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Follow Up: Indy | Records, and Typecasts

Posted on 2004-07-30
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Last Modified: 2010-04-05
Hello there,
This Q is a Follow up of http:Q_21063476.html

I tried to add a field to the TLoginSizes and as soon as I did it the compiler stopped at "TLoginSizes(mr.DataLen).UsernameLen:=Length(Edit1.Text);" with the message "invalid typecast"

type
  TLoginSizes = record
    UsernameLen: Word;
    PasswordLen: Word;
    HexArraylen: Word;     //    New field...
  end;

  TMsgRec = record
    RecType: Byte;
    Command: Byte;
    DataLen: Cardinal;
  end;
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Question by:Cerf
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4 Comments
 
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by:Cerf
ID: 11681700
This Q has turned into "extremely urgent" for me... Raising to 500p
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Author Comment

by:Cerf
ID: 11681866
I see what you wrote previously,

>"it's a kindof an optimization .. I only send one record and the data once with an existing record type
DataLen is a cardinal field - 4 bytes
that TLoginSizes record has two word fields - 2x 2bytes
so the datasize is the same and using that typecast it can represent various things
if you take a look at TRect you'll see a similar thing"

and I guess it is about  the size of cardinal and 3 word fields = 6bytes but I don't really understand what are you doing there...
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Accepted Solution

by:
Wim ten Brink earned 500 total points
ID: 11685451
Use packed records...

TLoginSizes and TMsgRec might appear to be only 6 bytes byt don't have to be, just because the way Delphi aligns these fields. It's not a very safe typecast anyway and will be invalid in future Delphi versions so you better not use it. PErhaps a variant record is a better option. Example:

type
  TYourRecord = record
    case boolean of
      False: (RecType: Byte; Command: Byte; DataLen: Cardinal);
      True: (UsernameLen: Word; PasswordLen: Word; HexArraylen: Word);     //    New field...    
  end;

Or something similar. I don't have Delphi available right now so can't check...

If you're using global variables, you might also think of using the 'absolute' keyword:

var
  LoginSizes: TLoginSizes;
  MsgRec: TMsgRec absolute LoginSizes;

As a final solution, consider using pointers.
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by:Cerf
ID: 11686428
Thanks Alex!

I already fixed it changing a bit the comm protocol between the server and the clients to a simplier thing. As a friend told me...
"KISS" keep it simple, stupid :) so I did and works fine.
I only send the header record (TMsgRec) which is fixed size for sure and don't do any typecasting... the rest is interpreted and manipulated by the server. However, your post makes sense.

Thanks for interesting,

Cërf.

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