[DUG] Sizeof record gives error

[Robo]

I think your TLogData contains storage for an integer (TLogType, 4 bytes),
and a pointer to an array (4 bytes).

However, what you need is enough memory to store the entire array, not just
the pointer, so you should get the size of the array to get the correct
size.

On Thu, Aug 25, 2011 at 11:40 AM, David Moorhouse (DUG) <
delphi at moorhouse.net.nz> wrote:

> I have the following code snippet
>
> <code>
> type
>  PConstArray = ^TConstArray;
>  TConstArray = array of TVarRec;
>
> function CreateConstArray(const Elements: array of const): TConstArray;
>
> type
>  TLogType = (ltError, ltWarn, ltInfo);
>  PLogData = ^TLogData;
>  TLogData = record
>    LogType: TLogType;
>    LogArgs: TConstArray;
>  end;
>
> ....
>
> procedure TUserClass.Log(const LogType: TLogType; const Args: array of
> const );
> var
>  LogData: PLogData;
> begin
>    // size of record TLogData does not work
>    GetMem(LogData, sizeof(TLogData));
>    LogData.LogType := LogType;
> // blows up on next line
>    LogData.LogArgs := CreateConstArray(Args);
> //  ... do some other stuff with the LogData item finally calling FreeMem
> end;
>
> function CreateConstArray(const Elements: array of const): TConstArray;
> var
>  I: Integer;
> begin
>  SetLength(Result, Length(Elements));
>  for I := Low(Elements) to High(Elements) do
>    Result[I] :=  // assign a TVarRec here
> end;
> </code>
>
> The code that assigns the memory only assigns 8 bytes - and an access
> violation ensues.  If I replace the call to "sizeof" with the number 16,
> the code works fine.
>
> My understanding of dynamic arrays was that the compiler created a 4 byte
> field before the first element that contained the length of the array.
>
> So why does the sizeof  function not reflect this ?  And why do I need 16
> bytes not 12  (4 for LogType + 4 for length of array + 4 for array
> pointer)?
> Also regardless of the number of items in the open array parameter, 16
> bytes works, so it does not relate the length of the TConstArray.
>
> Your thoughts ?
>
> David
>
>
>
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