Am 10.04.2012 22:16, schrieb Vincent Snijders:
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.
This is indeed the problem.
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);
Your pointer to v1 is already invalid here, because the second
"add_vert" already changed the location of the array and thus the new
pointer locations are also different.
add_edge(v1, v2);
readln;
end.
A better solution if you want to keep the "vertex array" would be to
have an array of "PVertex" instead of "TVertex". You then need to do a
"New(Result)" in your "add_vertex" function and add that "Result" to the
enlarged array. Similiary you'll need to do a "Dispose(...)" in some
"remove_vertex" procedure.
Regards,
Sven
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