Re: RPG IV Pointers

[Jim Langston <jlangston@xxxxxxxxxxxxxxxx>]

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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