TReader.Position
lazarus / 62-General

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Tesla2k · 2021-08-26 09:46

Hello,
I am trying to port this code from Delphi 7. I check if I am at the
end of the data with Reader.Position. Unfortunately this property does
not exists in FPC. Is there an other way to check if I a at the end of
the stream?

Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i  
Stream:=TMemoryStream.Create;  Reader:=TReader.Create(Stream,1024); 
//Load Data to Stream  Stream.Position:=0;  repeat       
Reader.ReadInteger();              Reader.ReadVariant;  until
Reader.Position=Stream.Size;

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Remy Lebeau · 2021-08-26 22:27

Quote from: jamie on August 26, 2021, 11:23:23 pmCode: Pascal
 [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i Until
Reader.EndOFList;
Checking if the stream contains an end marker is not the same thing as
checking if the stream is at the end of its available data.

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Remy Lebeau · 2021-08-26 23:02

Quote from: jamie on August 27, 2021, 12:35:08 amSo why isn't the
"Stream.Position >= Stream.Size" being used in that case ?

Because TReader has an internal buffer.  If you just look at
Stream.Position by itself, you won't take into account any data that
hasn't been read from the buffer yet.

Quote from: Tesla2k on August 26, 2021, 11:46:41 amIs there an other
way to check if I am at the end of the stream?

In both Delphi and FreePascal, TReader has an internal buffer. 
TReader.Position in Delphi subtracts the amount of unread data in the
buffer from the TStream's current Position, which is the total amount
of data that has been buffered.

But in FreePascal, TReader does not provide a direct way for users to
determine the amount of unread data in that buffer.  However, I can
think of a couple of indirect ways you can approach this.

TReader uses an internal TBinaryObjectReader to handle its buffer, and
has a public Driver property to access that object.  And fortunately,
TBinaryObjectReader's data members are declared as protected, not
private.  So you can use an accessor class, or a class helper, to
reach them at runtime, eg:

Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i type 
TBinaryObjectReaderAccess = class(TBinaryObjectReader)  end;   { or: 
TBinaryObjectReaderHelper = class helper for TBinaryObjectReader   
function GetPosition: Longint;    property Position: Longint read
GetPosition;  end;  }   TReaderHelper = class helper for TReader   
function GetPosition: Longint;    property Position: Longint read
GetPosition;  end; {function TBinaryObjectReaderHelper.GetPosition:
Longint;begin  with TBinaryObjectReader(Self) do    Result :=
FStream.Position - (FBufEnd - FBufPos);end;} function
TReaderHelper.GetPosition: Longint;begin  with
TBinaryObjectReaderAccess(Driver as TBinaryObjectReader) do    Result
:= FStream.Position - (FBufEnd - FBufPos);  // or: Result := (Driver
as TBinaryObjectReader).Position;end; ... Stream :=
TMemoryStream.Create;Reader := TReader.Create(Stream, 1024);//Load
Data to StreamStream.Position := 0;repeat  Reader.ReadInteger; 
Reader.ReadVariant;until Reader.Position = Stream.Size; // should work
now...
You could then take this a step further by deriving your own class
from TBinaryObjectReader, and then deriving a class from TReader to
override its virtual CreateDriver() method, eg:

Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i type 
TMyBinaryObjectReader = class(TBinaryObjectReader)    function
GetPosition: Longint;    property Position: Longint read GetPosition; 
end;   TMyReader = class(TReader)    function CreateDriver(Stream:
TStream; BufSize: Integer): TAbstractObjectReader; override;   
function GetPosition: Longint;    property Position: Longint read
GetPosition;  end; function TMyBinaryObjectReader.GetPosition:
Longint;begin  Result := FStream.Position - (FBufEnd -
FBufPos);end; function TMyReader.CreateDriver(Stream: TStream;
BufSize: Integer): TAbstractObjectReader;begin  Result :=
TMyBinaryObjectReader.Create(Stream, BufSize);end; function
TMyReader.GetPosition: Longint;begin  Result :=
TMyBinaryObjectReader(Driver).Position;end; ... Stream :=
TMemoryStream.Create;Reader := TMyReader.Create(Stream, 1024); // be
sure to declare Reader as TMyReader, not TReader!//Load Data to
StreamStream.Position := 0;repeat  Reader.ReadInteger; 
Reader.ReadVariant;until Reader.Position = Stream.Size; // should work
now...

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Tesla2k · 2021-08-28 08:49

Thank You very much! I will use the version with the TReaderHelper
class.

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marcov · 2021-08-28 10:34

Please file a bug for the missing property.

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Remy Lebeau · 2021-08-31 00:16

Quote from: marcov on August 28, 2021, 12:34:07 pmPlease file a bug
for the missing property.

A ticket already exists, created 11 years ago, closed and locked 3
weeks ago, so can't add comments to it:


#16765: Position property is missing from TReader (and TWriter)