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Explain the work with pointers in the following case

sassas081597
sassas081597 asked
on
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Last Modified: 2010-04-06
Hallo All

I have the following problem:

A function (for example GetBits(var P: Pointer)) returns me a pointer to a
number of bits (how many I know from the function GetSize : LongInt).
All I need is to read the Bits from P  twice: once as an array of Byte and
once as an array of LongInt. Who can explain me how can I get the 5th Byte
or the 3rd LongInt from the memory allocated for P.

Best wishes
Alexander
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Commented:
Declare an array of each type that you want as a pointer type.

TYPE
   taByte = ARRAY [0..99] OF BYTE;
   taLInt = ARRAY [0..99] OF LONGINT;

   paByte = ^taByte;
   paLInt = ^taLInt;

VAR
  BArray: paByte;
  IArray: paLInt;

You pass BArray or IArray in the call (or whatever you want) and then use the array index:

  Something := BArray^[34];   {This assumes that BArray contains a valid pointer}

Hope this helps,

Ian C.

Author

Commented:
I do not know at design time the length of the array of bits. It can be 2 bytes as well as 100000 bytes. It depends on the result of the function getSize: LongInt. May be that it returns a number that exceeds the maximum length of an array.
Commented:
Here's how to do it for LongInt.  Just change your pointer type for BTYEs.


var
  p : pointer;
  lp : ^LongInt;
  larray : array[1..10] of LongInt;
  value : LongInt;
begin
  larray[1] := 100;
  larray[2] := 200;
  larray[3] := 300;
  larray[4] := 400;
  p := @larray; { start of array memory - first element }

  { this is what your interested in }

  lp := p;
  Inc(lp); { point to second }
  Inc(lp); { point to third }
  Value := lp^;  { Value will be 300 }
end;

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Author

Commented:
I think you are right - this code is accepted by compiler. May be you can explain me why the code

lp:=lp+SizeOf(LongInt)

does not work in this case - the compiler stops. I think we write the same code.

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