[fpc-pascal] Re: Problems with assigning pointers

[Darius Blaszyk]

Here's a minimal example that has the bug. First of all the first output is wrong as it says 0,50 instead of 20,50 (so the x item is overwritten). The second printed output differs from the first, so again the x item is overwritten. I'm clueless.

program test;

{$mode objfpc}{$H+}

type
  TVertex = record
    x: double;
    y: double;
  end;
  PVertex = ^TVertex;

  TEdge = record
    v1: PVertex;
    v2: PVertex;
  end;

var
  vert_count: integer = 0;
  vert_list: array of TVertex;
  edge_count: integer = 0;
  edge_list: array of TEdge;

function add_vert(x, y: double): PVertex;
begin
  Inc(vert_count);
  SetLength(vert_list, vert_count);

  vert_list[vert_count - 1].x := x;
  vert_list[vert_count - 1].y := y;

  Result := @vert_list[vert_count - 1];
end;

procedure add_edge(v1, v2: PVertex);
begin
  Inc(edge_count);
  SetLength(edge_list, edge_count);

  WriteLn(v1^.x,' ',v1^.y); // this
  edge_list[edge_count - 1].v1 := v1;
  WriteLn(v1^.x,' ',v1^.y); // outputs the same thing as this
  edge_list[edge_count - 1].v2 := v2;
end;

var
  v1: PVertex;
  v2: PVertex;

begin
  v1 := add_vert(20, 50);
  v2 := add_vert(220, 50);

  add_edge(v1, v2);

  readln;
end.

Regards, Darius



On Apr 10, 2012, at 7:04 PM, leledumbo wrote:

> Can't reproduce with the following program:
> 
> type
>  TVertex = record
>    x: double;
>    y: double;
>  end;
>  PVertex = ^TVertex;
> 
>  TEdge = record
>    v1: PVertex;
>    v2: PVertex;
>  end;
> 
> var
>  edge_count: LongWord = 0;
>  edge_list: array of TEdge;
> 
> procedure add_edge(v1, v2: PVertex);
> begin
>  inc(edge_count);
>  SetLength(edge_list, edge_count);
>  WriteLn(v1^.x:1:4,' ',v1^.y:1:4); // this
>  edge_list[edge_count - 1].v1 := v1;
>  WriteLn(v1^.x:1:4,' ',v1^.y:1:4); // outputs the same thing as this
>  edge_list[edge_count - 1].v2 := v2;
> end;
> 
> var
>  a: PVertex;
> begin
>  New(a);
>  a^.x := 2.56;
>  a^.y := 3.14;
>  add_edge(a,nil);
>  Dispose(a);
> end.
> 
> 
> --
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