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SetLength / Move - causes memory corruption

Today I came across a problem that leads to damage to my array. 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 think memory damage after the latter SetLengthis only a symptom of poor use Move. Can someone explain to me what I'm doing wrong and how to use it correctly Movein this case?

In the original problem, I remove the elements from BoundryInfo in a loop, so I call Finalize(BoundryInfo[BoundryLength])

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2 answers

In your code

Move(BoundryInfo[PointIndex+1], BoundryInfo[PointIndex], 
  ((BoundryLength - 1) - PointIndex) * SizeOf(BoundryInfo[PointIndex]));

Copy the pointer BoundryInfo[PointIndex+1]to BoundryInfo[PointIndex]. This pointer is another dynamic array, you need to monitor the reference count.

I.e:

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().
+10

Move , . , . .

for i := 0 to high(BoundaryInfo)-1 do
  BoundaryInfo[i] := BoundaryInfo[i+1];
SetLength(BoundaryInfo, Length(BoundaryInfo)-1);
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