Op 10 april 2012 22:06 heeft Darius Blaszyk <dhkblas...@zeelandnet.nl>
het volgende geschreven:
> 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];
I think this is not correct. If you increase the size of vert_list,
then the array may be relocated and Result isn't a valid pointer
anymore.
> 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.
>
Vincent
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