Street."  But such a definition carries a built-
in assumption that Main Street is zero-based,
and in a linear computer memory no more than one
object can begin at "address zero."  If you want
to find the house three times as far from the
start of the street, you really want to compute
"Main Street Origin plus three times (123 Main
Street minus Main Street Origin)."

--
Eric.Sosman@sun.com
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