RE: RPG IV Pointers

[Colin Williams <colin.williams@xxxxxxxxxxxxxxxxx>]

Personally, I like to think of it as a data structure with a pointer
attached to it, that I can set to any value.

Got some data in a user space, set the pointer to the start of the user
space, and look at the data via the data structure
Got some data in another data structure, set the pointer to the start of
the other data structure, and look at the other data structure via the
data structure

Is a null pointer any pointer that has not been set yet?
-----Original Message-----
From: Jim Langston [mailto:jlangston@conexfreight.com]
Sent: Tuesday, February 29, 2000 11:28 PM
To: RPG400-L@midrange.com
Subject: Re: RPG IV Pointers


MyField, in this case, is also just a pointer?  Something to the effect
in C of
*char  MyPointer;
(Or is that char *MyPointer, I always get those two mixed up).

>From what I understand you as saying... We set up MyPointer as a
pointer.  In this case to use C terminology, a null pointer.  Then, we
set up MyField to be a pointer of type char, and, MyField is
going to "follow" MyPointer.  Wherever MyPointer is pointing to is
where the data for MyField is going to reside?  So, basically, we are
saying that wherever MyPointer points at, we can look at it as 50 bytes
of character.  Very similar, in effect, of a data structure.

Ahh, okay...  So, we can move our pointer around (MyPointer) to point
anywhere we want, but can't really access the data it is 
pointing to.  To
access that data, we use MyField, which just defines the type 
of data that
MyPointer is looking at.

So, in RPG we can address a pointer either as a pointer (MyPointer) or
as data (MyField), but not both.  So, we really need two 
pointers, one to
move the place it is point to around, and one to actually 
access the data.

Is that the way it is?

Regards,

Jim Langston

Joep Beckeringh wrote:

> Jim,
>
> Your first example:
> > D MyPointer                         S                              *
> This tells the compiler to define a variable called MyPointer of type
> pointer.  In other words: storage is allocated, which you can 
refer to by
> its name MyPointer.  The RPG compiler is a friendly one and 
even initializes
> your variable.  So its initial value is *NULL, in other words 
it doesn't
> point anywhere (yet).
> > D MyField                            S                        50
> 50 bytes of storage are allocated for your variable MyField.
> > C                                              Eval     MyPointer =
> %Addr(MyField)
> The address of the (first of the) 50 bytes you know by the 
name of MyField
> is assigned to MyPointer.  Only after an assignment like this is your
> pointer useable.
>
> Your second example:
> > D MyPointer                         S                              *
> Same as above.
> > D MyField                            S                        50
> Based(MyPointer)
> No storage is allocated.  You only instruct the compiler to 
make the 50
> bytes of storage that MyPointer is going to point to 
addressable as MyField.
> You have to assign a pointer value to MyPointer before you 
can do anything
> with MyField.
>
> Joep Beckeringh

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