[fpc-devel] The slice function and bug #2856

[Ales Katona]

Peter Popov wrote:

> Hi All
> As you now, the Slice function (internal to the Delphi compiler) is 
> not  currently implented in fpc (see bug  
> http://www.freepascal.org/bugs/showrec.php3?ID=2856). I am wondering 
> how  easy it is to implement/circumvent this problem. In these regard, 
> can  someone shed light on the following two things:
>
> 1. Are open array parameters in fpc functions and procedure 
> implemented in  the same way as in Delphi. That is,
>
> procedure MyFunc(x: array of integer)
>
> is actually compiled as
>
> procedure MyFunc(count: Integer; x: ^Integer)
>
> and inside MyFunc, x is treated as a zero based arrays of size count, 
> and  count is only accessible through High(x).
>
> Are fpc open arrays done in the same way?
>
> 2. What is the internal structure of a dynamic array/open array in 
> fpc.  Maybe, given a pointer and a array size, one can hack a 
> temprorary dynamic  array structure (probably without typecasting) and 
> pass it as an open  array parameter. Even without typecasting, this 
> will let me compile  existing delphi code.
>
> Thanks
> Peter
>
Array slicing is already possible with the use of absolute eg:

program Project1;

{$mode objfpc}{$H+}

procedure Crap(var A: array of Integer);
var
  b: array[1..3] of Integer absolute A[2];
  i: Integer;
begin
  for i:=1 to 3 do b[i]:=0;
end;

var
  a: array of Integer;
  i: Integer;
begin
  SetLength(a, 6);
  for i:=0 to 5 do
    a[i]:=i + 1; // 1 to 6
  Crap(a);
  for i:=0 to 5 do
    Write(a[i], ' ');
  Writeln;
end.

Output is 1 2 0 0 0 6

Ales