Today I've stumbled on a problem which causes my array to be corrupted. Here is a reproducible test case:
unit Unit40;
interface
type
TVertex = record
X, Y: Double;
end;
TEdge = record
V1, V2: TVertex;
end;
TEdges = array of TEdge;
type
TBoundryInfo = array of TEdges;
procedure MemoryCorrupt;
implementation
procedure MemoryCorrupt;
var
BoundryInfo: TBoundryInfo;
i, PointIndex, BoundryLength: Integer;
begin
BoundryLength := 57;
PointIndex := 0;
SetLength(BoundryInfo, BoundryLength);
for i := 0 to BoundryLength - 1 do
begin
if i <> 17 then
begin
SetLength(BoundryInfo[i], 1);
BoundryInfo[i][0].V1.X := 1;
BoundryInfo[i][0].V2.X := 1;
BoundryInfo[i][0].V1.Y := 1;
BoundryInfo[i][0].V2.Y := 1;
end else
begin
SetLength(BoundryInfo[i], 2);
BoundryInfo[i][0].V1.X := 1;
BoundryInfo[i][0].V2.X := 1;
BoundryInfo[i][0].V1.Y := 1;
BoundryInfo[i][0].V2.Y := 1;
BoundryInfo[i][1].V1.X := 1;
BoundryInfo[i][1].V2.X := 1;
BoundryInfo[i][1].V1.Y := 1;
BoundryInfo[i][1].V2.Y := 1;
end;
end;
BoundryLength := 9;
SetLength(BoundryInfo, BoundryLength);
Move(BoundryInfo[PointIndex+1], BoundryInfo[PointIndex],
((BoundryLength - 1) - PointIndex) * SizeOf(BoundryInfo[PointIndex]));
Dec(BoundryLength);
Finalize(BoundryInfo[BoundryLength]);
SetLength(BoundryInfo, BoundryLength); //After this, arrays contains garbage
BoundryInfo[0][0].V1.X := 3;
end;
end.
I guess memory corruption after last SetLength
is only a symptom of bad usage of Move
.
Could someone explain to me what am I doing wrong and how to properly use Move
in this case?
In original problem I am removing elements from BoundryInfo in a loop, that is why I am calling Finalize(BoundryInfo[BoundryLength])
In your code,
Move(BoundryInfo[PointIndex+1], BoundryInfo[PointIndex],
((BoundryLength - 1) - PointIndex) * SizeOf(BoundryInfo[PointIndex]));
Will copy the pointer of BoundryInfo[PointIndex+1]
to BoundryInfo[PointIndex]
. This pointer is another dynamic array, you have to take care of reference counting.
That is:
SetLength(BoundryInfo[PointIndex],0); // release memory
Move(BoundryInfo[PointIndex+1], BoundryInfo[PointIndex],
((BoundryLength - 1) - PointIndex) * SizeOf(BoundryInfo[PointIndex]));
PPointerArray(BoundryInfo)^[BoundryLength-1] := nil; // avoid GPF
In short:
move()
;move()
.By using Move and subverting the dynamic array reference counting mechanism you are simply setting a trap for yourself. I would strongly recommend that you stick within the standard mechanisms, and let the compiler worry about the details. It will get them right every time.
for i := 0 to high(BoundaryInfo)-1 do
BoundaryInfo[i] := BoundaryInfo[i+1];
SetLength(BoundaryInfo, Length(BoundaryInfo)-1);
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