How do you access different data types using Pascal pointers? - delphi

Question

In general I have a problem figuring out how to handle pointer conversions in pascal. If I wanted a word from my byte pointer wouldn't I do something like:
type WordPtr = ^Word;
var P1, P2 : ^Byte; w, byteoffset : word;
begin
P1:=SomeGetMemVar; byteoffset:=8;
w:=WordPtr(#P1^[byteoffset])^; { invalid qualifiers at the [ }
w:=WordPtr(P1[byteoffset])^; { invalid qualifiers at the [ }
w:=WordPtr(P1+byteoffset)^; { type mismatch }
end;
Apparently not, because it doesn't work as shown in the comments.
How do I get this to work in Pascal? C would be:
uint16_t byteoffset=8;
uint8_t* P1=(uint8_t*) malloc(1000);
uint16_t w;
w=*((uint16_t*) (P1+byteoffset)); // method 1
w=*((uint16_t*) (&P1[byteoffset])); // method 2
Thanks!

Answer 1

In order to treat pointers as arrays, make sure that {$POINTERMATH ON} is enabled. It already is for PByte (don't use ^Byte directly), but not for most pointer types by default.
Try something more like this:
{$POINTERMATH ON}
var
P1 : PByte;
w, byteoffset : Word;
begin
P1 := SomeGetMemVar;
byteoffset := 8;
w := PWord(#P1[byteoffset])^;
//or:
w := PWord(P1+byteoffset)^;
end;
Alternatively, use Move() instead of casting pointers:
var
P1 : PByte;
w, byteoffset : Word;
begin
P1 := SomeGetMemVar;
byteoffset := 8;
Move(P1[byteoffset], w, SizeOf(w));
//or:
Move((P1+byteoffset)^, w, SizeOf(w));
end;
For older compilers that don't support {$POINTERMATH}, you can type-cast to PAnsiChar (which always supported pointer math by default):
var
P1 : PByte;
P2: PAnsiChar;
w, byteoffset : Word;
begin
P1 := SomeGetMemVar;
P2 := PAnsiChar(P1);
byteoffset := 8;
w := PWord(#P2[byteoffset])^;
//or:
w := PWord(P2+byteoffset)^;
end;
Or, just increment the PByte pointer manually:
var
P1, P2 : PByte;
w, byteoffset : Word;
begin
P1 := SomeGetMemVar;
byteoffset := 8;
P2 := P1;
Inc(P2, byteoffset);
w := PWord(P2)^;
end;

Answer 2

If your example is what you're trying to do, i.e. simply extract one byte of a word, you don't need to do all that pointer manipulation. Try this:
uses System.SysUtils;
type WordPtr = ^WordRec;
var w: word;
p1: WordPtr;
begin
p1=GetSomeMemVar;
w:=p1.Hi;
end;
WordRec is a record that has two byte fields in the space of a word, so that you can access each byte of the word. SysUtils also has LongRec and Int64Rec, and all have variant parts so that you can access separate bytes or separate words, etc, or treat them as arrays. And you can always create your own similar records.
If you want to pursue working with pointers, see https://docwiki.embarcadero.com/RADStudio/Sydney/en/Pointer_Math_%28Delphi%29