From: gvision@ntlworld.com
"Bill Leary" wrote in message
news:I_qdnVyLKtdghZ_fRVn-pw@giganews.com...
> "RadSurfer" wrote in message
> news:1107412767.319036.77740@f14g2000cwb.googlegroups.com...
> > OK! TP 5.5 appears to be somewhat like Qbasic and VBforDos...
> > to use GetMem, I 1st declare a "pointer" (whatever that really means),
> >
> > BlockPtr: pointer;
> >
> > getmem(BlockPtr, AmountRam);
> >
> > What exactly is this "pointer" ? A far* that is 24-some-bits, or
> > what?
First off, you should avoid untyped pointers as much as possible. If you
have some type declared - lets call it TSomeThing - and you are using
pointers to access instances of it (that is, normally instances allocated
from the heap), then you also define a "typed pointer" for it:
type
PSomeThing = ^TSomeThing;
TSomeThing = record
Whatever: Integer;
BigNose: String;
end;
Note that there are two type declarations here - PSomeThing and TSomeThing.
The same thing can be used for predefined types:
type
PInteger = ^Integer;
PWord = ^Word;
..etc
A pointer in TP is always a far (16:16) pointer. That is, a 16-bit segment
and 16-bit offset.
Secondly, always use "New" and "Dispose" (not GetMem and FreeMem) to alloced
and release "typed memory":
var
P: PSomeThing;
begin
New(P);
....
Dispose(P);
end;
> > Is it possible to convert/treat this "pointer" as a purely integer
> > value ?
You can convert it to Longint, since it is the same size, but this strongly
discouraged since the only part of a pointer that can rationaly be
manipulated is the offset part. You can declare the following "special"
type-cast type and use that to perform pointer manipilation:
type
PtrRec = record
Ofs: Word;
Seg: Word;
end;
Now if "P" is an instance of some pointer type you can do the following:
Inc(PtrRec(P).Ofs, X);
Where "X" is an ordinal value.
Note that in TP7/BP7 you can also do the following:
Inc(P, X);
Dec(P, X);
Where "P" is still a typed-pointer instance. In this case, The value of X
will multiplied by SizeOf(P^) before being added to (the offset part of) P.
I don't know if TP5.5 can do this though (TP5.5 is VERY old).
> You're not supposed to ever mess with a Pascal pointer other than to
> assign to it. If the C program you're translating does a lot of pointer
math,
> things like p = p + 5; with the assumption that 'p' will be advanced by 5
> elements of what 'p' points to, or p = p + (sizeof)thing, or even p++,
you're
> probably in deep trouble.
Not deep trouble (at least not in TP7), but he would be better to convert it
to an array and manipulate the array index variable.
> > Is type-casting ever done in TPascal ?
Yes, but when you know the language better, you will try to avoid them like
the plague because they are ofter the source hard-to-find run-time errors.
In short, pointer manipulation and type-casting are both major causes of
errors, and are two reasons why C programs never work properly ;-)
> > Quite the challenge me thinks.... :-)
Not really. Your big challenge is to get you head around strong-typing. Note
that this is the way new mainstraim languages are going - C# and Java are
both strongly typed. Pascal has had it since the early 70's. Once the penny
drops, you'll wonder how you ever managed without it.
--
Jay
Author of Graphic Vision
http://homepage.ntlworld.com/gvision/
--- SoupGate-Win32 v1.05
* Origin: you cannot sedate... all the things you hate (1:229/2)
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